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

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Summary

• 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.

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 An international team led by scientists from the Astrobiology Center in Japan, the University of Tokyo, the National Astronomical Observatory of Japan, and Tokyo Institute of Technology has successfully discovered a new extrasolar planet named Gliese 12 b through a collaboration between NASA’s TESS campaign and a strategic survey program (SSP) of the Subaru Telescope. Gliese 12 b has a size similar to Earth and Venus, and is orbiting around its host star, Gliese 12, with a period of 12.8 days. Despite its close proximity to its host star, the amount of radiation Gliese 12 b receives is comparable to that of Venus, because the host star is much cooler than the Sun. The planet may still retain a certain amount of atmosphere, making it one of the most suitable targets out of all of the planets discovered so far to investigate the atmosphere of a planet like Venus. It remains an open question why the surface environment of Venus – a sibling of Earth – became so harsh for life compared to that of Earth. In the near future, NASA’s JWST and extremely large telescopes, such as TMT, will be used to characterize the atmosphere of Gliese 12 b in detail, greatly improving our understanding of the conditions necessary for habitability. 

Is Earth a special planet with its wide variety of life? Or are planets bearing life common in this Universe? In order to answer these fundamental questions, we need to look for clues from other planets that are similar to Earth. In particular, Venus in the Solar System is an important target. Venus’s size and mass are very similar to those of Earth, so Venus is called “Earth’s sibling.” Nevertheless, its atmosphere is thick and dry and thus not like Earth’s. Why did Venus develop a surface environment that is significantly different from Earth’s? Although Venus’s insolation – the amount of light a planet receives from the host star – is slightly higher than Earth’s insolation, the answer to the above question remains unclear. Indeed, scientists don’t understand why a planet develops an environment suitable for bearing life. To better understand that question, it is essential to get hints from not only Venus but also an “exo-Venus,” which is a Venus-like planet outside the Solar System. 

Since the 1990s, more than 5,500 planets orbiting around stars other than the Sun have been discovered by various detection methods. In particular, the Kepler satellite launched by NASA in 2009 played a major role in the discoveries and was the first to discover planets with sizes comparable to or smaller than Earth. However, as these planets are hundreds of light years away from Earth, it is challenging to characterize their atmospheres in detail with the current or even up-coming telescopes. 

The current trend is to discover planets orbiting M type stars, which are less massive than the Sun, in the vicinity of the Solar System. This is because if the star is less massive or smaller, it is easier to detect a change in the host star’s velocity and brightness that originates from the orbital motion of a planet. The method to detect the velocity change is called the “Doppler” technique, while that to detect the brightness change is called the “transit” technique. 

To use the Doppler technique, astronomers carry out spectroscopic observations, in which stellar light is divided into many “rainbows.” A huge amount of light is required for this analysis. M-type stars are faint at visual wavelengths but bright at infrared wavelengths. So, the Subaru Telescope started a large program to search for planets via the Doppler technique in 2019 using the newly-developed infrared spectrograph, IRD. Between 2019 and 2022, the astronomers extensively monitored Gliese 12, a star located 40 light-years away in the direction of the concentration Pisces, as one of the targets of the IRD-SSP observing campaign. Gliese 12 is an M-type star one-fourth the size of the Sun, with a surface temperature of 3,000 ℃, which is 2500 ℃ cooler than the Sun. 

Gliese 12, was also observed by NASA’s TESS space telescope between August 2021 and October 2023. The TESS team detected signs of a planet candidate with a size similar to Earth and reported the detection in April, 2023. This report motivated the astronomers to start the follow-up observations for validating the candidate signal with the multi-color simultaneous cameras MuSCAT2 and MuSCAT3, which were developed by the Astrobiology Center (ABC) and the University of Tokyo. The analysis of the data taken with TESS and the MuSCAT series determined the orbital period of Gliese 12 b to be 12.8 days and the radius to be 0.96 Earth radii. Furthermore, the astronomers constrained the mass of Gliese 12 b to be less than 3.9 Earth masses by combining the Doppler velocity measurements taken with IRD and those with CARMENES on the Calar Alto 3.5 m telescope in southern Spain. 

