• 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.

Key Points of the Announcement:

• Global collaborative observations using space telescopes and ground-based observatories have led to the discovery of six transiting planets around the star HD 110067, located approximately 100 light-years away from the solar system.
• These six planets exhibit a commensurability where all adjacent planets have orbital periods that are expressed in simple integer ratios.
• This planetary system is unique by providing valuable insights into how planets form and evolve. Observing the atmosphere of each planet will expectedly lead to the understanding of processes of planetary atmospheric acquisition and the influence of stellar radiation on atmospheric dissipation and chemical evolution.

Overview:

An international research team, including Professor Norio Narita (Visiting Professor at the National Institutes of Natural Sciences Astrobiology Center) from the Graduate School of Arts and Sciences at the University of Tokyo, and Project Assistant Professor Akihiko Fukui, along with the MuSCAT team, discovered six transiting planets around the star HD 110067 through collaborative observations using space telescopes and ground-based observatories.

These six planets exhibit a relationship where all adjacent planets have orbital periods that are expressed in simple integer ratios (commensurability). This gives clues to how planets formed and migrated within a protoplanetary disk. Additionally, observing the atmospheres of these planets in the future will expectedly lead to the understanding of processes of planetary atmospheric acquisition and the influence of stellar radiation on atmospheric dissipation and chemical evolution.

This discovery was realized through collaborative observations using the Transiting Exoplanet Survey Satellite (TESS) by NASA, the CHaracterising ExOPlanets Satellite (CHEOPS) by the European Space Agency (ESA), and multiple ground-based telescopes including the multi-color simultaneous imaging cameras MuSCAT2 and MuSCAT3 developed by the MuSCAT team.
The research findings will be published in the scientific journal “Nature” on November 29, 2023 (British time).

Announcements

HD 110067, a star with about 80% of the mass and radius of the Sun, is located about 100 light years away in the direction of the constellation Camelopardalis. The star was observed by NASA’s TESS to monitor its brightness change for about 27 days each in March-April 2020 and February-March 2022; the TESS observations revealed that the transit-induced dimming occurred with a period of about 9.11 and 13.67 days, respectively. However, there were many other transit-like dimming events in the TESS data, and it was not known how many transit planets there were around this star or what the period of each planet was. The international research team worked to solve this mystery through hypothesis based on consideration and verification by observation.

The research team first focused on the shape of the transit (depth and duration of dimming). This is because transits by a given planet have the same shape each time. The team found that there were two pairs of transits with the same shape in the TESS data, one observed in 2020 and the other in 2022. However, the period is not necessarily two years, since TESS did not observe the periods between about two years. The time interval between two transits observed about 2 years apart divided by a natural number is a candidate for the true period. Observations made by ESA’s CHEOPS during the times of transits predicted by these candidate cycles confirmed that one of the two transits occurred with a period of about 20.52 days.

If you look closely at the periods of the three confirmed planets (9.11, 13.67, and 20.52 days), you will notice that the period ratios of the neighboring planets are simple integer ratios of 2:3, respectively. Such a simple integer ratio of periodicities of celestial bodies orbiting the same celestial body is called an “exhaustive relationship. For example, Neptune and Pluto have an orbital period ratio of 2:3, and Jupiter’s satellites Io, Europa, and Ganymede have an orbital period ratio of 1:2.

Considering the fact that there are three planets with such an exhaustive relationship from the viewpoint of planet formation, it is thought that at the time of formation of this planetary system, multiple planets were trapped in orbits of mean-motion resonance (Note 3) with an exhaustive relationship with each other and migrated to their current orbits while maintaining this relationship within the protoplanetary disk. Then, it is natural to assume that the remaining transiting planets’ periods also have an exhaustive relationship with each other. Therefore, the research team considered that the true period of the other transit, which was observed about two years apart, has an exhaustive relation to the period of about 20.52 days, i.e., the time interval between the two observed transits divided by a natural number has a simple integer ratio of about 20.52 days. We then found a period of about 30.79 days as the only solution that satisfies such a condition.

Even after the cycles of the four planets were identified, two transits remained in the 2022 TESS data, each with a different shape. Since each of these two had only one transit, the true period was not known. The research team therefore considered 50 different scenarios, assuming that the period of the fifth planet has an exhaustive relationship to the period of the fifth planet, which is approximately 30.79 days, and that the period of the sixth planet has an exhaustive relationship to the period of the fifth planet. Specifically, we considered five scenarios with period ratios of 1:2, 2:3, 3:4, 4:5, and 5:6, and two scenarios with the two observed transits not knowing whether they were the fifth or sixth planet, respectively. From these scenarios, based on the absence of transits in the existing TESS data and on astrodynamic considerations, the research team concluded that the period of the fifth planet is about 41.06 days, which is 3:4 compared to about 30.79 days, and the period of the sixth planet is about 54.77 days, which is 3:4 compared to the period of the fifth planet The sixth planet’s period is about 54.77 days, which is 3:4 relative to the fifth planet’s period.

