Key Points

• By combining direct imaging and radial velocity observations from ground-based telescopes with precise astrometric acceleration data from a space telescope, the team discovered a companion orbiting a nearby red dwarf (about 55 light-years from Earth) and determined its mass (about 60 times that of Jupiter) and orbital semi-major axis (about 4.3 astronomical units) with high precision.
• This achievement was made possible by integrating radial velocity monitoring from the Subaru Telescope’s Infrared Doppler instrument (IRD) as part of the IRD Strategic Program (IRD-SSP), high-contrast imaging with the Keck Telescope, and astrometric data from the Gaia telescope.
• The newly detected companion is inferred to be a late-T-type brown dwarf and exhibits about 30% variability in near-infrared brightness, making it a promising “benchmark object” for future studies of atmospheric clouds and circulation.
• While previous methods combining Hipparcos and Gaia astrometric acceleration with direct imaging have been used to detect and constrain the masses of companions, this study represents the first successful application of Gaia-only acceleration data to a faint nearby red dwarf system, beyond Hipparcos’ detection limits, resulting in the precise characterization of a brown dwarf companion.

Results:

 M dwarfs, or red dwarfs, are the most common type of star in our galaxy, accounting for more than half of all stars in the Milky Way. These small, cool stars are key targets for understanding the processes of stellar and planetary formation and evolution. However, because M dwarfs are intrinsically faint, detailed observations have historically been limited, and early surveys suggested that more than 70% of them were single stars. Recent advances in observational techniques, however, have revealed that this picture was incomplete: the frequency of low-mass stellar and substellar companions, such as brown dwarfs, may have been significantly underestimated. Understanding how often such companions occur—and their mass distribution—is essential for distinguishing the similarities and differences between planet formation and star formation.

 An international research team led by the Astrobiology Center, California State University Northridge, and Johns Hopkins University has now discovered a brown dwarf companion, J1446B, orbiting the nearby M dwarf LSPM J1446+4633 (hereafter J1446), located about 55 light-years from Earth (Figure 1). J1446B has a mass of about 60 times that of Jupiter and orbits its host star at a distance roughly 4.3 times the Earth–Sun separation, completing one orbit in about 20 years. Remarkably, near-infrared observations revealed brightness variations of about 30%, suggesting dynamic atmospheric phenomena such as clouds or storms.

 The key to this discovery was the combination of three complementary observational techniques: (1) radial velocity measurements from long-term infrared spectroscopic monitoring with Subaru’s IRD instrument, (2) high-resolution near-infrared imaging with the W. M. Keck Observatory using advanced adaptive optics with a pyramid wavefront sensor, and (3) precise astrometric acceleration measurements from the Gaia mission. By integrating these datasets and applying Kepler’s laws, the team was able to determine the dynamical mass and orbital parameters of J1446B with unprecedented accuracy (Figure 2). Radial velocity data alone cannot break the degeneracy between mass and orbital inclination, but adding direct imaging and Gaia astrometry resolves this ambiguity. The Subaru IRD-SSP program provided essential RV data, while Keck’s state-of-the-art adaptive optics enabled the direct detection of the companion at a very small separation from its host star.

 Previous studies have demonstrated the power of combining Hipparcos and Gaia astrometric acceleration with direct imaging to detect and characterize companions. However, Hipparcos was unable to measure the positions of faint red dwarfs like J1446. This study is the first to apply Gaia-only acceleration data to such a system, successfully constraining the orbit and dynamical mass of a brown dwarf companion.

 This discovery provides a critical benchmark for testing brown dwarf formation scenarios and atmospheric models. Future spectroscopic observations may even allow researchers to map the weather patterns of this intriguing object. The result highlights the power of combining ground-based and space-based observatories to uncover hidden worlds beyond our solar system.

“Direct Imaging Explorations for Companions from the Subaru/IRD Strategic Program II; Discovery of a Brown-dwarf Companion around a Nearby Mid-M-dwarf LSPM J1446+4633” by Uyama et al. (DOI: 10.3847/1538-3881/ae08b6).

Research Funding:

This research was supported by JSPS KAKENHI (Grant Numbers: 24K07108, 24K07086).[1]  The development and operation of IRD were supported by JSPS KAKENHI (Grant Numbers: 18H05442, 15H02063, and 22000005).

Notes:

*1: Brown dwarfs typically have masses between about 13 and 80 times that of Jupiter and cannot sustain hydrogen fusion like stars. They are sometimes referred to as “failed stars” in popular science, but their formation processes remain poorly understood. Like gas giants such as Jupiter, they cool over time, making them important targets for studies of planet formation.