What kind of planet is Gliese 12 b? The orbital period of this planet, that is to say one year on this planet, is just 12.8 days. This translates to a distance between the star and the planet of only 0.07 au, where one au corresponds to the Earth–Sun distance. However, the amount of insolation Gliese 12 b receives is only 1.6 times higher than that of Earth, or similar to that of Venus (which is 1.9 times higher than Earth’s), thanks to the low temperature of the host star. Nevertheless, even with such a relatively weak insolation, the planetary surface would be hot enough to start the runaway evaporation of liquid water from the surface. 

Meanwhile, whether liquid water can be stably retained on the surface of a planet depends on the composition and thickness of the atmosphere. For example, even if the surface temperature of a planet is appropriate, the planet cannot retain water as a liquid on the surface if the atmosphere is too thin. However, the characteristics of the atmospheres of extrasolar planets have been poorly understood. 

A well-known system for study of planetary atmospheres is the TRAPPIST-1 system, a cool M-type star with seven terrestrial planets. Among the planets around TRAPPIST-1, the second-closest planet to the star, TRAPPIST-1 c, is very similar to Gliese 12 b and Venus in size (1.1 Earth radii) and insolation (2.2 times Earth’s insolation). However, recent observations by the James Webb Space Telescope (JWST) revealed that the atmosphere of TRAPPIST-1 c is at least not as thick as that of Venus. TRAPPIST-1 is active enough to release strong radiation such as X-ray and ultraviolet light, and high-energy particles like stellar winds. Most of the planet’s atmosphere might have been dissipated by this high-energy radiation in the past. 

In contrast, the X-ray luminosity of Gliese 12 is an order of magnitude weaker than that of TRAPPIST-1. In addition, the distance between Gliese 12 b and its host star is more than 4 times larger than that between TRAPPIST-1 c and its host. Accordingly, the effect of high-energy radiation on Gliese 12 b is much weaker than that on TRAPPIST-1 c, making it possible that Gliese 12 b might retain a certain amount of atmosphere compared with TRAPPIST-1 c. 

Given that Gliese 12 is a neighbor of the Sun, Gliese 12 b is an ideal target for atmosphere characterizations with JWST and future 30-m class telescopes, alongside TRAPPIST-1. In the future, by observing the atmosphere of Gliese 12 b and comparing it with those of Venus and TRAPPIST-1 c, scientists will be able to reveal how the atmospheres of terrestrial planets vary depending on the radiation environments around the host stars. 

Although Venus currently does not retain liquid water on the surface, it might have in the past. Likewise, it cannot be fully ruled out that liquid water is present on Gliese 12 b’s surface. “Follow-up observations with JWST and future ground-based observations with 30-m class telescopes for transit spectroscopy are expected to determine whether Gliese 12 b has an atmosphere and whether the atmosphere contains molecular components associated with life such as water vapor, oxygen, and carbon dioxide,” says Masayuki Kuzuhara, a project assistant professor of the Astrobiology Center (ABC). 

There results were published in the Astrophysical Journal Letters on May 23, 2024 (Kuzuhara, Fukui et al. “Gliese 12 b: A temperate Earth-sized planet at 12pc ideal for atmospheric transmission spectroscopy“). 

The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i. 

(Related Links) 

NAOJ May 24, 2024 Press Release

Subaru telescope, Press Release

NASA May 23, 2024 Press Release 

W. M. Keck Observatory May 23, 2024 Press Release

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In a breakthrough discovery, the Subaru Telescope’s powerful extreme adaptive optics system has imaged a massive benchmark gas giant planet around the nearby, bright star HIP 99770. The object, HIP 99770 b, is the first extrasolar planet jointly discovered by direct imaging and the new method of indirect detection, precision astrometry. This new approach for finding imageable planets simultaneously measures their weight, orbits and even their atmosphere. It prefigures the way that we will someday identify and characterize an Earth twin around a nearby star. 

