A team of astronomers, led by Sayyed Ali Rafi, a graduate student from the University of Tokyo, has recently discovered evidence of water vapor (H₂O) in the atmosphere of the hot Saturn, HD 149026 b (Figure 1). This exoplanet, located about 250 light years from Earth in the Hercules constellation, is a type of hot gas giant similar in size to Saturn but orbits extremely close to its host star. It orbits a metal-rich evolved star, HD 149026, nearly 10 times closer than Mercury’s orbit around the Sun, resulting in a year that lasts only about 2.9 days! This proximity causes the temperatures of such hot gas giants to soar above 1500 Kelvin. Specifically, HD 149026 b has an equilibrium temperature of approximately 1700 Kelvin, hot enough to melt even the strongest steel.

 This paper will be published in The Astronomical Journal on August 5th, 2024.

To detect atmospheric signatures from the planet, the team used a technique called transmission spectroscopy. When the planet transits or passes in front of its host star relative to the observer on Earth, some of the star’s light passes through the planet’s atmosphere. This starlight is absorbed by various gases in the atmosphere, creating a planetary absorption spectrum that is imprinted on the stellar spectrum. By separating the stellar spectrum from the planetary spectrum, such as by subtracting the spectrum observed outside of transit (where there’s no atmospheric absorption from the planet), the atmospheric signatures of the planet can be identified.

One major challenge in observing exoplanetary atmospheres is the extremely high contrast between the bright star and the dim planet. This makes the planet’s atmospheric signatures difficult to detect, often buried below the stellar photon noise. The strength of the planet’s signatures would be stronger if we observed planets with either higher temperatures (resulting in more extended atmospheres and easier detection), closer distances to their host stars (making it easier to separate the stellar and planet spectra), or a combination of both. Hot gas giants possess both these properties, making them ideal targets for transmission spectroscopy observations, though their atmospheric signatures remain challenging to detect.

“We can boost the exoplanet signal by combining the information of hundreds or thousands of weak spectral absorption lines that are individually resolved in high-resolution spectroscopy using cross-correlation. This is one of the most successful methods used to characterize the atmosphere of exoplanets so far allowing us to take a peak of the atmosphere of alien worlds”, explains Dr. Stevanus Kristianto Nugroho from Astrobiology Center, who co-authored this study.

Using this technique, the team analyzed high-resolution transmission spectroscopy archival data from CARMENES, a high-resolution spectrograph installed at the 3.5-meter Calar-Alto Observatory in Spain. They focused on the near-infrared wavelength range (0.97 – 1.7 μm) of the spectrograph to search for signs of H₂O and HCN (hydrogen cyanide), which have strong absorption features in this range. “We found an evidence of H₂O in HD 149026 b’s atmosphere at an S/N of 4.8 whilst we cannot find anything related to HCN (Figure 2)”, the lead author, Rafi, said. Last year, interestingly, the James Webb Space Telescope (JWST) also detected H₂O on HD 149026 b, but from its dayside rather than during transit. This complementary finding supports the presence of water vapor in the planet’s atmosphere, indicating that water is present in different regions of the planet’s atmosphere. The team emphasized that their discovery, however, still needs to be confirmed by more transit observations follow-up. As for HCN, the non-detection, the team outlined in their work, might be attributed to the data’s S/N which perhaps might not be enough to detect the molecule.

So, why search for H₂O and HCN? In the atmosphere of hot gas giants like HD 149026 b, if the carbon-to-oxygen ratio (C/O) is less than one (indicating that oxygen is more abundant than carbon), H₂O and carbon monoxide (CO) are the most abundant oxygen and carbon-bearing species. If the C/O ratio is greater than one, H₂O becomes less abundant, and HCN becomes more prevalent, alongside CO, whose abundance remains relatively constant. By finding and determining the abundance of these gases, scientists can measure the atmospheric C/O ratio, which is crucial to infer the formation and evolution history of gas giant planets like HD 149026 b. “This detection of water vapor on a hot Saturn-like exoplanet offers new clues about its atmospheric dynamics and orbital properties and takes us one step closer to understanding planetary formation”, explains Dr. Alejandro Sánchez-López from Instituto de Astrofisica de Andalucia and co-authored this work.

Studying the atmosphere of HD 149026 b is particularly important due to its unique characteristics. This planet has an anomalously large core, estimated to be up to around 110 Earth masses, which challenges existing planet formation models such as gravitational instability and core accretion. These models typically predict much smaller cores for gas giants, so forming a core of this size suggests unusual conditions or processes. Several theoretical scenarios have been proposed, and any follow-up atmospheric observations of this planet could help support one of these theories or even suggest a new one.

Graduate students play a pivotal role in the field of exoplanetary research, as demonstrated by the work of Rafi and his team. Their work highlights the significant contributions these young researchers make. Graduate students’ ability to conduct such study using data from the state-of-art instrument is essential for advancing our understanding of distant worlds, thus highlighting their important role in this rapidly evolving scientific discipline.

PUBLICATION

Journal: The Astronomical Journal
”Evidence of Water Vapor in the Atmosphere of a Metal-Rich Hot Saturn with High-Resolution Transmission Spectroscopy”
Authors: S. A. Rafi, S. K. Nugroho, M. Tamura,  et al.
DOI: 10.3847/1538-3881/ad5be9
URL: https://doi.org/10.3847/1538-3881/ad5be9

The post Graduate Student Found Evidence of Water Vapor in the Atmosphere of a Hot Saturn first appeared on Astrobiology Center, NINS.

• Our team discovered mini-Neptunes^*1 around four red dwarfs^*2, which are named TOI-782, TOI-1448, TOI-2120, and TOI-2406, using observations from a global network of ground-based telescopes with MuSCATs and the TESS space telescope^*3.
• These four mini-Neptunes are close to their parent stars, and the three of them are likely to be in eccentric orbits (TOI-782 b, TOI-2120 b, TOI-2406 b).
• These mini-Neptunes are not rocky planets like Earth but may be Neptune-like planets.

abstract

An international team of astronomers, led by Yasunori Hori and Teruyuki Hirano from Astrobiology Center and Akihiko Fukui and Norio Narita from The University of Tokyo, has reported the discovery and follow-up of four short-period mini-Neptunes around red dwarfs older than one billion years. At least three of these mini-Neptunes are likely to be in eccentric orbits. The fact that these mini-Neptunes have maintained non-zero eccentricities for billions of years after their birth suggests that they may not be rocky planets like Earth but Neptune-like planets that are less susceptible to tidal deformation. This study should provide a clue to the origins and elusive interior structures of mini-Neptunes.

 This paper was published in The Astronomical Journal on May 30, 2024.

introduction

Planets between the size of Earth and Uranus/Neptune, known as mini-Neptunes, are not found in our Solar System. However, mini-Neptunes are relatively common outside the Solar System and are promising targets for atmospheric characterization by the James Webb Space Telescope. What do mini-Neptunes look like?

results

We have discovered four transiting^*4 short-period mini-Neptunes orbiting red dwarfs (TOI-782, TOI-1448, TOI-2120, and TOI-2406) through follow-up observations with ground-based telescopes with MuSCATs (a series of Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets^*5). These mini-Neptunes have radii about 2-3 times that of Earth and orbital periods of less than eight days. In addition, our radial velocity measurements^*6 of their parent stars, obtained with the IRD (InfraRed Doppler) on the Subaru telescope, indicate that the upper limit on the masses of these four planets is less than 20 times the mass of Earth. The relationship between the measured radii and the upper mass limits of these mini-Neptunes suggests that they are not rocky planets like Earth. Their interiors likely contain volatiles such as icy materials like H₂O and atmospheres. 

We also found that at least three of these four mini-Neptunes (TOI-782 b, TOI-2120 b, TOI-2406 b) are likely to be in eccentric orbits. In general, the orbit of a short-period planet around a red dwarf should be circular due to tidal dissipation. However, three short-period mini-Neptunes around red dwarfs have maintained non-zero eccentricities for billions of years. One possible interpretation of this is that their interiors are not susceptible to tidal effects. The mass-radius relationship of these four mini-Neptunes suggests that they are not rocky planets. Thus, the interiors of these mysterious mini-Neptunes may be similar to those of Neptune. Short-period mini-Neptunes are promising targets for atmospheric observations with the James Webb Space Telescope. Further detailed follow-up observations are expected to improve our understanding of the internal compositions and atmospheres of short-period mini-Neptunes.

acknowledgments

This research was supported by Grant-in-Aid for Scientific Research (KAKENHI: Grant-in-Aid for Scientific Research No. JP18H05439, JP18H05442).