The MuSCAT team participated in this campaign and accurately captured the beginning of the transit with MuSCAT2 on Tenerife Island, Spain, and the end of the transit with MuSCAT3 on Maui Island, USA (Figure 3). The MuSCAT team participated in this campaign and accurately captured the beginning of the transit at MuSCAT2 in Tenerife, Spain, and the end of the transit at MuSCAT3 in Maui, USA (Figure 3). The transit was a challenging observation with a depth of attenuation of only about 0.1%, a transit duration of more than 5 hours, and a large forecast error. The collaboration between MuSCAT2 and MuSCAT3, which are mounted on time-delayed telescopes, demonstrated its great power. This campaign observation confirmed that the period of the fifth planet is about 41.06 days.

The other is the analysis of TESS data from 2020, which was not included in the analysis, because scattered light from the Moon and the Earth can be mixed into the observation field of view depending on the observation direction and time of year, and such data is highly noisy. Although such data are acquired, they are not usually analyzed. However, the research team thought that if the above hypothesis was correct, the transits of the fifth and sixth planets should be included in the 2020 TESS data, so they analyzed the data that had been excluded from the analysis. In fact, the team confirmed that the transits were at the times predicted by the hypothesis.

As described above, the research team solved the mystery of the complex transit observed by TESS based on the hypothesis and verification, and revealed that HD 110067 is a sextuplet planetary system in which the orbital periods of all neighboring planets are in an euclidean relationship. The existence of a seventh planet and beyond has not yet been confirmed, but its existence has not been ruled out, and further exploration is expected to continue. The radii of the six planets are 1.9 to 2.9 times that of the Earth, suggesting that they are not rocky planets like the Earth, but rather small Neptune-like planets with hydrogen atmospheres (Neptune’s radius is about four times that of the Earth).

By 2023, more than 5,000 exoplanets have already been discovered, but only a handful of planetary systems like HD 110067, with three or more planets in an ectoparallel relationship, have been discovered. Such planetary systems provide clues for deeper theoretical consideration of how planets formed and migrated within the protoplanetary disk. HD 110067 is also the brightest star of the five or more transit planets discovered around the same primary star. Transit planets orbiting a bright star are suitable for atmospheric observations, and moreover, the presence of multiple transit planets in the same planetary system makes it possible to observe and compare the atmospheres of these planets. Therefore, this sextuplet will be an excellent target for planetary atmospheres in the future, and is expected to allow us to study how the exoplanets acquired their atmospheres in the protoplanetary disk and how light from the star affected the dissipation and chemical evolution of the planetary atmospheres.

○Related information :

Press Release (1): Discovery of an Earth-Size Planet with Possible Volcanic Activity – Exoplanet LP 791-18d Heated by Tidal Forces (2023/05/18)

Press Release (2): Discovery of Super Earths in the Habitable Zone” (2022/09/07)

Press Release (3): Discovery of an Earth-like Exoplanet Suitable for Detailed Atmospheric Studies” (2021/03/05)

https://www.c.u-tokyo.ac.jp/info/news/topics/files/20210305naritanosobun01.pdf

research grant

This research was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI: project number JP18H05439), Japan Science and Technology Agency (JST) CREST (project number JPMJCR1761), and National Institutes of Natural Sciences Astrobiology Center Satellite (project number AB The research was supported by the Japan Science and Technology Agency (JST) CREST (JPMJCR1761) and the National Institutes of Natural Sciences Astrobiology Center Satellite (AB022006).

用語解説

（Note 1）MuSCAT team

Professor Narita and Assistant Professor Fukui have developed the MuSCAT series of multi-color simultaneous imaging cameras that can simultaneously observe transits in three or four wavelength bands for the 188 cm telescope in Okayama Prefecture, Japan; the 1.52 m telescope in Tenerife, Spain; the 2 m telescope in Maui, USA; and the 2 m telescope in New South Wales, Australia. The MuSCAT is a Multicolor Simultaneous Camera for studying MuSCAT stands for Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets, named after the famous Muscat fruit of Okayama Prefecture.

（Note 2）transiting planets

When an exoplanet passes in front of its host star, the apparent brightness of the host star dims slightly. This phenomenon is called transit, and a planet with an orbit that causes a transit is called a transit planet.

（Note 3）Exhaustive number relations and mean kinetic resonance

The period of revolution or rotation of two celestial bodies should be a simple integer ratio. When the orbital periods of two celestial bodies have an exhaustive relationship, they are said to be in a state of mean-motion resonance. When the orbital periods of two objects have an exhaustive relationship, they are said to be in mean motion resonance.

（Note 4）TESS（Transiting Exoplanet Survey Satellite）

TESS is NASA’s satellite program to search for exoplanets by transit, designed by researchers at the Massachusetts Institute of Technology.TESS was launched on April 18, 2018, and has carried out a two-year plan to search for transit planets in nearly the entire sky. A second extension plan is currently underway, which will continue observations until at least 2024. In the five years to date, TESS has discovered more than 6,000 transit planet candidates.