“We are now in a new era for imaging other worlds,” says Thayne Currie, lead author of the ground-breaking paper published in Science.

Direct imaging is a method that will someday reveal an Earth-like exoplanet around a nearby star. In the past 14 years, large ground-based telescopes equipped with adaptive optics (AO) to sharpen starlight have taken key steps towards this goal, revealing the first direct images of Jupiter-like gas giant exoplanets. These discoveries draw from so-called blind surveys: targets are selected based on system properties like age and distance but are otherwise unbiased. Unfortunately, the low yields of these blind surveys show that exoplanets we can image with current telescopes are rare. 

Direct imaging searches focused on stars showing dynamical evidence for a planet may greatly increase the rate of imaging discoveries. Precision astrometry — measuring the position and motion of stars on the sky — could identify which stars are being pulled by the gravitational influence of an unseen companion and thus may host planets we can image. 

An international research team led by Subaru Telescope, the University of Tokyo, the University of Texas-San Antonio, and the Astrobiology Center of Japan report the world’s first joint direct imaging and astrometric discovery of an exoplanet, using Subaru Telescope’s extreme adaptive optics system (SCExAO; Note 1) coupled with its near-infrared spectrograph (CHARIS) combined with astrometry from European Space Agency’s Gaia mission and its predecessor, Hipparcos. The planet was imaged around the nearby bright star HIP 99770, located in the constellation Cygnus.

“Once we knew which star to look at, Subaru’s extreme adaptive optics system was able to sharpen starlight so well that our infrared instruments could see the faint planet hinted at by Gaia and Hipparcos” notes Olivier Guyon, the Principal Investigator of SCExAO.

The planet – HIP 99770 b – is about 100,000 times fainter than the star it orbits. Its CHARIS spectrum, combined with follow-up imaging from the W.M. Keck Observatory, reveals an atmosphere shaped by water and carbon monoxide, with a temperature about 10 times hotter than Jupiter’s. Its atmosphere resembles an older and slightly less cloudy counterpart to the atmospheres of the first imaged planets, HR 8799 bcd.

By jointly analyzing data from the Subaru Telescope, Keck, Gaia and Hipparcos, the team was able to directly measure the planet’s mass and constrain its orbit. HIP 99770 b is about 14-16 times the mass of Jupiter in our own Solar System, and orbits a star that is nearly twice as massive as the Sun. The planet’s orbit is three times larger than Jupiter’s around the Sun or just over half of Neptune’s distance from the Sun. However, it receives nearly the same amount of light as Jupiter because it’s host star is far more luminous than the Sun. 

“Combining direct imaging from Subaru and Keck with precision astrometry tells us far more about planets like HIP 99770 b than was previously possible,” says Currie.

The discovery has broader implications for the field of extrasolar planets. HIP 99770 b was detected as a part of a SCExAO direct imaging program using Gaia data to identify stars being gravitationally pulled by unseen planets. While many results are currently unpublished, their detection rate so far appears much higher than from previous blind surveys.

“This approach is a better way to find planets that we can then image and study in detail. As our instruments are improving, more will be found,” says Guyon.

The combined approach will also allow us to find an Earth-like planet around a nearby star with upcoming ground-based observatories like the Thirty Meter Telescope or space-based ones like the Habitable Worlds Observatory. Such a planet will be much closer to its star than any planet imaged to date and so will spend a large amount of time either in front or behind that star, making it impossible to see. 

“The indirect detection method will point us to a star around which a rocky, terrestrial planet could be imaged. Once we know when to look, we hope to learn whether this planet has an atmosphere compatible with life as we know it on Earth,” says Motohide Tamura, Professor of the University of Tokyo.

The Subaru Telescope and the W. M. Keck observatory are located at the summit of Maunakea in Hawai`i, an inactive volcano known for its unsurpassed qualities as an astronomy site and its deep personal and cultural significance to many Native Hawaiians.