Publication

Journal: The Astronomical Journal
”The Discovery and Follow-up of Four Transiting Short-Period Sub-Neptunes Orbiting M dwarfs”
Authors: Hori, Y., Fukui, A., Hirano, T. et al. (2024)
DOI: 10.3847/1538-3881/ad4115
URL: https://iopscience.iop.org/article/10.3847/1538-3881/ad4115

*1: Mini-Neptunes or sub-Neptunes are planets between the size of Earth and Neptune (about 4 times the radius of Earth).
*2: M-type stars with effective temperatures below ~3,800K.

*3: NASA’s space telescope, the Transiting Exoplanet Survey Satellite (TESS).

*4: Transit is a phenomenon caused by a planet partially blocking starlight as it passes in front of the star.

*5: MuSCAT series are multi-color cameras mounted on 1~2m class grand-based telescopes.

*6: The gravitational pull of a planet causes its parent star to wobble. The radial velocity method (or the Doppler method) uses the apparent variations in the velocity of a star in the direction of the line of sight to detect an unseen planet.

The post The Discovery of Enigmatic Mini-Neptunes in Unexpectedly Eccentric Orbits first appeared on Astrobiology Center, NINS.

Summary

The team led by researchers from the Astrobiology Center discovered an “approximately Earth-sized planet” K2-415b orbiting the red dwarf star K2-415 located 71 light-years away from Earth with a period of about 4 days (see Figure 1) by the “transit method”, which takes advantage of planets passing in front of stars causing a slight eclipse. Using the Subaru Telescope, they placed constraints on its mass and other physical parameters, confirming that its composition is consistent with those of a terrestrial (rocky) planet. Until now, very few transiting planets have been found around stars as light and cool as K2-415, making the K2-415 system a valuable observational target for studying the atmospheres and orbital characteristics around such cool stars. Moreover, K2-415b is currently the closest planet to Earth among the planets (including candidates) discovered by the Kepler satellite operating between 2009 and 2018, making it an excellent observational target for future observations using instruments like the James Webb Space Telescope (JWST). This achievement was published online in The Astronomical Journal on February 27, 2023（Hirano et al, 2023, “An Earth-sized Planet around an M5 Dwarf Star at 22 pc”）

Research Background

More than 5,300 exoplanets have been discovered so far, most of which are found around stars similar to our Sun in mass and surface temperature (solar-type stars). On the other hand, red dwarfs, which are less than half the mass of the Sun, are known to be the most common type of planets in our galaxy, but since red dwarfs are particularly faint and difficult to observe in visible light, the atmospheric and orbital characteristics of the planets around them are not as well understood as those around solar-type stars. The atmospheres and orbits of the planets around them are not as well known as those around solar-type stars. Transit planet systems, in which a planet passes in front of its star, are important targets to study the atmospheres and orbits of planets, but few transit exoplanets have been found, especially around late M-type dwarfs, which have masses less than 0.2 times that of the Sun.

Research Findings

The research team analyzed in detail the data acquired from 2017 to 2018 by the second transit exoplanet mission K2, launched in 2009 by NASA’s Kepler satellite, and discovered a transit planet “candidate” orbiting a red dwarf star K2-415, located 71 light years from Earth, with a period of 4.02 days (Figure 2). The data was analyzed in detail using a proprietary method, and a transit planet “candidate” was discovered orbiting a red dwarf star K2-415 with a period of 4.02 days, located 71 light years away from the Earth (Figure 2). The team conducted follow-up observations of K2-415 using the Subaru Telescope and other telescopes from 2018 to 2021 in order to confirm that the candidate is a real planet. K2-415 is a very cold star with a mass about 0.16 times that of the Sun and an effective surface temperature below 3,000 degrees Celsius, making it faint in visible light and difficult to observe with conventional visible light instruments. The team confirmed that K2-415 is a real planet (named K2-415b) with a radius 1.02 times that of the Earth and a surface temperature of about 100 to 140 degrees Celsius, based on precise changes in line-of-sight velocity.

K2-415 was also observed by the Transiting Exoplanet Survey Satellite (TESS), the successor to the Kepler satellite, at the end of 2021, and the transit by K2-415b was independently detected from observations of stellar brightness changes over a period of about 80 days (Figure 3). The team analyzed the combined data from K2 and TESS to precisely determine the planetary radius, period, etc.

K2-415 is one of the lightest and coldest stars with an Earth-size planet, and only four such transit planetary systems (Note 2), including the famous TRAPPIST-1 system, have been discovered so far. K2-415b is a particularly valuable target for studying the characteristics of planets around low-temperature red dwarfs, since detailed spectroscopic observations of transits can provide information on planetary atmospheres, orbits, and so on. K2-415 is about 71 light years from Earth, which is quite close to Earth for a star with a transit planet (i.e., the star is relatively bright), which will be an advantage for future observations. The Kepler satellite has detected thousands of planets and their candidates during its observations from 2009 to 2018, and the newly discovered K2-415b has been confirmed to be the closest planet to Earth among those discovered by the Kepler satellite so far (Note 3).

The atmospheres and orbits of terrestrial planets around low-temperature stars are not well understood due to the paucity of previous observations. Now that a near-Earth sample has been observed, JWST will be able to study the atmospheres of these planets in more detail. The large ground-based telescope will also provide information on the orbits of these planets, and the study of their atmospheres and orbits will help us to understand terrestrial planets around low-temperature stars, which are worlds different from our Earth.

Annotation:

1) Ref: July 2, 2018 ABC release Searching for a second Earth, new instrument IRD is up and running!

2) Only four systems with Earth-like transit planets in stars cooler than K2-415 have been found so far: TRAPPIST-1, LP 791-18, LHS 1140, and Kepler-42.

3) Apart from the discoveries by the Kepler satellite, transit planet systems closer to the Earth than K2-415 have been found in recent years, mainly by TESS observations. However, there are only about 14 cases of systems with Earth-like planets like K2-415.

Publication

Journal：The Astronomical Journal

“An Earth-sized Planet around an M5 Dwarf Star at 22 pc”

DOI：10.3847/1538-3881/acb7e1

Authors：Teruyuki Hirano, et al.

The post Discovery of the Closest Terrestrial Planet among Planet Candidates by the Kepler Satellite first appeared on Astrobiology Center, NINS.

As of 2022, the Transiting Exoplanet Survey Satellite (TESS) (Note 1) is searching for exoplanets using the phenomenon called “transit,” in which a planet passes in front of its star. TESS uses four ultra-wide-field cameras to observe a 24° × 96° region of the sky for 27.4 days at a time, looking for periodic dimming of stars during transits. The low-temperature star (red dwarf; Note 2) LP 890-9, where the planet was discovered this time, was found by TESS to have a period of 2.73 days, and the name of the transit planet candidate “TOI-4306.01” was released to the world on July 21, 2021.

The Japanese MuSCAT team (Note 3) and the Belgian SPECULOOS team (Note 4), which are participating in the TESS Follow-up Observing Program (TFOP), the official follow-up observation program of TESS, will independently carry out follow-up observations after August 2021 to confirm whether this planet candidate The TESS team has been working independently since August 2021 on follow-up observations to confirm whether this planetary candidate is real or not. The periodic dimming found by TESS can also occur when two stars (binary stars) are obscuring each other.

The MuSCAT team confirmed that TOI-4306.01 is a planet (LP 890-9 b) by October 2021, based on multicolor transit observations with the MuSCAT3 four-color simultaneous imaging camera at Haleakala Observatory on Maui and line-of-sight velocity observations with Subaru’s IRD infrared Doppler instrument. The SPECULOOS team has confirmed that TOI-4306.01 is a planet (LP 890-9 b).

On the other hand, the SPECULOOS team has continuously observed LP 890-9 since August 2021, including the non-transit time of TOI-4306.01, and discovered another period of dimming (another transit planet candidate) in October and November 2021. Although the SPECULOOS team’s data could not narrow down the planet’s orbital period to one, the MuSCAT team, in cooperation with the SPECULOOS team, performed follow-up observations with MuSCAT3 and found that this transit planet candidate is a real planet (LP 890-9 c) with an orbital period of about 8.46 days. The MuSCAT team is led by Tokyo University’s Professor Masahiro Nakamura, who is also a member of the MuSCAT team.