（Note 5）CHEOPS（CHaracterising ExOPlanets Satellite）

CHEOPS is a space telescope dedicated to the observation of transit planets, conceived by Swiss researchers and launched by ESA on December 18, 2019. The main objective is to observe transits of known transit planets with high precision and to determine with high accuracy the time at which the transit occurred and the radius of the planet. Originally planned for 3.5 years, the first extension plan has been approved and observations will continue until at least 2026.

論文情報

Journal：Nature

Title：A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067

Authors：Rafael Luque*, Hugh P. Osborn, Adrien Leleu, et al. including Norio Narita and John H. Livingston

DOI：10.1038/s41586-023-06692-3

URL：https://www.nature.com/articles/s41586-023-06692-3

関連リンク

東京大学プレスリリース

科学技術振興機構プレスリリース

The post Discovery of Xextuplets Planets in Resonance first appeared on Astrobiology Center, NINS.

Key Points:

• A new extra-solar planet (exoplanet) LP 791-18d was discovered through collaborative global observations using space and ground-based telescopes.
• LP 791-18d is speculated to exhibit active volcanic activity similar to Jupiter’s moon Io.
• Located near the inner edge of the habitable zone, LP 791-18d potentially retains an atmosphere, making it an intriguing planet for studies on the origin of life.

Overview:

An international research team including Professor Norio Narita (Graduate School of Arts and Sciences, The University of Tokyo; Visiting Professor at Astrobiology Center, National Institutes of Natural Sciences), Project Assistant Professor Akihiko Fukui, and Project Researcher Mayuko Mori, discovered the Earth-sized exoplanet LP 791-18d around a red dwarf star located approximately 90 light-years away through research by a combination of space and ground-based telescope observations (see Figure 1). This planet d has an elliptical orbit influenced by the gravitational pull from a larger, outer adjacent planet c, suggesting that d is covered with volcanoes akin to Jupiter’s moon Io. Future observations of this planet’s atmosphere are anticipated, and the study of how crustal activity affects planetary atmospheres could yield significant findings. This discovery was realized through collaborative observations over NASA’s Transiting Exoplanet Survey Satellite (TESS), NASA’s Spitzer Space Telescope, and multiple ground-based telescopes including the MuSCAT and MuSCAT2 multi-color simultaneous imaging cameras developed by researchers from The University of Tokyo and the Astrobiology Center (see Figures 2 and 3).

This research was published in the journal “Nature” on May 17, 2023 (British Summer Time).

Announcements

〈Research Background〉

The red dwarf star LP 791-18 is located approximately 90 light years away from our solar system in the direction of the constellation Cop. The transit planet search satellite TESS (Transiting Object Survey Satellite) has previously discovered planets b and c around this star. Planet b has a radius about 1.2 times that of the Earth and an orbital period of about 0.94 days, while planet c has a radius about 2.5 times that of the Earth and an orbital period of about 4.99 days.

〈Research〉

Research DescriptionThe transit of a new planet, LP 791-18d, was discovered by a series of 127-hour observations with the Spitzer Space Telescope. The planet d is located in an orbit between planets b and c (Figure 1), orbiting its star with an orbital period of 2.75 days. Its radius is estimated to be approximately 1.03 Earth radii, very similar to that of the Earth.

To find out what kind of planet this planet is, a number of teams participating in the TESS Follow-up Observing Program (TFOP), the official follow-up observation program of TESS, including the MuSCAT team from Japan (Note 4), conducted transit observations of planets c and d using ground-based telescopes. Observations of the transits of planets c and d were made using ground-based telescopes.

Each orbit around LP 791-18 brings planets d and c closer to each other. At this time, their mutual gravitational influence causes the transit time to deviate from the fixed orbital period. The MuSCAT team and other ground-based telescopes have measured the transit time of each planet and found that the mass of planet d is about the same as that of the Earth and the mass of planet c is about 9 times that of the Earth. The results of the observations are as follows

The gravitational force exerted by planet c slightly deforms the orbit of planet d into an elliptical shape. While orbiting in this elliptical orbit, the planet d is slightly deformed by the tidal forces (Note 5) exerted on it from its star. This deformation creates friction inside the planet, possibly heating the planet and causing active volcanism on the planet’s surface. This is the same heating mechanism used by Jupiter’s moon Io, which exhibits the most active volcanic activity in the solar system.

〈今後の展望〉

LP 791-18d is a planet located near the inner boundary of the habitable zone, where tidal forces from the star cause its rotation period to coincide with its orbital period, and it is thought to always face the same side toward the star. The “day side” of the planet receiving light from the star is likely too hot for liquid water to exist, but if volcanic activity is occurring, the planet may have an atmosphere and liquid water may exist on the “night side” of the planet due to water vapor condensation in the atmosphere.

Planet c is scheduled to have its atmosphere observed by the James Webb Space Telescope (Note 6), the newest space telescope that began observations last year. In addition, the research team believes that the newly discovered planet d could also be an important target for planetary atmosphere observations.