*2: This refers to an observational technique that uses the Doppler effect: the motion of a star causes shifts in its spectral lines, which can be measured to detect companions.

*3: The Subaru Telescope and the Keck Telescope are large-aperture (8 – 10 m class) observatories located at the summit of Maunakea on the island of Hawai‘i.

*4: The Gaia spacecraft, launched in 2013, is an astrometric mission designed to create a detailed 3D map of stars in the Milky Way. Its extremely precise positional measurements enable the detection of companions and planets through the astrometric method, which relies on subtle stellar motions. Hipparcos, launched in 1989, was Gaia’s predecessor and provided the first space-based astrometric catalog.

Publication Information:

Journal: The Astronomical Journal
Title: Direct Imaging Explorations for Companions from the Subaru/IRD Strategic Program II; Discovery of a Brown-dwarf Companion around a Nearby Mid-M-dwarf LSPM J1446+4633
Authors: Uyama, T.; Kuzuhara, M.; Beichman, C.; Hirano, T.; Kotani, T.; An, Q.; Brandt, T. D. et al.
DOI: 10.3847/1538-3881/ae08b6

Related links:

Subaru telescope, NAOJ, October 20, 2025 Press release

W. M. Keck Observatory, October 20, 2025 Press release

The post Discovery of a Brown Dwarf Orbiting a Red Dwarf through the Synergy of Ground- and Space-based Observatories first appeared on Astrobiology Center, NINS.

Abstract

As atmospheric observations of exoplanets become increasingly precise, it is more important than ever to correctly account for the effect of starspots on host stars. An ideal opportunity to study starspots arises when a transiting planet passes directly across them—a phenomenon known as a spot-crossing transit.

An international research team led by scientists at the Astrobiology Center (Tokyo, Japan) has combined ground-based observations to reveal the detailed properties of the starspots and the orbital geometry of the planetary system TOI-3884.

Background

NASA’s James Webb Space Telescope (JWST) has revolutionized the study of exoplanet atmospheres. Atmospheric observations primarily rely on transits—when a planet passes in front of its host star and blocks a fraction of its light. By comparing the transit depth at different wavelengths, astronomers can identify the atoms and molecules in the planet’s atmosphere. JWST now enables the detection of subtle transit depth differences as small as 0.01%. However, this unprecedented precision also makes it necessary to account for effects that were previously hidden in the noise, such as those caused by starspots—cooler, darker regions on the stellar surface. Starspots can mimic or obscure atmospheric signals, making it crucial to understand and correct for their impact.

TOI-3884 is a red dwarf star located about 140 light-years away. It hosts the planet TOI-3884b, a “super-Neptune” about six times the radius of Earth. Remarkably, TOI-3884b’s transits show a persistent spot-crossing signal (Fig. 1). Such systems are extremely rare and provide a valuable opportunity to simultaneously study both the properties of starspots and the system’s orbital geometry.

Previous studies (Almenara et al. 2022; Libby-Roberts et al. 2023) produced conflicting results regarding key parameters of the TOI-3884 system, such as the stellar inclination and rotation speed. The present study aimed to resolve these discrepancies using more precise ground-based observations.

Results

To capture the spot-crossing transits, the team used the multicolor MuSCAT3 and MuSCAT4 instruments mounted on the Las Cumbres Observatory (LCO) 2-meter telescopes. Between February and March 2024, they observed three transits and successfully detected clear spot-crossing signals (Fig. 2). The color dependence of the signal provides critical information about starspot temperature.

Light curve analysis revealed that the starspots are about 200 K cooler than the stellar surface (3150 K) and cover roughly 15% of the visible stellar disk. Also, the three transit light curves show changes in the shape of the spot-crossing signal. Because these variations occurred over a short timescale, they are more likely caused by stellar rotation than by spot evolution.

To confirm this, the team carried out a photometric monitoring campaign using the global network of LCO 1-meter telescopes. From December 2024 to March 2025, they measured the star’s brightness variations several times per night and detected clear periodic fluctuations (Fig. 3). This revealed, for the first time, that the stellar rotation period is 11.05 days.

The measured rotation period was consistent with the spot position shifts inferred from the transit observations, enabling the team to obtain a unique solution for the system geometry. They found that the stellar spin axis and the planet’s orbital axis are misaligned by about 62°, revealing that TOI-3884 is a highly tilted planetary system. Such large tilts are typically attributed to past gravitational interactions with massive planets or stellar companions—yet no such companions are known to exist, making this system particularly intriguing.