The IRD line-of-sight velocity measurements provided strong constraints on the mass of the planet candidate and were decisive in confirming that the two objects orbiting LP 890-9 are real planets,” said Professor Noriyasu Narita of the University of Tokyo, who led the MuSCAT team.

The two discovered exoplanets LP 890-9 b and LP 890-9 c are super-Earths (Note 5) with radii of 1.32 and 1.37 Earth radii, respectively. Planets with these radii are theoretically considered rocky planets slightly larger than Earth. The outer of the two, LP 890-9 c, lies within the so-called habitable zone, a region where the distance from the main star (LP 890-9) meets the conditions for liquid water to be retained on the planet’s surface. The reason a planet with an orbital period of less than 10 days, i.e., in close proximity to its host star, is in the habitable zone is that its host star is a small star with a radius about 15% that of the Sun and a surface temperature of only about 2600 degrees Celsius (the Sun is about 5500 degrees Celsius).

LP 890-9 c has only just been discovered, and at this point we do not know what kind of world it is or whether it is a habitable world. However, since LP 890-9 c is a transit planet, future follow-up observations of the transit will allow us to study its atmospheric composition, cloud cover, and other atmospheric properties in detail. The nature of the atmosphere greatly affects the stability of liquid water on the surface. Even if future observations show that life is unlikely on this planet, it is important to study the atmospheric properties of rocky planets in the habitable zone to determine what kind of existence our planet has in the universe. In this respect, this discovery provides an important research target for further study in the future.

The results of this research were published in the online edition of the European scientific journal Astronomy & Astrophysics on September 7, 2022. (Delrez et al. “Two temperate super-Earths transiting a nearby late-type M dwarf“)

This research was supported by Grant-in-Aid for Scientific Research (KAKENHI: Grant-in-Aid for Scientific Research (KAKENHI: JP15H02063, JP17H04574, JP18H05439, JP18H05442, JP19K14783, JP21H00035, JP21K13975, JP21K20376, JP22000005), Grant-in-Aid for Young Scientists (JP20J21872), Japan Science and Technology Agency (JST) CREST (JPMJCR1761), National Institutes of Natural Sciences (NINS) Astrobiology Center Project (AB031010, AB031014), and social welfare corporation Azusa Tomokai. The project was conducted with support from the Azusa Yuukai social welfare corporation.

For more information, please see the University of Tokyo press release.


(Note 1) Transiting Exoplanet Survey Satellite (TESS) is a NASA satellite program led by the Massachusetts Institute of Technology. The plan has been to explore almost all of the transiting planets in the sky over a two-year period. It is currently in its fifth year of observations, with the second phase of the extension program underway. During the four years of the first extension plan, more than 5,000 transit planet candidates have been discovered.

(Note 2) A star with a surface temperature below about 3,500 degrees Celsius is called a red dwarf. In fact, nearly 80% of all stars in the universe are red dwarfs, and many of the stars in the vicinity of our solar system are also red dwarfs. Because they are smaller than the Sun and have a lower surface temperature, the habitable zone is located closer to the stars than in the case of the solar system.

(Note 3) The MuSCAT series are instruments that can observe transits in three or four wavelength bands simultaneously, and are mounted on the 188 cm telescope in Okayama, the 1.52 m telescope in Tenerife, Spain, and the 2 m telescope in Maui, USA (the instrument names are MuSCAT, MuSCAT MuSCAT stands for Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets and is named after a specialty of Okayama Prefecture.

(Note 4) SPECULOOS is a project led by researchers at the University of Liège in Belgium to search for transit planets orbiting in the habitable zone around red dwarf stars. cOOl Stars, after the name of a traditional Belgian biscuit.

(Note 5) Planets with a radius 1 to 1.5 times that of the Earth and slightly larger than the Earth are called Super Earths. Theoretically, a planet with this radius is very unlikely to be a small gaseous planet (subneptune) with a hydrogen atmosphere (it cannot sustain a hydrogen atmosphere), so it is considered to be a rocky planet.

About Subaru Telescope
Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan (NAOJ) and supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) under the Large-Scale Scientific Frontier Initiative. Mauna Kea, where Subaru Telescope is located, is a precious natural environment and an important place in Hawaiian culture and history, and we are deeply grateful for the opportunity to explore the universe from Mauna Kea.

■Related Links

• The University of Tokyo September 7, 2022 Press Release
• Subaru Telescope September 7, 2022 Press Release
• National Astronomical Observatory of Japan Press Release (Japanese) September 7, 2022

The post Discovery of a Super-Earth in the Habitable Zone first appeared on Astrobiology Center, NINS.

Research on exoplanets, which has made great progress in recent years with the discovery of giant planets around stars similar to our Sun, is now focusing on red dwarfs, which are lighter than our Sun. Red dwarfs, which account for three-quarters of the stars in our galaxy and exist in large numbers in the vicinity of our solar system, are excellent targets for the discovery of exoplanets in our neighborhood. The discovery of nearby exoplanets, with detailed observations of their atmospheres and surface layers, will allow us to discuss the presence or absence of life in environments that are very different from those of our solar system.

However, red dwarfs are very faint in visible light due to their low surface temperature of less than 4000 degrees. Conventional planet searches using visible light spectrometers have only discovered a limited number of planets around very nearby red dwarfs, such as Proxima Centauri b. In particular, red dwarfs with surface temperatures below 3,000 degrees (late red dwarfs) have been discovered. In particular, there have been no systematic planet searches for red dwarfs (late red dwarfs) with surface temperatures below 3000 degrees (Note 1).

Red dwarfs are important targets for studying life in the universe, but are difficult to observe because they are too faint in visible light. In order to solve the difficulty of spectroscopic observation of red dwarfs, a planetary survey using a high-precision spectrograph in the infrared, where red dwarfs are relatively bright, has been long awaited (Note 2).

The Astrobiology Center has successfully developed the world’s first high-precision infrared spectrograph for an 8-meter class telescope. This is the Subaru Telescope’s IRD (InfraRed Doppler) instrument. Using the Doppler method, the IRD can detect minute wobbles in the velocity of a star, about the speed of a person walking (Note 3, Note 4).

Using this IRD, a project (IRD-SSP) to strategically observe late red dwarfs and search for planets is starting in 2019. This is the world’s first systematic planetary search around late red dwarfs and is an international project involving about 100 researchers from Japan and abroad. During the first two years, screening observations were conducted to discover “stable” red dwarfs with low noise, where even small planets can be detected (Note 5). Currently, the project is in the phase of intensive observations of about 50 promising late red dwarfs that have been selected through screening.

Translated with DeepL.com (free version)

The first exoplanet was discovered by IRD-SSP around Ross 508(*6), a red dwarf star one-fifth the mass of the Sun, located about 37 light years away from Earth. This is the world’s first exoplanet discovered by a systematic search using an infrared spectrometer (Note 7). This planet, Ross 508b, has a minimum mass of only about four times that of the Earth (Note 8). Its average distance from its central star is 0.05 times the Earth-Sun distance, and it is located at the inner edge of the habitable zone. Interestingly, the planet is likely to have an elliptical orbit, in which case it would cross the habitable zone with an orbital period of about 11 days (Figures 1 and 2).

Planets in the habitable zone retain water on their surfaces and may harbor life. Ross 508b will be an important target for future observations to verify the possibility of life habitability of planets around red dwarfs. Spectroscopic observations of molecules and atoms in planetary atmospheres are also important, although the current telescope lacks the resolution for direct imaging observations due to the close distance between the planet and the star. In the future, they will be the target of life search observations by 30-meter class telescopes.

The IRD-SSP is expected to continue discovering new planets.

The significance of the first planet discovered by SSP-IRD is explained by the paper’s lead author, Dr. Hiroki Harakawa, a researcher at NAOJ’s Subaru Telescope in Hawaii, as follows: “Ross 508b is the first planet discovered by SSP-IRD. Ross 508b is the world’s first successful detection of a super-Earth using only near-infrared spectroscopic data. Prior to this, near-infrared observations alone were not accurate enough for the detection of light planets such as super Earth, and verification by highly accurate line-of-sight velocity measurements in visible light was necessary. This study shows that IRD-SSP alone is capable of detecting planets and clearly demonstrates the advantage of IRD-SSP in that it can search with high precision even for late red dwarfs that are too faint to be observed with visible light.”