Active volcanism on a planet may be responsible for pumping materials into the atmosphere that would normally be trapped inside the planet’s crust. Such materials include carbon, which is important for life. Detection of the atmospheric composition of a planet would allow us to study in depth the effects of planetary crustal activity on planetary atmospheres. This could lead to the study of the origin of life, which is important from an “astrobiology” perspective.

Research Grants

This work was supported by Grants-in-Aid for Scientific Research (KAKENHI: Grant-in-Aid for Scientific Research, JP17H04574, JP18H05439), Grant-in-Aid for Young Scientists (JP20J21872), Japan Science and Technology Agency (JST) Strategic Creative Research Promotion Program PRESTO (JPMJPR1775), CREST (JPMJCR1761), and National Institutes of Natural Sciences Astrobiology Project (AB031010). JPMJCR1761), and National Institutes of Natural Sciences, Center for Astrobiology Project (Project No. AB031010).

Annotation

(Note 1) Red dwarf

A star with a surface temperature below about 3,500 degrees Celsius is called a red dwarf. About 80% of the 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, which is the region that can hold liquid water on the surface of a planet, has a shorter period than that of the Sun.

(Note 2) Transit planet search satellite TESS

When an exoplanet crosses in front of its star, the brightness of the star dims slightly. TESS is NASA’s satellite project to search for exoplanets by transit. The plan has been to search for transit planets over a two-year period. A second extension plan is currently underway, which will continue observations until at least 2025. Over the past five years, TESS has discovered more than 6,000 transit planet candidates.

(Note 3) Spitzer Space Telescope

This space telescope was launched by NASA in 2003 and retired in 2020. It is capable of high-precision infrared observations and played a major role in the observation of exoplanets.

(Note 4) MuSCAT Team

The MuSCAT series of multi-color simultaneous imaging cameras that can observe transits in three or four wavelength bands simultaneously, developed by Professor Narita and Project Assistant Professor Fukui for 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, MuSCAT2, and MuSCAT3), MuSCAT stands for Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets, after the famous Okayama (Note 4) Tidal force

(Note 5) Tidal force

A force that changes the shape of celestial body A when it is subjected to gravitational force from another celestial body B. On Earth, the moon causes the ebb and flow (shape change) of the oceans. On Jupiter’s moon Io, the friction inside the moon caused by tidal forces heats the interior of the moon, resulting in active volcanic activity.

(Note 6) James Webb Space Telescope A 6.5 m aperture space telescope launched by NASA, launched on December 25, 2021, to begin scientific observations in 2022. The telescope is capable of imaging, spectroscopic, and photometric observations in the visible, near-infrared, and mid-infrared regions with previously unattainable precision.

Publication

〈Journal〉  Nature

〈Title〉  A temperate Earth-sized planet with tidal heating transiting an M6 star

〈Authors〉  Merrin S. Peterson, Björn Benneke, Karen Collins et al. 

〈DOI〉  10.1038/s41586-023-05934-8

〈URL〉  https://www.nature.com/articles/s41586-023-05934-8

Related Links

University of Tokyo Press Release

Japan Science and Technology Agency (JST)Press Release

The post Discovery of Earth-sized Exoplanet with Potential Volcanic Activity  – A Tidally-heated exoplanet LP 791-18d 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.

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.

Key Points of the Announcement:

• By multi-color transit observations with space and ground-based telescopes (Note 1), they discovered the first giant planet candidate orbiting a “white dwarf” (Note 2), which is a remnant after burnout of stellar death, with a period of 1.4 days.
• This discovery demonstrates that planets can exist near white dwarfs without being destroyed.
• The discovery of this unharmed planet sheds light on the potential existence of habitable planets around white dwarfs.

Abstract:

An international research team, including Project Professor Norio Narita from the Astrobiology Center (Professor at the Komaba Institute for Science, Graduate School of Arts and Sciences at the University of Tokyo), discovered the first candidate giant planet orbiting a white dwarf by multi-color transit observations with NASA’s Transiting Exoplanet Survey Satellite (TESS) launched in April 2018 (Note 3), NASA’s Spitzer Space Telescope retired in January 2020, and the multi-color simultaneous imaging camera MuSCAT2 developed by Project Professor Narita (Note 4).

This object, WD 1856 b, orbiting the white dwarf WD 1856+534 (hereinafter WD 1856: Note 5) located approximately 80 light-years away from the Solar system, is revolving with a period of 1.4 days, has a radius similar to that of Jupiter, and a mass estimated to be less than 14 times that of Jupiter. While examples of orbiting debris believed to be “asteroids” that are remnants after planet destruction have been previously discovered around white dwarfs, this finding marks the first discovery of an unharmed giant planet candidate near a white dwarf. This discovery provides the first evidence that exoplanets can exist near white dwarfs without being destroyed.

The research findings were published online in the international scientific journal “Nature” on September 17, 2020 (at 0:00 am Japan Standard Time).