“Maunakea is the best place on the planet Earth to see other worlds. We are extremely grateful for the privilege of being able to study the heavens from this mountain,” says Currie.

These results appeared as Currie et al. “Direct Imaging and Astrometric Detection of a Gas Giant Planet Orbiting an Accelerating Star” in Science on April 13, 2023.

(Note 1) With ground-based telescopes, the images of celestial objects appear out of focus and shaky, as if looking out from underwater, due to the effects of the Earth’s atmosphere. Extreme adaptive optics corrects the turbulence caused by the Earth’s atmosphere in real time with exceptional precision, making the Subaru Telescope produce extremely sharp images.

(Related Links)

NAOJ April 14, 2023 Press Release 

Subaru Telescope April 13, 2023 Press Release

The University of Tokyo April 14, 2023 Press Release 

The University of Texas-San Antonio April 13, 2023 Press Release

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A brown dwarf orbiting the Sun-like star HIP 21152 was discovered using the Subaru Telescope’s Extreme Adaptive Optics System. HIP 21152 B was found to be the lightest brown dwarf with an accurately determined mass, approaching the mass of a giant planet. HIP 21152 B is expected to be an important benchmark object for studying the evolution of giant planets and brown dwarfs and their atmospheres.

Brown dwarfs (Note 1) are an interesting type of object not found in our Solar System, with masses somewhere between those of stars and planets. Brown dwarfs are also important for studying the evolution of giant planets and their atmospheres, because Jupiter-like planets and lighter brown dwarfs are expected to have similar characteristics.

Brown dwarfs  drift alone in space or orbit around stars. While thousands of brown dwarfs have been found since the first discovery in 1995, companion-type brown dwarfs are rare, with a frequency of only a few per 100 stars. For this reason, astronomers have been racking their brains for an efficient way to find companion brown dwarfs.

An international team including astronomers from the Astrobiology Center; the National Astronomical Observatory of Japan; the University of California, Santa Barbara; and NASA has developed a new method to efficiently discover companion brown dwarfs and giant planets. Furthermore, they applied that method to imaging surveys with the Subaru Telescope. This search adopts information on the “proper motion” of stars in our Galaxy, which is the motion of stars with their own unique velocities. When a companion object orbits a star, the proper motion of the host star is accelerated by the gravity from the companion. However, the velocity change caused by a light companion such as a brown dwarf or planet is very small, making it challenging to measure the change precisely.

However, a turning point came with ESA’s astrometry satellite Gaia (Note 2), the successor to the Hipparcos satellite. By measuring the difference between the measurements from the two satellites, it is now possible to derive minute accelerations in proper motion (Figure 2 left). Using data from both telescopes, the research team analyzed the acceleration of proper motion for stars near the Sun, and selected stars that may be accompanied by giant planets or brown dwarfs. They then proceeded with direct imaging observations using Subaru Telescope’s high contrast instruments, SCExAO and CHARIS, leading to the discovery of a brown dwarf “HIP 21152 B” orbiting the star HIP 21152.

The team determined the orbit of HIP 21152 B by combining a total of four direct imaging observations by the Subaru Telescope and Keck Telescope, line-of-sight velocity observations of the star HIP 21152 by HIDES on the Okayama 188-cm Reflector Telescope, and the proper motion data from Gaia and Hipparcos. The companion’s mass is derived from the orbit, as indicated by Kepler’s law. The actual orbital analysis (Figure 2, right) determined the mass of HIP 21152 B to be 22­–36 Jupiter masses. Brown dwarfs with such accurately determined masses are rare (Note 3). HIP 21152 B was also found to be the lightest brown dwarf among those with accurately determined masses, approaching planetary masses (Note 4).