Professor Fumie Sato of Tokyo Institute of Technology, who leads the IRD-SSP, said, “It has been 14 years since the IRD project began. We have continued our development and research with the hope of finding a planet exactly like Ross 508b. The IRD-SSP will continue to search for planets around low-temperature red dwarfs, which have not yet been explored, with the aim of making new discoveries,” says Professor Sato of the Tokyo Institute of Technology. We are very enthusiastic about this project.

This research result was published in the “Publications of the Astronomical Society of Japan” on June 30, 2022 (Harakawa et al. “A Super-Earth Orbiting Near the Inner Edge of the Habitable Zone around the M4.5-dwarf Ross 508“).

(Note 1) The transit method, which detects changes in stellar brightness as a planet crosses the front of a star, does not require as many photons as the spectroscopic Doppler method, so the search for planets around red dwarfs using the transit method has been gaining momentum in recent years. TESS satellites can detect terrestrial planets around red dwarfs (early red dwarfs) with relatively high temperatures.

(Note 2) For example, the brightness of the Sun seen from a distance of 30 light years is 5th magnitude in visible light and 3rd magnitude in infrared light. On the other hand, the lightest late red dwarf is very faint in visible light at magnitude 19, but relatively bright in the infrared at magnitude 11.

(Note 3) The transit method can only detect planets whose planetary orbits are along the line of sight, whereas the Doppler method can detect planets without regard to their placement on the celestial plane as such. It is also an important method in that it can determine the “mass” of a planet.

(Note 4) IRD has achieved cutting-edge results in determining the orbits of terrestrial planets and young planets and in detecting planetary atmospheres, in addition to planet detection, by taking advantage of its infrared observation capabilities.

(Note 5) Red dwarfs have high surface activity, such as flares, and this surface activity can cause changes in the stellar line-of-sight velocity even if no planets exist. Therefore, only stable red dwarfs with low surface activity are targets for the search for small planets such as the Earth. (In fact, infrared observations are more advantageous for planet detection for the same accuracy because the effect of surface activity is reduced in infrared compared to visible light. However, it is important to observe red dwarfs with as little activity as possible in both visible and infrared light in order to detect light planets such as the terrestrial planets.)

(Note 6) Ross 508 is the 508th object on the list of stars with large intrinsic motion published by Frank Elmore Ross (1874-1960).

(Note 7) To confirm that the periodic wobble of Ross 508 was indeed caused by a planet, the IRD-SSP team identified several indicators of stellar activity that could be mistaken for a planet (e.g., changes in stellar brightness and some emission line shape indicators) and showed that the periods of these indicators were clearly different from planetary periods. The team showed that the periods of these indicators are distinctly different from the planetary periods. This is a much more difficult task than using the Doppler method to confirm planetary candidates reported earlier by the transit method, but it is an indispensable method for detecting non-transiting planets.

(Note 8) In principle, the Doppler method alone requires a lower limit for the mass of a planet. If a planetary system can be observed in transit, the planetary mass can be accurately determined in combination with the results of the Doppler method.

About Subaru Telescope
Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan (NAOJ) and supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the Large-Scale Scientific Frontier Initiative. Mauna Kea, where Subaru Telescope is located, is a precious natural environment and an important place in Hawaiian culture and history, and we are deeply grateful for the opportunity to explore the universe from Mauna Kea.

■Related Links

• National Astronomical Observatory of Japan August 1, 2022 Press Release
• Subaru Telescope August 1, 2022 Press Release
• Is this relationship a combination of a redder Sun and a heavier Earth? (Space Scoop – Space News for Kids)

The post Discovering A Planet Orbiting a Cool Star at Infrared – Could a “super-Earth” harbor life? first appeared on Astrobiology Center, NINS.

An international team led by researchers from the Astrobiology Center has utilized data from the exoplanet exploration project using the Subaru Telescope’s near-infrared high-dispersion spectrograph “IRD” (IRD-SSP) and revealed the chemical compositions of 13 cool stars. The IRD-SSP searches for planets around lower-mass and cooler stars than the Sun (cool stars), also known as “M-type dwarfs”. This study elucidates the characteristics of M-type dwarfs themselves before planet discovery and represents the first achievement utilizing IRD-SSP data.

The chemical composition of a star is the ratio of each element, such as iron, sodium, and magnesium, among the components that make up the star. This information is necessary to determine the characteristics of exoplanets when they are discovered in the future, as it is also used to determine the formation materials of planets (exoplanets) that may exist around the star. It is also an important indicator of when a star was born in the evolution of the galaxy. For this reason, there is a long history of spectroscopic studies of the chemical composition of stars with temperatures similar to those of the Sun (F-, G-, and K-type stars) in visible light. On the other hand, it has been difficult to measure the chemical composition of M-type dwarfs using conventional methods because they are very faint in visible light and their low temperature makes spectroscopic data complicated.

The team developed a unique method using the near-infrared spectra collected by the IRD-SSP to measure the chemical composition of an initial sample of 13 M dwarfs (specifically, the ratios of the abundances of sodium, magnesium, potassium, calcium, titanium, chromium, manganese, iron, and strontium to hydrogen). IRD is optimized for observing M dwarfs, which are brighter in the near-infrared than in visible light. In addition, the large aperture of the Subaru Telescope, one of the largest in the world, makes it possible to study particularly faint M dwarfs. The Subaru Telescope’s large aperture, the largest in the world, also made it possible to study particularly faint M dwarfs, and the combined use of the data from multiple observations of the same M dwarf at different times was an advantage over a one-time observation.

The measurements revealed that the thirteen M-type dwarfs in this study have chemical compositions similar to those of F-, G-, and K-type stars near the Sun. Combining the data from the European Space Agency’s Gaia satellite, we also examined their motion in the Galaxy, suggesting that M-type dwarfs, which are particularly metal-poor, tend to move differently from the Sun (Figure 2). This trend is also known for F-, G-, and K-type stars and may reflect the chemical evolution of the Galaxy. Among the targets in this study is a well-known M-type dwarf star called Barnard’s star, which is a relatively new type of star in the Galaxy. There are multiple reports of evidence that this star is a relatively old type of star in the Galaxy, and the detailed chemical composition measurements obtained for the first time by this observation are consistent with this.

This achievement is significant not only because it reveals the detailed chemical compositions of 13 M-type dwarfs, but also because it indicates that the chemical compositions of approximately 100 M-type dwarfs for which IRD is being used to search for planets can be measured in the near future. It is expected to reveal for the first time what kind of stars are M-type dwarfs in the vicinity of our solar system. In addition, when IRD discovers a planet in the future, the presentation of the chemical composition of the planetary material will also provide clues to the characteristics of that planet.

This research was published in the “Astronomical Journal” on January 18, 2022 (Ishikawa et al. “Elemental Abundances of nearby M Dwarfs Based on High-resolution Near-infrared Spectra Obtained by the Subaru/IRD Survey: Proof of Concept“).

About Subaru Telescope 
Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan (NAOJ) and supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the Large-Scale Scientific Frontier Initiative. Mauna Kea, where Subaru Telescope is located, is a precious natural environment and an important place in Hawaiian culture and history, and we are deeply grateful for the opportunity to explore the universe from Mauna Kea.

Related Links：
Subaru Telescope March 28, 2022 Press Release

The post Investigating the Chemical Composition of Unexplored Cool Stars from Exoplanet Search Data first appeared on Astrobiology Center, NINS.

Summary：

A research team led by researchers from the University of Tokyo and the Astrobiology Center of the National Institutes of Natural Sciences has discovered a new exoplanet “TOI-2285b” located near the solar system (138 light-years away) through a collaboration between the exoplanet exploration satellite TESS and ground-based telescopes. This planet is approximately 1.7 times the size (radius) of Earth and receives about 1.5 times the amount of irradiance that Earth receives from the Sun from its host star, which is weaker than most of previously discovered exoplanets. The planet is believed to have a slightly warmer environment than Earth, and if it has a layer of H2O in its interior and a hydrogen-dominated atmosphere, liquid water could exist on its surface. Detailed follow-up observations are feasible as the host star is bright, and future investigations into the planet’s mass and atmospheric composition are expected to provide detailed information about its internal composition.