*Normally, objects with masses less than 13 times that of Jupiter are called planets, and objects with masses greater than 13 times that of Jupiter are called brown dwarfs, so this presentation describes them as giant planet candidates in the sense that the possibility of brown dwarfs still remains.

Research Background：

When a star with a mass smaller than about eight times that of the sun ages and finishes nuclear fusion of hydrogen in its core, it becomes a “red giant” with an outer layer of hydrogen that expands to the level of the earth’s orbit. Finally, the outer layer is ejected, leaving behind “stellar cinders,” which are white dwarfs. Although more than 4,000 exoplanets orbiting stars other than the Sun have been discovered to date, no intact planets have yet been found around the white dwarfs that remain after the stars have reached the end of their lives, although some microplanets have been discovered that are thought to be the remnants of destroyed planets. However, no intact, undestroyed planets have yet been discovered.

Research Description：

Currently, in the field of exoplanet research, exoplanet searches are being conducted by NASA’s Transiting Exoplanet Survey Satellite (TESS), which uses the phenomenon of “transit” to search for exoplanets in almost the entire sky, mainly aiming to discover planets around nearby stars in the solar system. TESS (Transiting Exoplanet Survey Satellite) uses four ultra-wide-field cameras to observe a 24° x 96° region (called a sector) for 27.4 days at a time, looking for periodic dimming that occurs when a planet passes in front of its host star.

In the second year of TESS’ Sector 19 observations, the team discovered that the brightness of the region containing the white dwarf WD1856 (Note 6), located about 80 light years from the solar system, is dimming at a period of about 1.4 days. Initially, the automatic identification program used by the TESS team to detect exoplanets determined that the dimming signal was not due to a planet. This was because the auto-discrimination program only assumed planets around stars and did not assume planets around white dwarfs. Specifically, if it were a transit planet around a star, the dimming due to transit would have lasted at least 30 minutes, but this dimming lasted only about 8 minutes, so it was determined not to be a planet. However, in the process of visually confirming all the dimming signals, researchers realized that it might be a planet around a white dwarf, and it was selected as a candidate for a transit planet.

For each transit planet candidate discovered by TESS, additional observations are made to confirm whether or not it is a real planet. These confirmatory observations include “multicolor transit observations” to determine if WD1856 is indeed the one that is emitting light and if WD1856’s emission is the same at all wavelengths from visible light to infrared. The reason for this is that the planet does not emit light itself, so at any wavelength it will only attenuate a fraction of the area of the white dwarf hidden by the planet. The confirmatory observations were made with the Spitzer Space Telescope (which will be retired after this observation in January 2020) and ground-based telescopes. The Japanese team performed the multicolor transit observations using the MuSCAT2 simultaneous multicolor imaging camera, which was developed with the support of the National Astrobiology Center of the National Institutes of Natural Sciences (see Figure 1).

This additional multicolor transit observation confirmed that it is indeed WD1856 that is dimming, and that the rate of dimming is nearly identical at all observed wavelengths. It was concluded that WD1856 b is a giant planet candidate with a mass about the same size as Jupiter and smaller than Jupiter by a factor of 13.8 (although the possibility that it is a brown dwarf cannot be completely ruled out, it is highly likely to be a giant planet).

Until now, examples of “microplanets,” which are thought to be remnants of destroyed planets, have been found orbiting around white dwarfs. However, this is the first time that an intact, undestroyed giant planet candidate has been discovered. This discovery is the first demonstration that exoplanets may exist undestroyed even near white dwarfs.

The discovery of WD1856 b suggests one interesting possibility. The discovery of WD1856 b suggests an interesting possibility: that intact life-supporting planets (rocky planets that can retain liquid water on their surfaces) can also exist around white dwarfs. If a rocky planet could form in the “right” orbit near a white dwarf without being destroyed, as WD1856 b was, it could provide a suitable environment for life for billions of years (see Note 2).

Moreover, in fact, life habitable planets around white dwarfs are known to be good targets for studying the presence of signs of life by observing the light transmitted through the planet’s atmosphere during transit. In a specific estimate, if there were a life-habitable planet around a white dwarf like WD 1856, five transits with NASA’s James Webb Space Telescope (JWST), scheduled to launch in 2021, would detect water vapor and carbon dioxide molecules in the planet’s atmosphere, It is estimated that 25 transits will detect oxygen, ozone, and other molecules that could be called signs of life.

Although the actual discovery of a life-supporting planet around a white dwarf will depend on future exploration, the discovery of WD1856 b may shed light on the possibility of such a planet.

This research was supported by the Japan Science and Technology Agency (JST) Strategic Creative Research Promotion Program “PRESTO: Development and Application of Intelligent Measurement and Analysis Methods by Integrating Measurement Technology and Advanced Information Processing” under the research theme “Search for a Second Earth by Simultaneous Multi-color Imaging Observation and High Precision Analysis” (Researcher: Kenpo Narita, Subject No.: JPMJPR1775): JPMJPR1775).