HIP 21152 B will be an important source for characterizing the atmospheres of brown dwarfs and giant planets. The team also obtained the spectrum of HIP 21152 B (Figure 3), showing that its atmospheric characteristics can be classified as being in the transition stage between two brown dwarf spectral types, L-type and T-type. Strong absorption from methane is shown in the atmosphere of a T-type brown dwarf, while an L-type brown dwarf shows little of it in the atmosphere. This spectral transition is strongly related to atmospheric temperature and the presence of clouds. Interestingly, the well-known directly-imaged planets around HR 8799 show a similar spectrum. In this respect, it is again important that the most fundamental characteristics of HIP 21152 B, namely its mass and age, are accurately determined. Masayuki Kuzuhara, a project assistant professor at the Astrobiology Center, who led the research, says, “This result can provide an important clue to understand the atmospheres of giant planets and brown dwarfs based on how and when they show atmospheric characteristics similar to those seen in the planets of the HR 8799 system and HIP 21152 B. It is expected that HIP 21152 B will play an important role as a benchmark for future progress in astronomy and planetary science.”

As this observation project is still ongoing, even more discoveries are expected. The Subaru Telescope’s direct imaging instruments continue to be improved, making new observational capabilities ready for science operation. With the progress in the efficient exploration and the development and improvement of Subaru Telescope’s instruments, various important discoveries will continue to be made in the future.

These results were published in the Astrophysical Journal Letters on July 27, 2022 (Kuzuhara et al., “Direct-imaging Discovery and Dynamical Mass of a Substellar Companion Orbiting an Accelerating Hyades Sun-like Star with SCExAO/CHARIS“.) It was also featured in AAS Nova, which highlights outstanding research in the AAS journals (Featured Image: First Images of a Substellar Companion in the Hyades).

(Note 1) There are several definitions of brown dwarfs, but in general, brown dwarfs are considered to be objects with masses between 13 and 80 times that of Jupiter. Objects with such masses do not fuse hydrogen (unlike stars) but do fuse deuterium (unlike planets). In contrast, heavy planets and light brown dwarfs are very similar, and it is thought that there is no need to distinguish between them except for their mass.

(Note 2) Gaia is a space telescope launched in 2013 for high-precision astrometry. It provides unprecedented positional and radial velocity measurements for about one billion astronomical objects.

(Note 3) So far, the main method used to estimate the mass of brown dwarfs has been the “evolutionary model.” Evolutionary models predict the luminosity and temperature of a brown dwarf as it ages. Then the observed luminosity and temperature are used to determine the mass of the brown dwarf using these models. However, this method could yield an inaccurate mass due to uncertainties in the evolutionary model and the age (generally, the age of the brown dwarf is assumed to be equal to that of the host star or the cluster). HIP 21152 B belongs to the Hyades cluster, so its age is accurately determined, but the evolutionary model remains uncertain. The mass of HIP 21152 B estimated from the evolutionary model is 1.3 times larger than the mass determined from the orbital analysis.

(Note 4) A European research team independently succeeded in imaging HIP 21152 B (myScience article). Meanwhile, the study led by Kuzuhara is the first to prove that HIP 21152 B orbits its host star and to derive its dynamical mass.

(Note 5) A web tool provided by the University of Geneva is used as a reference for displaying the absorption wavelengths of the molecules.

(Related Links)

Subaru telescope, Jan. 23, 2023 Press Release

W. M. Keck Observatory January 23, 2023 Press Release

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Yu Komatsu, a researcher at the Astrobiology Center (ABC), and his collaborators estimated for the first time by numerical simulation how photosynthesis-derived fluorescence could be detected as a biosignature of life in future observations of extrasolar planets and discussed in detail based on our knowledge of photosynthesis. The results suggest that although fluorescence detection will be difficult with a future planned 6-meter aperture space telescope, some conditions and features will facilitate the identification of planets around ultra-cool dwarfs such as TRAPPIST-1. The results, which were obtained through discussions across multiple disciplines from biology to astronomy, were published in the online edition of the American scientific journal “The Astrophysical Journal” on 11th January, 2023 (Komatsu et al., 2023).

The search for life on exoplanets is one of the most important themes in the field of astrobiology. As evidence for the existence of such life, it is expected to detect biosignatures (Note 1) that show characteristic patterns of photosynthesis-derived light. One of these is the red edge (Note 2), a spectroscopic feature of the light spectrum reflected by vegetation. For example, the light environment of planets around stars lighter than the Sun (M dwarfs), which are currently the target of observation, is very different from that of the Earth in our solar system, and it is under discussion how the red edge appears.