Research Background：

More than 4,000 exoplanets have been discovered by the Kepler Space Telescope of the National Aeronautics and Space Administration (NASA), which was active from 2009-2018, using the transit method (Note 2). These include many warm and small exoplanets that are expected to harbor life (Figure 1). However, most of the planetary systems discovered by the Kepler space telescope are located more than 500 light years away from the solar system, and it has been difficult to obtain detailed information such as the mass and atmospheric composition of the planets because their main stars are faint. The TESS space telescope, the successor to the Kepler space telescope, is currently searching for exoplanets around bright stars in the entire sky, and it is expected that subsequent follow-up observations of planets around bright stars discovered by the TESS search will provide detailed information on the mass and atmospheric composition of the planets. The TESS search for exoplanets around bright stars is expected to yield detailed information on the planet’s mass and atmospheric composition through follow-up observations.

On the other hand, due to limitations such as resolution and observation period, TESS observations alone can only discover “candidate” planetary objects. Therefore, in order to discover true planets, it is necessary to verify the authenticity of the discovered planetary candidates through detailed observations using ground-based telescopes. Therefore, a research team led by researchers from the University of Tokyo and the National Astrobiology Center of the National Institutes of Natural Sciences (NINS) is now using the MuSCAT series (Note 3) of multicolor imaging instruments mounted on three 2-meter class telescopes in Japan and abroad, and the infrared The IRD (*4) on the 8.2-meter Subaru Telescope in Hawaii is being used to verify planetary candidate objects discovered in the TESS search.

研究の成果：

The research team discovered TOI-2285b, a planet orbiting a star relatively close to our solar system (138 light-years away), from among the candidate planets they observed. It orbits around a low-temperature (3200 degrees Celsius) star with a period of about 27 days.

It is very important to observe the transit at multiple wavelengths in order to verify whether the candidate planet discovered by TESS is a real planet or not. However, since the transit of TOI-2285b occurs only once every 27 days, the opportunity to observe it from the ground under favorable conditions (nighttime and clear skies) was very limited. The research team developed three MuSCAT series instruments that can simultaneously observe the transit at multiple wavelengths and deployed them on three telescopes in Japan and abroad, which enabled them to confirm that TOI-2285b is a planet ahead of the rest of the world. Furthermore, by using the IRD, one of the world’s most accurate infrared Doppler instruments for measuring planetary masses, we succeeded in obtaining an upper limit for the mass of the planet (19 times the mass of the Earth).

The distance between TOI-2285b and the main star is only about 1/7 of the distance between the Earth and the Sun, but due to the low temperature of the main star, the amount of solar radiation the planet receives from the main star is estimated to be about 1.5 times that received by the Earth from the Sun. This insolation is moderate compared to many other exoplanets discovered so far, but it is still strong enough to quickly dry up the water on the planet’s surface if the planet were a rocky planet with a thin atmosphere like the Earth. On the other hand, if a layer of H2O exists outside the planet’s central core and a hydrogen-based atmosphere covers the outer layer (Note 5), part of the H2O layer may be stable as a liquid. The research team simulated the temperature and pressure inside TOI-2285b assuming such an internal composition, and found that there is indeed a possibility of liquid water (ocean) in the surface layer of the planet (top image).

Research Findings:

In order to determine whether liquid water actually exists in the surface layer of TOI-2285b in the future, it is important to first accurately measure the mass of the planet and then constrain the internal composition of the planet in combination with the already known information on the radius and insolation of the planet. To measure the mass of a planet, the main star must be bright enough, but since TOI-2285b is orbiting a star in the solar system’s neighborhood and appears bright in the infrared, it is possible to actually measure the mass using an infrared Doppler instrument attached to a large telescope such as IRD. Although this study has only obtained an upper limit for the mass of the planet, further observations are expected to enable the precise measurement of the planet’s mass and a closer look at the planet’s internal composition. In addition, next-generation telescopes such as the James Webb Space Telescope (JWST), which is scheduled for launch in December 2021, are expected to investigate the composition of the planet’s atmosphere to determine whether water and other molecules exist in the atmosphere.

The discovery of TOI-2285b is an important step toward the future “search for traces of life” on exoplanets. In the future, it is expected that next-generation large space telescopes and giant ground-based telescopes will be able to search for molecules such as water and oxygen in the atmospheres of warm exoplanets, which could be traces of life. On the other hand, in order to obtain reliable evidence of traces of life, it is not enough to observe only one or two planets; it is considered important to observe as many planets as possible. However, the number of promising planets (small, warm planets in the vicinity of the solar system) for observation is still very limited at this time (Figure 1). Since TESS is scheduled to continue its search until at least 2022, it is expected that the number of planets that are equal to or more promising than TOI-2285b can be further increased in the future by collaborating with ground-based telescopes as in this case. TOI-2285b or even more promising planets in the future.

The results of this research were published in the online edition of the journal “Publications of the Astronomical Society of Japan” on December 6, 2021. This research was conducted as part of the Grant-in-Aid for Scientific Research on Innovative Areas “Elucidation of the Formation and Evolution of Planetary Atmospheres and their Diversity” (PI: Dr. Taiyo Ikoma, Project Leader: JP18H05439) and the Japan Science and Technology Agency (JST) Strategic Basic Research Promotion Program “PRESTO” in the research area “Development and Application of Intelligent Measurement and Analysis Methodology by Integrating Measurement Technology and Advanced Information Processing.

JP17H04574), and the project “Discovery Confirmation and Characterization of Candidate Life-supporting Exoplanets Discovered by TESS” (PI: Noriyasu Narita, Project No.: AB031010) of the Center for Astrobiology, National Institutes of Natural Sciences, Japan.

Publication Journal：

Journal: Publications of the Astronomical Society of Japan (Online version: December 6)
Paper Title: TOI-2285b: A 1.7 Earth-radius Planet Near the Habitable Zone around a Nearby M Dwarf
Authors: Fukui, A.*, Kimura, T., Hirano, T., Narita, N., et al.
DOI: 10.1093/pasj/psab106

Terminology：

(Note 1) Officially known as the Transiting Exoplanet Survey Satellite, it was launched in 2018 to search for planets orbiting bright stars in the entire sky using the transit method (Note 2). The search is currently scheduled to continue until 2022.

(Note 2) This method captures the periodic dimming of the main star observed when a planet passes in front of the main star (transit).This method can be used to determine the radius and orbital period of a planet.

(Note 3) Instruments capable of simultaneously observing transits in three or four wavelength bands of visible light (named MuSCAT, MuSCAT2, and MuSCAT3, respectively) are installed on the 188cm telescope in Okayama Prefecture, a 1.52m aperture telescope in Tenerife, Spain, and a 2m aperture telescope in Maui, USA. MuSCAT2, and MuSCAT3, respectively). In this study, the transit signals observed by TESS were confirmed using MuSCAT2 and MuSCAT3.

(Note 4) An infrared spectrometer that can measure planetary masses with high precision using the Doppler method. By obtaining the upper limit of the mass, we confirmed that the transiting object is not a star but a planet (with a mass less than 13 times that of Jupiter).

(Note 5) The existence of an H2O layer in the interior of a planet is predicted from the theory of planet formation. On the other hand, it is highly likely that a hydrogen-based atmosphere existed at least in the early stages of planet formation, but it may have been stripped away by high-energy electromagnetic waves (X-rays and ultraviolet rays) emitted from the host star later on.

Related Links：

・The University of Tokyo Release
・Japan Science and Technology Agency release

The post Discovery of a Low-Irradiance Small Exoplanet Near the Solar System first appeared on Astrobiology Center, NINS.

A research team led by researchers from the Astrobiology Center and the University of Tokyo discovered “ultra-short-period planets” with orbital periods of less than a day around cool stars by observations using the Subaru Telescope’s near-infrared spectrograph IRD and other instruments, and revealed that they consist of mainly iron and rock.
The planets discovered around two cool stars, TOI-1634b and TOI-1685b, are super-Earths (Note 1), roughly 1.5-2 times the size of Earth. Particularly, TOI-1634b is one of the terrestrial planets with the largest radius (1.8 Earth radius) and mass (10 Earth mass) among the ultra-short-period planets discovered to date. Located at the boundary between rocky and gas planets, and given the rarity of such discoveries around cool stars, it is said that the most valuable objects were discovered in investigating how planets with “years” shorter than a day on Earth are formed.