Publication：

Journal：Nature

Title：“A Giant Planet Candidate Transiting a White Dwarf ”

Authors(* is the responsible author)：

Andrew Vanderburg*, Saul Rappaport, Siyi Xu, Ian Crossfield, Juliette Becker, Bruce Gary, Felipe Murgas, Simon Blouin, Thomas Kaye, Enric Palle, Carl Melis, Brett Morris, Laura Kreidberg, Varoujan Gorjian, Caroline Morley, Andrew Mann, Hannu Parviainen, Logan Pearce, Elisabeth Newton, Andreia Carrillo, Ben Zuckerman, Lorne Nelson, Greg Zeimann, Warren Brown, Rene Tronsgaard, Beth Klein, George Ricker, Roland Vanderspek, David Latham, Sara Seager, Joshua Winn, Jon Jenkins, Fred Adams, Björn Benneke, David Berardo, Lars Buchhave, Douglas Caldwell, Jessie Christiansen, Karen Collins, Knicole Colon, Tansu Daylan, John Doty, Alexandra Doyle, Diana Dragomir, Courtney Dressing, Patrick Dufour, Akihiko Fukui, Ana Glidden, Natalia Guerrero, Kevin Heng, Andreea Henriksen, Chelsea Huang, Lisa Kaltenegger, Stephen Kane, John Lewis, Jack Lissauer, Farisa Morales, Norio Narita, Joshua Pepper, Mark Rose, Jeffrey Smith, Keivan Stassun, Liang Yu 

DOI：10.1038/s41586-020-2713-y

Abstract URL：https://www.nature.com/articles/s41586-020-2713-y

Terminology：

Note 1: Multicolor Transit Observations
The eclipse phenomenon where a planet passes in front of a star is called “transit”. It occurs when the orbit of an exoplanet happens to cross in front of its host star. Planets that transit are called “transiting planets”. Observing the transit by multiple wavelengths is called multi-color transit observations. The multi-color observations are known for distinguishing whether transiting planet candidates are genuine planets, and Professor Norio Narita, supported by JST SAKIGAKE, conducts searches for extra-solar terrestrial planets by multi-color transit observations.

Note 2: White Dwarf
A star with a mass smaller than about eight times that of the Sun, after hydrogen nuclear fusion in its core ceases, expands into an object called a “red giant,” where the outer layers composed of hydrogen extend to about the orbit of Earth that are eventually ejected outward. A white dwarf is the remnant left behind in the core after this process. White dwarfs are very dense objects, with masses similar to the Sun but size comparable to that of Earth.
White dwarfs are initially very hot objects with surface temperatures reaching 100,000 degrees, but they gradually cool over about 2 billion years to temperatures similar to around 6,000 K (or 5,700 degrees Celsius), similar to a main-sequence star like the Sun. Subsequently, over approximately 8 billion years, their temperature slowly drops further to about 4,000 K.
Due to their small size comparable to Earth, the radiation energy of a white dwarf is much lower compared to a star even if the surface temperature is comparable to a star, making rotation periods of habitable planets around white dwarfs shorter periods than about 10 hours. Consequently, if a rocky planet exists in an orbit closer than about 10 hours around a white dwarf, that planet could remain a potentially habitable planet for several billion years.

Note 3: Transiting Exoplanet Survey Satellite (TESS)
TESS is NASA’s satellite project led by the Massachusetts Institute of Technology. Launched on April 18, 2018, TESS has conducted a plan to survey almost the entire sky for transiting exoplanets for two years. During its initial two-year mission, TESS discovered over 2,000 candidate transiting exoplanets. The project has since been granted an extended mission, currently in its third year of observations.

Note 4: MuSCAT2
MuSCAT2 is a multicolor simultaneous imaging camera developed by Professor Narita and Project Assistant Professor Fukui under the support of the Astrobiology Center, National Institutes of Natural Sciences. It is installed on the 1.52-meter Carlos Sánchez Telescope at the Teide Observatory on the island of Tenerife, Spain. MuSCAT2 enables simultaneous observations of celestial objects in four colors: blue light (400nm-550nm), red light (550nm-700nm), and two near-infrared wavelengths (700nm-820nm, 820nm-920nm). It is used to verify whether candidate transiting exoplanets discovered by TESS are genuine planets.

Note 5: WD1856 (WD1856+534)
WD1856 is a white dwarf located approximately 80 light-years away in the constellation Draco, forming a triple star system with the two red dwarf stars G 229-20 A and G 229-20 B. Although the exact age of this triple system is unclear, WD1856 is estimated backward to have become a white dwarf approximately 6 billion years ago, based on its current age.

WD1856 has a surface temperature of around 4,400 degrees Celsius, with a mass about half that of the Sun and in contrast a size only approximately 1.4 times that of Earth (about 1/80th the size of the Sun). Due to the low radiation energy of a white dwarf, WD1856 b, with its 1.4-day orbital period, is estimated to have a surface temperature of around -110 degrees Celsius, similar to the cold temperature of Jupiter in the solar system.

