– January 7, 2026 (London time) – One of the biggest recent surprises in astronomy is the discovery that most stars like the Sun harbor a planet between the size of Earth and Neptune within the orbit of Mercury — sizes and orbits absent from our solar system. These ‘super-Earths and sub-Neptunes’ are the galaxy’s most common planets, but their formation has been shrouded in mystery. Now, an international team of astronomers has found a crucial missing link. By weighing four newborn planets in the V1298 Tau system, they’ve captured a rare snapshot of worlds in the process of transforming into the galaxy’s most common planetary types.

“What’s so exciting is that we’re seeing a preview of what will become a very normal planetary system,” says John Livingston, the study’s lead author from the Astrobiology Center in Tokyo, Japan. “The four planets we studied will likely contract into ‘super-Earths’ and ‘sub-Neptunes’—the most common types of planets in our galaxy, but we’ve never had such a clear picture of them in their formative years.”

The study focused on V1298 Tau, a star only about 20 million years old—a blink of an eye in cosmic time compared to our 4.5-billion-year-old Sun. Orbiting this young, active star are four giant planets, all between the sizes of Neptune and Jupiter, caught in a fleeting and turbulent phase of rapid evolution. This system appears to be a direct ancestor of the compact, multi-planet systems found throughout the galaxy. Like the Rosetta Stone that helped scholars decipher Egyptian hieroglyphics, V1298 Tau helps us decode how the galaxy’s most common planets came to be.

For a decade, the team used an arsenal of ground- and space-based telescopes to precisely measure when each planet passed in front of the star, an event known as a transit. By timing these transits, astronomers detected that the planets’ orbits were not perfectly regular. Their orbital configuration and gravity cause them to tug on each other, slightly speeding up or slowing down their celestial dance. These tiny shifts in timing, called Transit-Timing Variations (TTVs), allowed the team to robustly measure the planets’ masses for the first time.

“For astronomers, our go-to ‘Doppler’ method for weighing planets involves making careful measurements of the star’s velocity as it’s tugged by its retinue of planets.” said Erik Petigura, a co-author from UCLA. “But young stars are so extremely spotty, active, and temperamental, that the Doppler method is a non-starter.” By using TTVs, we essentially used the planets’ own gravity against each other. Precisely timing how they tug on their neighbors allowed us to calculate their masses, and sidestep the issues with this young star.”

The results were remarkable. The planets, despite being 5 to 10 times the radius of Earth, were found to have masses of only 5 to 15 times that of our own world. This makes them incredibly low-density—more like planetary-sized cotton candy than rocky worlds.

“The unusually large radii of young planets led to the hypothesis that they have very low densities, but this had never been measured,” said Trevor David, a co-author from the Flatiron Institute who led the initial discovery of the system in 2019. “By weighing these planets for the first time, we have provided the first observational proof. They are indeed exceptionally ‘puffy,’ which gives us a crucial, long-awaited benchmark for theories of planet evolution.”

This puffiness helps solve a long-standing puzzle in planet formation. A planet that simply forms and cools down over time would be much more compact. The team’s analysis reveals that these planets must have undergone a dramatic transformation early in their lives, rapidly shedding much of their initial atmospheres and cooling dramatically when the gas-rich disk around their young star disappeared.

“These planets have already undergone a dramatic transformation, rapidly losing much of their original atmospheres and cooling faster than what we’d expect from standard models,” explains James Owen, a co-author from Imperial College London who led the theoretical modeling. “But they’re still evolving. Over the next few billion years, they will continue to lose their atmosphere and shrink significantly, transforming into the compact worlds we see throughout the galaxy.”

“I’m reminded of the famous ‘Lucy’ fossil, one of our hominid ancestors that lived 3 million years ago and was one of the key ‘missing links’ between apes and humans,” added Petigura. “V1298Tau is a critical link between the star/planet forming nebulae we see all over the sky, and the mature planetary systems that we have now discovered by the thousands.”

The V1298 Tau system now serves as a crucial laboratory for understanding the origins of the most abundant planets in the Milky Way, giving scientists an unprecedented glimpse into the turbulent and transformative lives of young worlds. Understanding systems like V1298 Tau may also help explain why our own solar system lacks the super-Earths and sub-Neptunes that are so abundant elsewhere in the galaxy.

“This discovery fundamentally changes how we think about planetary systems,” adds Livingston. “V1298 Tau shows us that today’s super-Earths and sub-Neptunes start out as giant, puffy worlds that contract over time. We’re essentially watching the universe’s most successful planetary architecture in the making.”