Observations have revealed that about 1% of exoplanets (exoplanets) are ultra-short-period planets (planets with orbital periods of less than one day). Ultra-short-period planets are thought to have formed in outer orbits and then moved to inner orbits due to interactions with other planets.

Most of the ultrashort-period planets observed so far are small planets with radii less than 1.5 times that of the Earth, and their internal composition is known to be similar to that of the Earth, consisting mainly of iron and rocks. However, most of these closely examined ultrashort-period planets are known only around sun-like stars (solar-type stars), and only a few have been observed around low-temperature, small-mass stars. Low-temperature stars are known to have a high frequency of multiple small planets, so the frequency of ultrashort-period planets may also be high. Detailed studies of the frequency and characteristics of ultrashort-period planets around low-temperature stars are expected to improve our overall understanding of the origin of ultrashort-period planets.

The research team focused on two low-temperature stars TOI-1634 and TOI-1685, which are transit planet candidates (Note 2) detected by NASA’s Transiting Exoplanet Survey Satellite (TESS). The stars are only about 50% of the mass of the Sun, so we analyzed the TESS data, followed up the transit observations with the MuSCAT series (Note 3), and then conducted spectroscopic observations with the IRD (InfraRed Doppler) spectrograph on the Subaru Telescope. IRD is a spectrograph that precisely measures the line-of-sight velocity of stars, and is uniquely optimized for observing low-temperature stars that appear brighter in the infrared than in visible light.

The team analyzed the line-of-sight velocities observed by the IRD in detail and confirmed that the ultra-short-period planets TOI-1634b and TOI-1685b actually revolve around each other with periods of 0.989 days (TOI-1634b) and 0.669 days (TOI-1685b), respectively. Furthermore, the amplitude of the line-of-sight velocity change revealed that TOI-1634b and TOI-1685b have masses about 10 times and 3.4 times that of the Earth, respectively (Note 4). Theoretical estimation of planetary compositions based on these planetary masses and planetary radii determined from transit observations (approximately 1.8 Earth radii for TOI-1634b and 1.5 Earth radii for TOI-1685b) revealed that both planets, like Earth, have internal compositions mainly consisting of iron and rocks (Figure 1). Figure 1). This means that two planetary systems with super Earth-like compositions orbiting close to low-temperature, low-mass stars have been discovered.

TOI-1634b is one of the largest ultra-short-period planets confirmed to have an internal composition similar to that of the Earth in terms of radius and mass, and it is very interesting that such a planet was found around a star much lighter than the Sun. The “mass-radius” relationship (Figure 1) also indicates that both planets lack thick hydrogen atmospheres. In the absence of primordial atmospheres composed of gas from protoplanetary disks, secondary atmospheres composed of gas ejected by the planets may have formed on both planets. This is an interesting observation target for studying how the atmospheres of terrestrial planets orbiting in close proximity to their stars evolve.

Both planetary systems are relatively close to Earth, about 100 light-years away, and are particularly bright among low-temperature stars with ultrashort-period planets, making them strong candidates for the next generation of telescopes. Assistant Professor Teruyuki Hirano (Astrobiology Center, National Institutes of Natural Sciences / National Astronomical Observatory of Japan), the first author of the paper, says, “In the future, we expect that the planetary system discovered in this research will be observed by the James Webb Space Telescope (JWST) and other spacecraft to study the planetary atmosphere and orbit in detail, which will bring us closer to understanding the origin of the still mysterious ultra-short period planets The TESS mission is also expected to help elucidate the origins of the still-enigmatic ultra-short-period planets. The project to intensively follow up the planetary candidates identified by TESS with IRD is still ongoing, and many unique planets should be confirmed by IRD within a year or two,” he said.

This research was published in the Astronomical Journal of the United States on September 23, 2021 (Hirano et al. “Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions”).

(Note 1) “Super-Earths” refer to exoplanets larger than Earth but smaller than Neptune. They typically have masses no larger than about 10 times that of Earth and diameters no larger than about twice that of Earth. There is no planet with such mass and size in the solar system and exoplanet observations first revealed the existence of such planets.

(Note 2) “Transit” is a phenomenon in which a star appears periodically fainter as a planet passes in front of it, and exoplanetary systems in which transits are observed are called transit planetary systems. The transiting planet candidates detected by TESS are confirmed to be real transiting planets only after follow-up observations using other telescopes.

(Note 3) Follow-up observations of transits have been made using the 188cm telescope in Okayama Prefecture, the 1.52m telescope at Teide Observatory in Tenerife, Spain, and the MuSCAT, MuSCAT2, and MuSCAT3 multicolor simultaneous imaging cameras on the 2m telescope at Haleakala Observatory in Maui, USA. The transits were tracked using the MuSCAT, MuSCAT2, and MuSCAT3 cameras mounted on the 2-meter telescope. The parameters such as orbital period and planetary radius, which were tentatively determined by TESS, were determined precisely by these follow-up observations for each of the planets.

(Note 4) When a planet orbits a star, the planet’s gravity causes the star to wobble slightly. This wobble is captured as a periodic change in the line-of-sight velocity of the star by the line-of-sight velocity method, and the larger the mass of the planet, the larger the amplitude of the line-of-sight velocity change. The masses of the two planets were determined by IRD follow-up observations.

About Subaru Telescope：

Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan (NAOJ) and supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the Large-Scale Scientific Frontier Initiative. Mauna Kea, where Subaru Telescope is located, is a precious natural environment and an important place in Hawaiian culture and history, and we are deeply grateful for the opportunity to explore the universe from Mauna Kea.

Publication：

Journal：Astronomical Journal
“Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions“
Authors：Teruyuki Hirano, Noriyasu Narita, et al.

■Related Links

Discovery of an Earth-like Planet with a “Year” Less Than One Day Long Around a Low-Temperature Star(NAOJ Hawaii Observatory, September 27, 2021 Press Release)

Discovery of an Earth-like Planet with an “Annual” Length of Less Than a Day Around a Low-Temperature Star（University of Tokyo, September 27, 2021,Press Release）

The post Discovery of Terrestrial Planets with “Years” Shorter than a Day around Cool Stars first appeared on Astrobiology Center, NINS.

An international team led by researchers from the Astrobiology Center of the National Institutes of Natural Sciences has discovered hydroxyl radical (OH) molecules in the day side atmosphere of the exoplanet WASP-33b. This exoplanet is a giant gas giant called “Ultra Hot Jupiter” orbiting around its star (also called the main star) in an orbit much more inner than that of Mercury in the solar system (Figure 1). As a result, the planet’s atmosphere can reach temperatures of more than 2,500 degrees Celsius, hot enough to melt most metals. Dr. Nugroho of the Center for Astrobiology and lead author of the paper said, “This is the first detection of OH on a planet outside our solar system. This discovery not only marks the detection of molecules in exoplanetary atmospheres, but also the beginning of a detailed understanding of the chemistry of planetary atmospheres,” he said.

In the Earth’s atmosphere, OH results primarily from reactions between water vapor and oxygen atoms. This is also known as “atmospheric detergent” and plays an important role in removing potentially harmful substances to life, such as methane and carbon monoxide, from the atmosphere. OH plays an important role in determining atmospheric composition through its interaction with water vapor and carbon monoxide in the planetary atmosphere. is thought to result mostly from the breakdown of water vapor due to the high temperatures of planetary atmospheres. Dr. Ernst of Queen’s University said, “Our data support the idea that the extremely high temperatures of the planetary atmosphere dissociate water, since water vapor is thought to be scarce.”

This discovery was achieved by using the IRD, a high-dispersion spectrograph in the near-infrared light, newly installed on the Subaru Telescope in the summit region of Mauna Kea, Hawaii, at an altitude of 4200 meters. This instrument can detect atoms and molecules in stars and planets as absorption lines in the spectrum. As an exoplanet orbits its host star, the velocity of the exoplanet relative to Earth varies with time. This is the same principle as the “Doppler effect” that causes the pitch of sirens to change when an ambulance or other vehicle passes by us; instead of pitch, the color changes slightly. This allows us to distinguish whether the atomic and molecular features in the spectrum are due to the main star or the planet. Because the light from the planets is so weak, it is not possible to directly separate the light from the main star and the light from the planets in normal observations, but this special method has allowed us to separate the signals due to OH molecules from Ultra Hot Jupiters for the first time.