Note 6: Area including WD1856
The reason I say “region” here is that because TESS has an ultra-wide field of view, the field of view covered by a single pixel of the detector is large, and other bright stars were mixed in the same pixel, so we could not determine that WD1856 was dimmed.

Related Links：

The University of Tokyo Press Release
Japan Science and Technology Agency (JST) Press Release

The post Discovery of an Unharmed Gas-Giant Candidate around a Burned-out Star first appeared on Astrobiology Center, NINS.

Abstract：

A research team composed of the Astrobiology Center, the University of Tokyo, Japan Science and Technology Agency, National Astronomical Observatory of Japan, and Institute of Astrophysics of the Canary Islands developed a new multi-color simultaneous imaging camera MuSCAT2. This instrument was installed on the 1.52-meter telescope (Carlos Sánchez Telescope) at the Teide Observatory in Tenerife, Spain, renowned as one of the best astronomical observation sites in the world.

MuSCAT2 is an observational instrument primarily aiming to confirm whether exoplanet candidates reported by NASA’s Transit (Note 1) Exoplanet Survey Satellite (TESS; Note 2) launched in April 2018, are actually real planets or not. To evaluate the camera’s performance, the research team observed known exoplanet transits and employed cutting-edge statistical analysis techniques. Consequently they demonstrated that MuSCAT2 can achieve world-leading photometric precision (the precision to investigate a variability in brightness) at four simultaneous color channels. This level of precision enables the confirmation of second Earths (potentially habitable planets) orbiting nearby red dwarfs (Note 3) discovered by TESS.
Furthermore, through an agreement between the Astrobiology Center andInstitute of Astrophysics of the Canary Islands, a substantial amount of observation time for MuSCAT2, totaling over 162 nights annually until 2022, has been secured. With Tenerife Observatory’s clear-sky rate of approximately 70%, this is equivalent to the confirmation observations of over 100 planets annually. Expectations are high for MuSCAT2 to discover numerous scientifically intriguing planets from the beginning era of TESS.

Key Points of the Announcement：

• Completion of MuSCAT2, a multicolor simultaneous imaging camera capable of observing variations in brightness at four wavelength bands (four colors) simultaneously.
• We achieved world-leading photometric precision across four colors as a result of the engineering runs.
• Collaboration with NASA’s Transit Exoplnaet Survey Satellite TESS enables the discovery of second Earth planets orbiting nearby red dwarfs.

Background：

By 2018, some 4,000 exoplanets have been discovered in stars other than our Sun. In particular, NASA’s transit planet-hunting satellite Kepler, launched in 2009, has discovered about 3,000 exoplanets by the end of its operations in November 2018, using a phenomenon called transit, in which a planet passes in front of its host star.

Kepler’s work has revealed an abundance of Earth-like planets in the universe, some of which are in habitable zones where the distance from the main star is just right to retain liquid water on the surface. However, Kepler mainly discovered planetary systems more than several hundred light years away from the solar system, making it difficult to study the properties of individual planets in detail, even if the existence of the planets and their statistical properties as a whole were known.

In order to discover planetary systems closer to our solar system, a new transit planet search satellite, TESS, was launched in April 2018, equipped with four cameras with an extremely wide field of view of 24° × 24°, which will observe more than 80% of the entire sky over the next two years and will be able to detect thousands of planetary systems that are close to our solar system. It is expected to observe more than 80% of the entire sky over the next two years and discover thousands of planetary systems that are close to our solar system. These are expected to include several hundred planets that are thought to be terrestrial planets, and dozens of planets in habitable zones.　However, the planet candidates discovered by the transit method include not only real planets but also fake ones called eclipsing binary stars, in which a star passes in front of another star. Therefore, as thousands of planet candidates are discovered by TESS, it has been a research challenge to efficiently distinguish between real planets and fake eclipsing binary stars (called “discovery confirmation observation”).

Research Description:

A research team led by the Astrobiology Center of the National Institutes of Natural Sciences, the University of Tokyo, the Japan Science and Technology Agency, the National Astronomical Observatory of Japan, and the Canarian Institute of Astrophysics has developed the MuSCAT2 multicolor imaging camera that can simultaneously observe changes in the brightness of celestial objects in four colors ranging from visible light to near-infrared light. The camera was installed on the 1.52-m telescope (Carlos Sanchez Telescope) at the Teide Observatory in Tenerife, Spain, which is known as one of the best astronomical observation sites in the world (Figure 1 and Figure 2).