PUBLICATION

Journal: Nature
”A young progenitor for the most common planetary systems in the Galaxy”
Authors: John H. Livingston, et al. 
DOI: 10.1038/s41586-025-09840-z
URL: https://www.nature.com/articles/s41586-025-09840-z

Related Links

• NAOJ press release Cotton Candy Worlds Evolve into Rock Candy Worlds

The post Astronomers Find Missing Link to Galaxy’s Most Common Planets first appeared on Astrobiology Center, NINS.

The discovery of two remarkable substellar companions orbiting distant stars has been announced by an international team of astronomersusing the Subaru Telescope’s sharp adaptive-optics imaging together with precision stellar measurements from space-based astrometry. This discovery is the first results from OASIS (Observing Accelerators with SCExAO Imaging Survey) program, which aims to find and characterize massive planets and brown dwarfs.

Only about 1% of stars host massive planets and brown dwarfs that can be photographed directly with current telescopes. Even in young planetary systems where these objects are still glowing hot with the energy of having just been formed, making them brighter and easier to detect, they are still much fainter than their host stars and are easily lost in the stellar glare. The key question for astronomers has been: where to look for these objects?

That is where OASIS comes in. The program uses measurements from two European Space Agency missions—Hipparcos and Gaia—to identify stars being tugged by the gravity of unseen companions. OASIS then targets these promising candidates with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system, which provides the exceptional precision and advanced technology needed to actually photograph these hidden companions.

“With OASIS, we are able to find, weigh, and track the orbits of massive planets and brown dwarfs around stars we never thought of looking at before,” says OASIS Principal Investigator (PI), Thayne Currie at The University of Texas at San Antonio (UTSA).

The first discovery, HIP 54515 b, is a gas giant planet with a mass just under 18 times that of Jupiter, orbiting a star twice the mass of the Sun.  HIP 54515 b orbits its star at approximately 25 astronomical units (au). This is roughly the same distance as between Neptune and theSun.  Because the planetary system is so distant—about 275 light-years from Earth—HIP 54515 b appears extremely close to its star in the sky, pushing the limits of current direct imaging technology.

“HIP 54515 b was imaged about 0.15 arc-seconds from its star. That’s roughly how small a baseball would appear from 100 km away, so we needed extremely sharp images enabled by Maunakea and SCExAO’s advanced technology,” says Currie.

HIP 54515 b adds to a growing trend of superjovian planets whose orbits are slightly less circular than those of lower-mass Jupiter-like planets. This may suggest that these planets have slightly different formation histories than the gas giants in our Solar System.

The second discovery, HIP 71618 B, also orbits a two-solar-mass star but is a brown dwarf, a type of object that forms like a star but lackssufficient mass to be a true star. HIP 71618 B is about 60 times as massive as Jupiter, orbits its star at an average distance slightly larger than Saturn’s orbit around the Sun, and follows a highly elongated, elliptical orbit. In addition to astrometry and SCExAO imaging, W. M. Keck Observatory imaging data were crucial to its discovery.  

While not a planet itself, HIP 71618 B may play an important role in future searches for Earth-like planets around other stars. This is because it meets the requirements for the Roman Space Telescope’s Coronagraph Instrument technology demonstration, planned for 2027.This experiment will be the first to test advanced planet imaging technologies in a space telescope to suppress the glare of Sun-like stars to see rocky, Earth-like planets ten billion times fainter. The Roman Coronagraph Technology Demonstration has strict requirements for its target stars, and until the discovery of HIP 71618 B, no system was known in the peer-reviewed literature to meet these criteria.

The discoveries of HIP 54515 b and HIP 71618 B showcase how combining space-based precision star-tracking and ground-based direct imaging can reveal planets and brown dwarfs that would otherwise remain hidden. The OASIS program continues to survey dozens of additional candidate systems, with more discoveries expected in the coming years which will deepen our understanding of how planets and brown dwarfs form and how their atmospheres evolve. The discoveries will also contribute to the development of technologies needed to detect habitable, Earth-like worlds in the future.  

“Thanks to innovative instruments like SCExAO and Maunakea’s world-leading astronomical observing conditions, Subaru Telescope will continue to be a preeminent observatory even as other telescopes come online, making breakthrough discoveries far into the future,” says Dr. Masayuki Kuzuhara (Astrobiology Center), who co-leads OASIS with Currie.

These results appeared in Currie & Li et al. “SCExAO/CHARIS and Gaia Direct Imaging and Astrometric Discovery of a Superjovian Planet 3–4 λ/D from the Accelerating Star HIP 54515” in the Astronomical Journal on December 3, 2025 and El Morsy et al. “OASIS Survey Direct Imaging and Astrometric Discovery of HIP 71618 B: A Substellar Companion Suitable for the Roman Coronagraph Technology Demonstration” in the Astrophysical Journal Letters on December 3, 2025.