Future Prospects

TOI-3884b is one of the prime targets for atmospheric studies with JWST and other telescopes. The detailed characterization of its starspots and orbital geometry from this study will be critical for correctly interpreting the results of atmospheric observations. 

Moreover, the findings also provide new insights into stellar magnetic activity. Large polar starspots are often thought to be linked to strong magnetic fields on rapidly rotating stars. However, TOI-3884 does not rotate particularly fast, yet it still hosts a giant polar spot. This suggests that polar spots may be especially common among red dwarfs. In addition to continuing detailed observations of TOI-3884, it will also be important to deepen our understanding of the general properties of starspots.

The paper was published in The Astronomical Journal on September 8, 2025.

Publication

Journal: Astronomical Journal
Title: Multiband, Multiepoch Photometry of the Spot-crossing System TOI-3884: Refined System Geometry and Spot Properties
Authors: Mayuko Mori, Akihiko Fukui, Teruyuki Hirano, Norio Narita, John H. Livingston et al.
DOI: 10.3847/1538-3881/ade2df
URL:https://doi.org/10.3847/1538-3881/ade2df

The post A Planet Crossing Starspots Reveals the Detailed Architecture of the TOI-3884 System first appeared on Astrobiology Center, NINS.

Key Points：

• Using the 8-meter telescope (VLT) at the European Southern Observatory, astronomaers have caught the protoplanet AB Aurigae b in the act of gathering material from its surrounding disk.
• The light spectrum from the planet looks similar to that seen in young stars actively accreting material, marking the first direct evidence of mass falling onto a protoplanet.
• This discovery provides strong support that AB Aurigae b is one of the youngest protoplanets ever observed, still embedded within its birth disk.

Abstract：

Small rocky planets like Earth, which can harbor life, and giant gas planets like Jupiter are born around stars like the Sun. Their birthplace is a thin, disk-shaped structure of gas and dust known as a protoplanetary disk. Protoplanetary disks are observed not only around Sun-like stars but also around more massive or lighter young stars. Since the 2010s, their detailed structures have been revealed by 8-meter class telescopes such as the Subaru Telescope (in visible and infrared light) and the ALMA Observatory (in radio wavelengths).

Although many planets have been inferred indirectly from fine structures in these disks—such as gaps or spiral arms—directly capturing newly formed planets (protoplanets) within the disks has so far been achieved only in a few cases, including PDS 70 b and c and AB Aurigae b (AB Aur b). This is thought to be because most protoplanets are embedded within the disk, and become more visible only when they carve gaps in the disk or are observed from directly above. Protoplanets are also considered to be actively gathering material from the surrounding disk as they grow. However, detailed spectroscopic observations of this mass accretion from an embedded disk have, until now, been limited to the PDS 70 system.

In the present study, an international team of researchers led by the Astrobiology Center (Japan) and the University of Texas at San Antonio (USA) succeeded in detecting hydrogen emission lines from AB Aur b using the multi-object spectrograph MUSE mounted on the VLT. These emission lines are interpreted as evidence of mass accretion from the circumplanetary disk onto the protoplanet.

Background:

Since the first discovery of planets beyond the Solar System in 1995, more than 6,000 exoplanets have been identified. Many of these planets have properties that differ significantly from the eight planets in our Solar System. How are such diverse exoplanets formed and evolved, and which of them could potentially become Earth-like planets capable of supporting life? To address these questions, it is essential to observe young planets in the very act of forming in their birthplaces. However, due to observational challenges, direct observations of planets only a few million years old have been extremely limited.

Small, rocky planets like Earth, which can harbor life, and giant gas planets like Jupiter are born around stars similar to the Sun. Their birthplace is a thin, disk-shaped structure of gas and dust known as a protoplanetary disk. Protoplanetary disks are found not only around Sun-like stars but also around both more massive and less massive young stars. Since the 2010s, their detailed structures have been revealed through observations with 8-meter class telescopes such as the Subaru Telescope and with the ALMA Observatory.
While numerous planets have been inferred indirectly from fine structures in these disks—such as gaps or spiral arms—directly capturing newly formed young planets (protoplanets) within the disks has so far been achieved only in a few cases, including the ~4-million-year-old PDS 70 b and c and the ~2-million-year-old AB Aurigae b (AB Aur b). The latter was discovered with the Subaru Telescope in 2022 (Note 1). This limited success is thought to result from the fact that most protoplanets are embedded within the disk, becoming more visible only when they carve gaps in the disk or are observed from directly above.