　MuSCAT2 ist ein Instrument, dessen Hauptzweck darin besteht, zu bestätigen, ob es sich bei den Planetenkandidaten, die von dem im April 2018 gestarteten NASA-Satelliten TESS für die Suche nach Transitplaneten entdeckt wurden, um echte Planeten handelt (Entdeckungsbestätigung). Um die Leistung dieses Instruments zu bewerten, beobachtete das Forscherteam tatsächlich bekannte Planetentransite und analysierte sie unter Anwendung der neuesten statistischen Methoden. Damit konnten sie zeigen, dass MuSCAT2 die weltweit höchste photometrische Genauigkeit (d.h. die Genauigkeit der Messung von Helligkeitsänderungen) in allen vier Farben gleichzeitig erreichen kann (Abb. 3). Diese photometrische Genauigkeit ist auch in der Lage, die Entdeckung der zweiten Erden (lebensfreundliche Planeten) zu bestätigen, die rote Zwerge in der Nachbarschaft des Sonnensystems umkreisen und die von TESS entdeckt wurden.　Darüber hinaus hat eine Vereinbarung zwischen dem Zentrum für Astrobiologie und dem Kanarischen Institut für Astrophysik mehr als 162 Teleskopnächte pro Jahr für MuSCAT2 bis 2022 gesichert. Die Aufklärungsrate am Teide-Observatorium liegt bei etwa 70 %, was der Möglichkeit entspricht, mehr als 100 Beobachtungen zur Bestätigung von Planetenentdeckungen pro Jahr durchzuführen. Dies lässt uns hoffen, dass MuSCAT2 in der kommenden Ära von TESS viele wissenschaftlich interessante Planeten entdecken wird, darunter auch eine zweite Erde.

Significance of this research：

TESS is currently conducting observations of the southern sky as of 2018, and will begin observations of the northern sky in the summer of 2019.With the completion of MuSCAT2, this research team can now lead the world in discovery confirmation observations of planetary candidates discovered by TESS’s observations of the northern sky.

The research team has previously developed the MuSCAT three-color simultaneous imaging camera for NAOJ’s 1.88-m telescope in Okayama, Japan. There is a 9-hour time difference between Okayama and Tenerife, and by installing the instruments in such time-differential locations, it will be possible to observe transits that occur at times when they cannot be observed by each other in a complementary manner. Also, if a transit cannot be observed at one location for a long period of time, continuous observation at Okayama and Tenerife Island will enable coverage of the transit for a long period of time.

Since no other research team in the world has multiple multicolor imaging cameras like this, the existence of MuSCAT and MuSCAT2 is expected to be a great advantage in the era of TESS, which requires a large number of discovery confirmation observations.

Future Development：

The team is developing MuSCAT3 for the U.S. telescope with the aim of establishing a system that will be able to perform 24-hour multi-color imaging observations by the summer of 2019. TESS will begin in the summer of 2019.

We hope that the MuSCAT series will play a major role in the TESS era.

Terminology Explanation：

Note 1: Transit
When a planet passes in front of a star, it causes periodic dimming of the star’s brightness. The method of discovering planets by observing these brightness variations over long time is called the transit method, and planets that cause transits are referred to as transiting planets.

Note 2: TESS
NASA’s Transiting Exoplanet Survey Satellite launched on April 18, 2018. Although the aperture size is smaller than the similar satellite Kepler launched in 2009, TESS has a wider field of view and is well-suited for discovering transiting planets closer to the solar system than Kepler. However, due to its larger field of view, TESS is expected to encounter more contamination from false positives such as eclipsing binaries and confirmation observations for discoveries are essential.

Note 3: Red Dwarf
A collective term for stars with absolute temperatures below approximately 3,800K. These stars have lower surface temperatures compared to the Sun, which has a temperature of about 5,800K, making them faint in visible light but brighter in near-infrared.Red dwarf stars account for 70-80% of stars in the universe, including those near the solar system, and they are primary targets for future searches for potentially habitable planets in the solar neighborhood.

Published Paper Information:

Journal Name: Journal of Astronomical Telescopes, Instruments, and Systems

Paper Title: MuSCAT2: 4-color Simultaneous Camera for the 1.52m Telescopio Carlos Sanchez

Authors (* indicates corresponding author):

*Norio Narita, Akihiko Fukui, Nobuhiko Kusakabe, Noriharu Watanabe, Enric Palle, Hannu Parviainen, Pilar Montanes-Rodriguez, Felipe Murgas, Matteo Monelli, Marta Aguiar, Jorge Andres Perez Prieto, Alex Oscoz, Jerome de Leon, Mayuko Mori, Motohide Tamura, Tomoyasu Yamamuro, Victor J. S. Bejar, Nicolas Crouzet, Diego Hidalgo, Peter Klagyivik, Rafael Luque, Taku Nishiumi

Research Group:

the University of Tokyo, Japan Science and Technology Agency, Astrobiology Center, National Astronomical Observatory of Japan, Institute of Astrophysics of the Canary Islands, and others

Research Support:

This research was supported by JSPS KAKENHI Grant-in-aid JP18H01265, JP17H04574, JP16K13791, JP15H02063 and JST CREST JPMJPR1775.

Related Links:

National Astronomical Observatory Press Release
University of Tokyo Press Release
Japan Science and Technology Agency (JST) Press Release

The post Completion of MuSCAT2, a New Multi-color Simultanoues Imaging Camera to Discover the Second Earth first appeared on Astrobiology Center, NINS.