By taking advantage of the unique capabilities of Subaru Telescope’s IRD, the team was able to detect a small amount of hydroxy molecules in the atmospheres of exoplanets. The IRD is an ideal instrument for studying the atmospheres of exoplanets in the infrared,” says Professor Tamura, Director of the Astrobiology Center and one of the people responsible for the development of the IRD. One of the goals of modern astronomy is to search for ‘Earth-like’ planets. The new atmospheric components that will be discovered will lead us toward this goal by deepening our understanding of exoplanets and techniques for studying their atmospheres,” says Gibson, assistant professor at Trinity University. This time, the target was an extremely hot planet, but with further development, we hope to be able to study the atmospheres of cooler planets and eventually a second Earth,” says Kawahara, who is an assistant professor at the University of Tokyo.

Professor Watson of Queen’s University said “These observations will provide a test bed for the next generation of Very Large Telescopes, such as the TMT (Thirty Meter Telescope) and ELT (European Very Large Telescope), which will look for signs of life on small rocky planets. And by developing this technique, we may get a hint at the oldest question of all: ‘Are we alone in the universe?’”

This result was published in The Astrophysical Journal Letters with a reputation for publishing groundbreaking discoveries, on March 23, 2021.

論文情報：

Nugroho et al. 2021, “First Detection of Hydroxyl Radical Emission from an Exoplanet Atmosphere: High-dispersion Characterization of WASP-33b Using Subaru/IRD”, Astrophysical Journal Letters

https://arxiv.org/abs/2103.03094

The post World First: Discovery of OH Molecule in an Exoplanet with the New Spectrograph at Subaru Telescope first appeared on Astrobiology Center, NINS.

• IRD, the world’s unique new exoplanet search instrument for the Subaru Telescope, has begun full-scale operation.
• For the first time in the world, it is now possible to detect stellar motion “in the infrared” at the speed of a human walk.
• For the first time, systematic searches for light planets such as the Earth became possible with the Subaru Telescope.

Abstract

The InfraRed Doppler (IRD, in Japanese), a new exoplanet search instrument for the Subaru Telescope, has been developed and built by a team led by researchers from the National Astrobiology Center of the National Institutes of Natural Sciences, the National Astronomical Observatory of Japan, the University of Tokyo, Tokyo University of Agriculture and Technology, and Tokyo Institute of Technology, In February 2018, the team began astronomical observations at the Subaru Telescope and successfully achieved first light (see note 1) of the instrument, taking advantage of all its capabilities.

　The IRD is an instrument designed to search for exoplanets using infrared light rather than visible light, which has been the predominant Doppler method (Note 2). Using the latest infrared and laser technology, IRD is currently the world’s most accurate infrared instrument for detecting changes in stellar velocity.

The IRD is currently the world’s most accurate infrared instrument for detecting changes in stellar velocity. With full-scale observations in the near future, we aim to use the unique features of IRD and the large aperture of the Subaru Telescope to discover Earth-like planets around light, low surface temperature stars (M-type stars: Note 3), which are also numerous near the Sun. Such planets will be excellent targets for the next-generation telescope’s search for life.

Please look forward to the new exoplanet search instrument, IRD, which has finally begun its mission!

Background

Exoplanets, one of the hottest research subjects in astronomy, have now transcended the boundaries of astronomy and become the subject of astrobiology, the study of life in the universe. Advances in exoplanet and astrobiology observation techniques are expected to lead to the detection of water, oxygen, and other substances necessary for the existence

of life on exoplanets in the near future.

Recent exoplanet searches, led by the Kepler space telescope, have discovered more than 5,000 exoplanets and candidates, some of which are habitable planets, or planets with the “potential” to be suitable for life. However, the number of habitable planets discovered so far is too small to study in detail, and the habitable planets found by the Kepler Space Telescope are too far from the Earth to study their characteristics in detail. Therefore, an important future challenge in astrobiology is to discover habitable planets that are close enough to Earth to study their characteristics in detail.

Liquid water, considered essential for the existence of life, can only exist in moderate environments. The distance from the parent star defined as “the region where liquid water can exist” is called the habitable zone (Figure 1). We can also imagine that a suitable planet for life would be a light planet like the Earth, mainly made of rock. However, the Doppler method (Note 2) and the transit method (Note 4), which are the main methods to search for exoplanets, detect planets by studying variations in the velocity and brightness of the parent star, but the smaller the planet to be found, the more precise observations are required. The higher the temperature of the parent star, the further away from the parent star the habitable zone becomes, and the farther away from the parent star, the more difficult it is to find a planet using the Doppler and transit methods. For example, it is very difficult to discover a planet about the same size as the Earth that orbits a star with a temperature similar to that of the Sun in one year. Therefore, it is very difficult to discover Earth-like planets in the habitable zone, and only a few such planets have yet been discovered.

M-type stars (Note 3), also known as red dwarfs, have a habitable zone close to the parent star due to their low surface temperature and faintness. In addition, because M-type stars are smaller in mass and size than other stars such as solar-type stars, the variability of the parent star caused by the planet is greater. This makes it easier to detect planets located in the habitable zone for M-type stars. In addition, M-type stars are often located near the Sun, making them very good targets for the discovery of Earth-like habitable planets. If we can discover habitable planets around M-type stars near the Earth, it will be easier to conduct detailed observations in the future.

　One of the most interesting objects to observe is a late M-type star (Note 3), which is a low-mass and low-temperature type of M-type star. Late M-type stars have the potential to discover a planet with a mass similar to that of the Earth in the habitable zone using current state-of-the-art technology. Therefore, Late M-type stars are very promising objects for the discovery of potentially life-supporting planets. However, late M-type stars are generally faint, which is a problem for astronomical observations. In order to make high-precision observations, it is necessary to collect enough light emitted by the observed object, which is difficult to do with faint objects. In addition, the search for exoplanets has been conducted mainly in visible light, but late M-type stars are generally faint in visible light, which has also made it difficult to observe late M-type stars. Late M-type stars at low temperatures are brighter in the infrared than in visible light, making observations in the infrared more advantageous.

Development of IRD and the science we aim to achieve

A research team consisting mainly of researchers from the National Astrobiology Center, National Astronomical Observatory of Japan, the University of Tokyo, Tokyo University of Agriculture and Technology, and Tokyo Institute of Technology has been developing a new instrument for exoplanet exploration called the InfraRed Doppler (IRD). The IRD will enable highly accurate Doppler observations in the infrared, which has not been possible in the past, using the most suitable infrared light for late M-type stars. The Doppler method, however, is less susceptible to this problem and is therefore more effective for searching for habitable planets that are close to the Earth. The Doppler method is effective for finding habitable planets near the Earth.

The IRD has a number of innovations for finding small planets, which will be discussed in more detail later. I will describe them in detail later.

(1) A very stable infrared spectrograph with high wavelength resolution and the ability to observe a wide range of wavelengths simultaneously (Note 5)
(2) A laser frequency comb (optical comb) that provides an accurate measure of stellar velocity
(3) mode scramblers that reduce light turbulence that can cause instability

, and more. Subaru Telescope’s large primary mirror can collect enough light even from faint Late M-type stars. Late M-type stars are faint in visible light but bright in infrared light because their surface temperature is only about half that of the Sun. The combination of the IRD, which can search for planets with high precision in the infrared, and the Subaru Telescope, which has a large aperture, is the best combination for finding habitable planets around Late M-type stars by the Doppler method.

First light on InfraRed Doppler (IRD) equipment

The IRD has been in planning, development, fabrication, testing, and installation on the Subaru Telescope for almost eight years. As a result, we succeeded in first light with the spectrograph alone in August 2017 (see note 1) and in a complete form combined with the optical com in February 2018.

IRD uses the Doppler method to observe planets, but the Doppler method requires the use of spectroscopy to obtain stellar spectra. Figure 3 shows the data for M-type stars obtained at First Light in February 2018. This figure is used to show why IRD can precisely measure the velocity of a star. On the two-dimensional image in Figure 3, you can see a line with darkened areas in some places, which is the spectrum of the star. In parallel with the spectrum of the star, you can see many small dots lined up in a regular pattern like dotted lines. This is the spectrum of the optical comb observed at the same time as the spectrum of the star as the reference light. This optical comb serves as a “precision scale” that is used as a reference for measuring the velocity of stars in IRD observations. Laser frequency combs are very new to astronomy, having been rarely used in astronomy until now.