OASIS is an international collaboration involving astronomers from institutions across the United States, Japan, Canada, Chile, and Europe, and is supported by the National Science Foundation for its scientific merit and NASA as Key Strategic Mission Support for the Roman Space Telescope. This research was supported by JSPS KAKENHI (Grant Numbers: 24K07108).

About the Subaru Telescope
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 Press Release
• Subaru Telescope Press Release

The post First Results from the Subaru Telescope’s OASIS Survey: Direct Imaging of New Worlds Around Unexplored Stars first appeared on Astrobiology Center, NINS.

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.

Key Highlights

• Field studies in Lake Akan revealed that marimo experience severe photoinhibition immediately after ice melt, caused by intense sunlight and low water temperatures.
• Despite this damage, marimo demonstrated strong resilience, recovering photosynthetic activity within 20–30 days through natural repair mechanisms.
• Climate change may extend these risky post-thaw periods, potentially threatening marimo survival due to prolonged light-induced stress.

Brief summary：

The marimo (Aegagropila brownii), a nationally designated Special Natural Monument of Japan, inhabits Lake Akan in Hokkaido, where environmental conditions fluctuate drastically with the seasons. Of particular concern is the period immediately after ice melt in early spring, when low water temperatures coincide with strong sunlight, posing a risk of severe damage to photosynthetic activity.

In this study, a research team led by the Astrobiology Center conducted a detailed assessment of marimo photosynthetic performance during this critical transition period, combining field observations with laboratory experiments. The results revealed that while marimo maintains healthy photosynthetic capacity in both summer and ice-covered winter conditions, their activity significantly declines just after ice melts. However, it was also found that marimo can recover this function over the following 20 to 30 days.

These findings provide valuable insight into the seasonal vulnerability of marimo and highlight the importance of spring as a critical period for conservation. The study was published in the international journal Phycological Research on September 29, 2025.

Main text：

【Background of the Study】

Marimo (Aegagropila brownii) (Note 1 & 2) is a freshwater green alga known for forming beautiful spherical aggregations. In particular, the large marimo found in Lake Akan, Hokkaido, are designated as a Special Natural Monument of Japan and represent a globally unique natural heritage.

During winter, Lake Akan is covered by thick ice and snow, which act like a “sunshade,” protecting marimo from harsh solar radiation. However, during seasonal transitions, which are just before ice formation and immediately after ice melt, the lake bottom experiences a harsh environmental condition characterized by low water temperatures (1–4°C) combined with strong sunlight. This creates what is known as a low water temperature and high light (LT-HL) environment (Note 3), which can severely stress photosynthetic organisms. Under such conditions, marimo may suffer “photoinhibition (Note 4),” a physiological impairment where photosystem II (Note 5), the protein complex responsible for photosynthesis, is damaged due to excess light energy.While the risk of photoinhibition under LT-HL conditions has been theoretically suggested, it remained unclear how marimo are actually affected in natural lake environments during these transitional periods. This study aimed to fill that knowledge gap through detailed field observations and laboratory experiments.

【Research Findings】

The research team conducted seasonal monitoring of water temperature and light conditions in Churui Bay of Lake Akan, along with seasonal sampling of marimo. Their photosynthetic performance was evaluated using a method called PAM chlorophyll fluorescence measurement (Note 6).

The results showed that marimo collected in summer (August) and in midwinter when the lake was fully ice-covered (March) maintained high photosynthetic activity, with F_v/F_m values—an indicator of photosystem II performance—around 0.6, indicating healthy physiological states.

In contrast, marimo collected immediately after ice melted (early April) exhibited significantly reduced F_v/F_m values, dropping to approximately 0.27 on the sun-exposed surface, clearly demonstrating severe photoinhibition. This suggests that marimo, which had acclimated to the dark conditions under ice, experienced substantial physiological stress due to sudden exposure to intense sunlight after thawing.

Nevertheless, marimo also demonstrated a remarkable capacity for recovery. By early May, 20–30 days after ice melt, F_v/F_m values had rebounded to around 0.55 as water temperatures began to rise. This recovery process was also verified in laboratory experiments, which showed that damaged marimo cells placed under low light began to recover their photosynthetic capacity.

【Future Perspectives】

This study, for the first time, reveals based on field data that the most vulnerable period for Marimo throughout the year is immediately after the ice thaws. Environmental changes during this period could have a significant impact on the long-term survival of Marimo.