Protoplanets are also considered to be actively gathering material from the surrounding protoplanetary disk as they grow. However, there have been no detailed spectroscopic observations of mass accretion from the embedded disk onto a protoplanet to date.

Findings：

An international team of researchers led by the Astrobiology Center (ABC), the University of Tokyo, National Astronomical Observatory of Japan (NAOJ), Kogakuin University, the University of Texas at San Antonio, and Peking University has successfully detected hydrogen emission (Hα line) from the protoplanet AB Aurigae b (AB Aur b) using the MUSE spectrograph on the VLT. This emission is interpreted as material falling onto the circumplanetary disk around the protoplanet (Figure 1).

Hydrogen emission is commonly observed around young stars and their protoplanetary disks. In the present case, the emission comes from material accreting onto the small disk surrounding the still-embedded protoplanet. Using MUSE, which allows high-resolution spectroscopic imaging of extended structures, the team was able to separate emission from the protoplanet and the protoplanetary disk. The high spatial (0.3 arcseconds) and spectral resolution (λ/Δλ ~ 3000) of MUSE under excellent Chilean seeing conditions made this possible.

The detected Hα emission at the position of AB Aur b shows an inverse P Cygni profile (Note 2), similar to that seen in young stars (T Tauri stars; Note 3, see figure 2) undergoing mass accretion. To date, AB Aur b is the only protoplanet with this type of emission. Its young age (~2 million years) and the large amount of surrounding material strongly support that AB Aur b is a protoplanet still in formation. Previously, only PDS 70 b and c showed Hα emission, but those planets are located in disk gaps; AB Aur b is still embedded in the disk, making this the first such observation with the infalling signature.

AB Aur b is about four times the mass of Jupiter (Note 4) and orbits at 93 AU from its star. Such a distant giant planet does not exist in the Solar System. Standard planet formation models cannot fully explain its formation so far from the star, before migration occurs. This discovery supports a scenario where massive planets can form via gravitational instability within the disk, providing insight into a type of giant planet not seen in our Solar System.

The results were published on September 2, 2025, in The Astrophysical Journal Letters (Currie et al., “Images of Embedded Jovian Planet Formation at a Wide Separation around AB Aurigae”).

Notes:

Note 1: For the discovery of the protoplanet around AB Aurigae using the Subaru Telescope, and for detailed observations of its surrounding protoplanetary disk with complex structures, please refer to the press releases from the Astrobiology Center and the Subaru telescope, NAOJ, on April 5, 2022, and from the Subaru telescope, NAOJ, on February 17, 2011.

Note 2: Inverse P Cygni profile: The term "P Cygni profile" refers to a characteristic spectrum seen in the star P Cygni, in which an emission line and an adjacent absorption line appear next to each other, indicating large amounts of gas being ejected from the stellar surface. An inverse P Cygni profile is the opposite pattern, where the order of the emission and absorption lines is reversed. This profile is also observed in T Tauri stars (see Note 3) undergoing gas accretion onto their surfaces.

Note 3: T Tauri stars: Young stars that have formed within a gas cloud and have cleared enough of the surrounding gas to be observed in visible light. Some are still accreting material from the surrounding gas. The name comes from the prototype T Tauri star in Taurus, which was first reported as a new variable star in 1945.

Note 4: Considering observational uncertainties, the mass of AB Aur b is estimated to be roughly 4–9 times that of Jupiter.

Paper Info.：

Journal：The Astrophysical Journal Letters
Title：“VLT/MUSE Detection of the AB Aurigae b Protoplanet with Hα Spectroscopy”
Author ：T. Currie et al.
DOI: 10.3847/2041-8213/adf7a0

The post A Glimpse of a Planet in Formation: AB Aurigae b Detected in H-alpha Light first appeared on Astrobiology Center, NINS.

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

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

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

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

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

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

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

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

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

PUBLICATION

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

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

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

abstract

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

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

introduction

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

results

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

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

acknowledgments

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

Publication

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

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

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

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

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

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

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

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

The post Discovery of an Exo-Venus: a Key to Find Extraterrestrial Life Earth Twin or Evil Twin 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.

“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

The post Subaru Images, Weighs, and Tracks Massive Benchmark Exoplanet first appeared on Astrobiology Center, NINS.

Summary

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

Research Background

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

Research Findings

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

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

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

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

Annotation:

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

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

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

Publication

Journal：The Astronomical Journal

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

DOI：10.3847/1538-3881/acb7e1

Authors：Teruyuki Hirano, et al.

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