The spectrum acquired on a two-dimensional image is transformed into a one-dimensional spectrum (light intensity per wavelength) by data processing, as shown in Figure 4. The upper panel of Figure 4 shows an enlarged image of a small portion of the spectrum of the M-type star (GJ436) obtained from observations, in which a large number of absorption lines are visible. These absorption lines are caused by the absorption of light at that wavelength by the gas in the atmosphere of the parent star, but when the star is accompanied by a planet, the wavelength of the absorption lines changes due to the Doppler effect of light as the velocity of the star changes. The presence of a planet can be investigated by examining the wavelength variation in detail with respect to the optical com spectra. The purpose of this first light was to test the performance of the IRD, and this data was also acquired for that purpose. We will continue to test the performance of the IRD using the Subaru Telescope in the future.

The IRD’s unique observational capabilities were also used in this test observation, and only with these instruments will we be able to discover habitable planets in late M-type stars. The following is a detailed description of those capabilities.

IRD Features for Discovering the Second Earth

Feature 1: Infrared spectrometer with high wavelength resolution, wide wavelength range, and high temperature stability

The key to finding planets using the Doppler method is the wavelength resolution of the spectrograph, which indicates “how finely light can be divided. The higher the wavelength resolution, the more absorption lines in the spectrum of a celestial body can be sharply captured. As a result, more absorption lines can be separated, substantially increasing the number of absorption lines whose variability can be studied. This allows the Doppler method to achieve high precision observations. Since absorption lines in the infrared spectra of late M-type stars are crowded, high wavelength resolution is important, and IRD uses infrared spectroscopy with very high wavelength resolution to search for planets. IRD’s infrared spectrograph has a wide wavelength range. The wider wavelength range of the spectrograph increases the number of available absorption lines, which improves the accuracy of the Doppler method. The spectrometer must be stable because the Doppler variation signal due to the planet is very small and difficult to capture. Therefore, IRD controls the temperature of the spectrograph with extremely high precision to minimize the noise that can occur when the instrument is unstable.

Feature 2: Laser frequency comb for extremely precise wavelength scaling

Das IRD verwendet einen Laserfrequenzkamm (optischer Kamm) als „Wellenlängenskala“. In der Vergangenheit wurde eine bestimmte Wellenlängenskala als Wellenlängenskala verwendet. In der Vergangenheit wurden Lampen und Jodzellen, die die Eigenschaften bestimmter Atome und Moleküle nutzten, als Wellenlängenskala verwendet, aber ihre Leistung war im Infraroten sehr begrenzt, was für die Beobachtung von Sternen des M-Typs von Vorteil ist. Laserfrequenzkämme, die in den letzten Jahren zunehmend in Bereichen wie der Präzisionsspektroskopie eingesetzt werden, senden eine sehr große Anzahl von „Laserstrahlen“ über einen überwältigend großen Wellenlängenbereich im Infraroten aus und dienen als Standards für verschiedene Wellenlängen. Auf diese Weise lassen sich Wellenlängen genauer als je zuvor bestimmen und die gesamte Bandbreite der Absorptionslinien im Sternspektrum nutzen.

Merkmal 3: Mode Scrambler zur Verringerung von Lichtturbulenzen beim Durchgang durch die Faser.

Ein weiteres wichtiges Merkmal von IRD ist eine Funktion namens Mode Scrambler. Bei der Messung sehr kleiner Dopplerverschiebungen mit einem Spektrographen ist bekannt, dass die Instabilität von Instrumenten wie Teleskopen und atmosphärische Turbulenzen das Licht (d. h. das Spektrum) von Himmelsobjekten stören können, was zu großem „Rauschen“ führt, das präzise Messungen behindert. Das IRD ist mit einem so genannten „Mode Scrambler“ ausgestattet, der dieses Rauschen reduziert, indem er das Licht mit Hilfe von Glasfasern usw. absichtlich stark „stört“. Die für astronomische Infrarotbeobachtungen geeigneten Fasern und Scrambler, die für die Zwecke des IRD verwendet werden, sind nicht gut bekannt, so dass das IRD-Team an verschiedenen Geräten arbeitet. Das IRD-Team hat bisher verschiedene Arten von Fasern und Mode-Scramblern getestet, da diese noch nicht gut bekannt sind. Die für diese Beobachtung ausgewählten Fasern und Modescrambler wurden auch verwendet.

Prospects

Der IRD, der erfolgreich zum ersten Mal beleuchtet wurde, steht Forschern auf der ganzen Welt seit August 2018 zur Verfügung. Es wird erwartet, dass die Beobachtungen mit dem Subaru-Teleskop in naher Zukunft ernsthaft beginnen werden. Das IRD-Team plant außerdem, mit einer Reihe japanischer und internationaler Exoplanetenforscher zusammenzuarbeiten, um ein Projekt zur Planetenjagd auf späte Sterne vom Typ M mit dem IRD und dem Subaru-Teleskop voranzutreiben. Das Projekt zielt darauf ab, bewohnbare Planeten um Sterne des späten M-Typs zu entdecken und Planetensysteme um Sterne des späten M-Typs zu charakterisieren. Sterne des späten M-Typs sind in unserer Galaxie reichlich vorhanden. Mehr als 500 von ihnen befinden sich in der Nähe unseres Sonnensystems in einer Entfernung von weniger als 30 Lichtjahren, so dass Exoplaneten in ihrer Umgebung eingehend untersucht werden können, sobald sie gefunden werden. Es wird erwartet, dass die Suche nach Planeten in späten M-Typ-Sternen mit IRD wertvolle Erkenntnisse für die Astronomie und Astrobiologie liefern wird. Auf diese Weise werden in Zukunft Beobachtungen mit dem IRD durchgeführt werden, um seine einzigartigen Eigenschaften optimal zu nutzen. Wir freuen uns auf das neue IRD-Instrument zur Suche nach Exoplaneten, das nun endlich seine Arbeit aufgenommen hat!

Annotation:

Note 1 First light
First time the light collected by a telescope is put into an astronomical instrument.

Note 2 Doppler method (line-of-sight velocity method)
This is a method of finding a planet by detecting the “stellar wobble” caused by the planet’s orbit around the star. In the spectrum of a star, there are many absorption lines due to atoms and molecules in the star’s atmosphere. When a star is slightly shaken by the gravitational pull of a planet, the wavelengths of these absorption lines are slightly shifted due to the Doppler effect of light. This periodic shift of wavelengths caused by planetary orbits can be used to determine the existence of a planet by observing the change in the velocity of the star relative to us over a long period of time. In the Doppler method, the more massive and close the planet is to us, the larger the stellar jolts become and the easier they are to observe. The first exoplanet (51 Pegasus) to be identified around an ordinary star was found by this method.

Note 3 (Late) M-type stars
M-type stars are stars with surface temperatures between 2,200°C and 3,800°C and masses between 0.08 and 0.6 times that of the Sun. Compared to the Sun, which has a surface temperature of about 5500 °C, M-type stars have a lower surface temperature than the Sun. In this section, we further distinguish the objects with surface temperatures below 3000 ºC among M-type stars, which have lower temperatures and lighter masses, as late M-type stars.

Note 4 Transit method
When a planet passes in front of a star, the light of the star periodically dims. This method is used to find a planet by observing this change in brightness over a long period of time. It is unlikely that a planet passes “just in front” of a star, so it is necessary to observe a large number of stars. On the other hand, the larger the planet, the greater the change in brightness. By observing many stars, Kepler was able to find thousands of planets.

Note 5 (Infrared) Spectrograph
A device that records light divided into various wavelengths using optical elements such as prisms and diffraction gratings in order to precisely study the “color (= wavelength)” of celestial light. To investigate the Doppler effect of light, it is necessary to divide the light into very fine wavelengths using a spectroscope. Spectrographs have different wavelengths of light that can be spectrally divided depending on their characteristics, but IRD is equipped with a spectrograph for spectroscopy of infrared light.

Co-presenting Institution

National Institutes of Natural Sciences National Astronomical Observatory of Japan Subaru Telescope
Graduate School of Science, The University of Tokyo

Announcer

Takayuki Kotani (Assistant Professor, Center for Astrobiology/ National Astronomical Observatory of Japan)
Motohide Tamura (Director, Center for Astrobiology/ Professor, Graduate School of Science, University of Tokyo/ Professor, NAOJ)
and others, IRD Development Team

The post Searching for a second Earth, the new IRD instrument is put into operation! first appeared on Astrobiology Center, NINS.