In recent years, due to the effects of climate change, a trend of delayed ice formation and earlier thawing has been reported in Lake Akan. This does not simply mean a longer warm period. Solar radiation is extremely strong on clear days from early spring (January to March), and under normal circumstances, a thick layer of ice and snow protects the Marimo. However, if the ice thaws earlier, the Marimo are exposed to this intense light for a longer period while water temperatures remain low. This raises concerns that the duration of exposure to the harsh LT-HL environment will be extended, making it easier for light-induced damage to accumulate. If the recovery from photoinhibition cannot keep pace, the entire Marimo population could weaken, potentially jeopardizing its future survival. Furthermore, the ice-covered period itself is a crucial element that supports the entire lake ecosystem, and its shortening or loss is expected to have a profound impact on the interactions among diverse organisms.

To pass this precious natural heritage on to future generations, it is crucial not only to protect the physical habitat but also to establish science-based conservation strategies that focus on the physiological impacts of climate change on Marimo, especially the light stress during this vulnerable period. The findings of this research extend beyond the specific biological community of Marimo in Lake Akan; they also hold significance as a model case for the universal challenge of how to conserve rare aquatic species facing complex stressors like climate change in other lake ecosystems, both in Japan and internationally.

This research also offers important insights from an astrobiological perspective. Elucidating the life strategies of Marimo, which endure and recover from sudden intense light stress upon ice thawing after dwelling in the darkness beneath the ice, provides critical clues for understanding the mechanisms by which life might survive in extreme extraterrestrial environments, such as icy celestial bodies. Furthermore, investigating the impact of Earth’s climate change on this unique ecosystem offers a valuable case study for considering how life might respond and leave its traces on planets that have experienced dramatic environmental shifts. This study is an attempt to address the fundamental astrobiological question, “How does life survive and evolve in extreme planetary environments?” by examining life forms here on Earth.

Glossary：

(Note 1) The Special Natural Monument “Marimo of Lake Akan”
Lake Akan is a dammed lake formed following the creation of the Akan Caldera and the ancient Lake Akan and was subsequently reshaped by volcanic activity from Mount Oakan. In the northern part of the lake, two bays, Churui Bay and Kinentanpe Bay, are home to extensive colonies of spherical marimo (Aegagropila brownii), an exceptionally rare phenomenon even on a global scale. Notably, Lake Akan is the only known lake in the world where giant spherical marimo exceeding 30 cm in diameter have been regularly observed.
Due to their rarity and high scientific value, the marimo of Lake Akan were designated a Natural Monument of Japan in 1921 and elevated to the status of Special Natural Monument in 1952. As of 2024, eight algal species have been designated as Natural Monuments in Japan, but Aegagropila brownii from Lake Akan remains the only algal species to hold the status of a Special Natural Monument.

(Note 2) Scientific Name of Marimo (Aegagropila brownii)
In 2023, a taxonomic study redefined the correct scientific name of marimo as Aegagropila brownii, replacing the widely used but now invalid name Aegagropila linnaei. This change does not reflect any alteration in the biology, ecology, or genetics of the organism itself, but rather a correction in nomenclature based on international rules of taxonomic classification.
The revision stemmed from a re-evaluation of the “type specimen,” which determines the official identity of a species. The name A. linnaei was based on the species Conferva aegagropila, originally described by Carl Linnaeus in 1753. However, the lectotype later selected to represent this species was found to be a marine alga, not the freshwater marimo. According to the International Code of Nomenclature for algae, fungi, and plants, this means that the name A. linnaei applies to the marine species and cannot be used for freshwater marimo.

Consequently, researchers reexamined historical literature to identify the oldest valid name applicable to the freshwater marimo. They concluded that Conferva brownii, based on a specimen collected in 1809 from a cave pool in Northern Ireland, fulfilled the criteria. As a result, the correct scientific name for marimo has been changed to Aegagropila brownii (Guiry & Frödén, 2023).
This update is purely taxonomic revision. It does not change the biological identity or conservation significance of marimo, but instead provides a more accurate scientific framework for recognizing and protecting this unique organism.

(Note 3) LT-HL (Low Water Temperature–High Light) Environment
A unique environmental condition characterized by low temperatures and high light intensity, commonly found in polar regions or freshwater lakes immediately after ice melt in spring. For plants and algae, such conditions can impose significant stress on the photosynthetic machinery.