In photosynthesis, light energy absorbed from sunlight is either used for photochemical reactions or released as fluorescence (Note 3) or heat (Figure 2). Remote sensing of the Earth has recently observed the fluorescence as well as the red edge. The red edge allows us to measure the amount of vegetation covering the planetary surface, whereas the fluorescence is used to estimate more detailed photosynthetic activity, such as stress conditions. We, therefore, tested the promise of photosynthetically derived fluorescence as an advanced biosignature in addition to the red edge.

In this study, we simulated how fluorescence appears in planetary spectra for a Sun-like star and an Earth-like planet orbiting two M-type dwarfs (GJ667C and TRAPPIST-1), respectively, assuming different planetary atmospheres and surface conditions. We used two light absorption and fluorescence spectra of photosynthetic organisms: typical vegetation with chlorophyll a and b (Chl) and purple bacteria with bacteriochlorophyll b (BChl). We determined the fluorescence intensity by appropriately scaling it according to the number of photons acquired under radiation fields in the habitat. Using these light absorption spectra, we also calculated the leaf reflection spectra by means of radiation transfer calculations (Note 4). In this way, we developed a model that consistently handles light absorption, fluorescence, and reflection, and investigated how they appear in planetary spectra.

Numerical simulations showed that, in the case of BChl, in the absence of clouds or strong absorbers around 1,000 nm, the fluorescence, together with the detection of red edges, can be a good biosignature to identify traces of photosynthesis (Figure 3). However, a noise model assuming NASA’s planned future 6 m aperture space telescope (previously considered as LUVOIR, now called the Habitable Worlds Observatory) around solar-type stars, we also found that it takes a very long observation time to identify the fluorescence. Even so, ultra-cool stars such as TRAPPIST-1 have strong absorption of vanadium oxide (VO), iron hydride (FeH), and potassium in the stellar atmosphere, and interestingly these lead to significantly larger apparent reflectance at wavelengths where the flux from the star is small due to this stellar absorption, and fluorescence emission from the planet. This may be a good feature for observing fluorescence at high dispersion by future large ground-based telescopes such as TMT and needs to be verified in the future. Furthermore, it is important to consider the conditions for large fluorescence emission from the physiological perspective of photosynthesis and to capture the nonlinear response of biological fluorescence relative to the incident light since fluorescence is also generated nonbiologically.
At ABC, young researchers actively collaborate across the boundaries of research fields between astronomy and biology, and observation, experiment, and theory. This study results from such activities and has been compiled as an academic paper. This is truly an achievement that links biology and astronomy, and theory and observation.

Footnotes :
(Note 1) Spectral features of atmospheric molecules such as oxygen, ozone, and methane, and the surface feature, e. g., due to vegetation.

(Note 2) A feature in which the reflectance spectrum of leaves increases sharply around 700 nm.

(Note 3) The light emitted when electronically excited states by light quenched to a low-energy state.

(Note 4) A calculation method that deals with light propagation.

Publication Information :
Journal: The Astrophysical Journal
Title: Photosynthetic Fluorescence from Earth-like Planets around Sun-like and Cool Stars

Authors: Yu Komatsu 1,2, Yasunori Hori 1,2, Masayuki Kuzuhara 1,2, Makiko Kosugi 1,2,3, Kenji Takizawa 1,3, Norio Narita 4,1, Masashi Omiya 1,2, Eunchul Kim 3, Nobuhiko Kusakabe 1,2, Victoria Meadows 5, Motohide Tamura 1,2,4
1) Astrobiology Center, 2) National Astronomical Observatory of Japan, 3) National Institute for Basic Biology, 4) University of Tokyo, 5) University of Washington

DOI: 10.3847/1538-4357/aca3a5

arXiv: https://arxiv.org/abs/2301.03824

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