(Note 4) Photoinhibition
Photosynthesis is the process plants and algae use to live, but if the light is too strong, their ability to perform it can decrease. This phenomenon is called “photoinhibition.”
Let’s compare the mechanism of photosynthesis to a “nutrient factory” that runs on sunlight as its energy source. This factory can produce nutrients efficiently with a moderate amount of light. However, if it is flooded with excessive light that far exceeds its processing capacity (as explained in (Note 3) LT-HL Environment), the machinery working on the front lines of energy conversion (as explained in (Note 4) Photosystem II) gets damaged and breaks down. This state, where the “machinery has broken down, and the entire factory’s productivity has slowed”, is photoinhibition.
For the Marimo, which spend the winter quietly under the dark ice, the intense sunlight immediately after the ice thaws is overwhelmingly strong, making it a major cause of this “photoinhibition.”

(Note 5) Photosystem II (PSII)
A protein-pigment complex that drives the initial step of photosynthesis by absorbing light energy to split water molecules and extract electrons. It functions as part of the “engine” of photosynthesis and is particularly vulnerable to damage under excessive light exposure.

(Note 6) PAM (Pulse-Amplitude-Modulation) Chlorophyll Fluorescence Measurement
This is a widely used, non-invasive technique to measure the health and efficiency of photosynthesis in plants, algae, and cyanobacteria. The method works by exposing a sample to controlled pulses of light and measuring the faint red light that is re-emitted from chlorophyll, a phenomenon known as fluorescence. The “Pulse-Amplitude-Modulation” (PAM) technique uses different light intensities (a weak measuring light, a strong saturating pulse, and actinic light to drive photosynthesis) to precisely quantify different aspects of the photosynthetic process.
By analyzing the fluorescence signals, scientists can calculate key parameters, most notably the maximum quantum efficiency of Photosystem II (PSII), expressed as F_v/_Fm. This value serves as a reliable indicator of the health of the photosynthetic apparatus. A high F_v/F_m value indicates a healthy, efficient system, while a low value suggests that PSII has been damaged by environmental stress, a condition known as photoinhibition. In this study, PAM measurements were crucial for quantifying the degree of stress the Marimo experienced and its subsequent recovery.

Funding :

This research was supported by JSPS KAKENHI Grants (Grant Numbers: 24K09493, 23H04961, and 23H02498). This work was also supported by the Sasakawa Scientific Research Grant from The Japan Science Society.

Publication

Journal: Phycological Research
Title: Photoinhibition Risk in Marimo (Aegagropila brownii) During Ice Transition Periods Based on Field Observations and Laboratory Assessments
Authors: Masaru Kono, Akina Obara, Yoshihiro Suzuki, Akitoshi Iwamoto, Keisuke Yoshida, Yoichi Oyama
DOI: 10.1111/pre.70013

The post Beneath the Ice: Spring Sunlight Triggers Photoinhibition and Recovery in Lake Akan Marimo 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

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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

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On the other hand, I feel that the field of “astrobiology,” the scientific study of “life in space,” has not yet penetrated society. Suddenly, “Aliens have been discovered! or “Let’s move to an exoplanet! Unfortunately, it is unlikely that we will be able to do so anytime soon, but astrobiology is a field that is concerned with the question, “Are we special in the universe? Are we special in the universe? As an Inter-University Research Center under the direct control of the National Institutes of Natural Sciences (NINS), ABC also conducts many joint research projects with other centers in Japan and abroad. In this page, we would like to introduce ABC and related astrobiology research in Japan.

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Overview
Dates: Friday, June 20, 2025 to Sunday, July 27, 2025
Venue: Astronomy and Science Information Space https://mitakatkjs.mall.mitaka.ne.jp
Mitaka Chuo Building 1F, 3-28-20 Shimorenjaku, Mitaka-shi, Tokyo
Hours: 11:00 am to 6:30 pm (open on Monday and Closed on Tuesdays, national holidays, and year-end and New Year holidays)
Organized by: National Astrobiology Center, National Institutes of Natural Sciences / National Astronomical Observatory of Japan

Related link:
National Astronomical Observatory of Japan　https://www.nao.ac.jp

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Summary

Recent advances in astronomical observations have found a significant number of extrasolar planets that can sustain surface water, and the search for extraterrestrial life on such planets is gaining momentum. A team of astrobiologists from Astrobiology Center, National Institute for Basic Biology, and SOKENDAI have proposed a novel approach for detecting life on ocean planets. By conducting laboratory measurements and satellite remote sensing analyses, they have demonstrated that the reflectance spectrum of floating vegetation could serve as a promising biosignature. Seasonal variations in floating vegetation may provide a particularly effective means for remote detection.The results of this research will be published in the journal Astrobiology on February 2, 2025.

Background 

Astronomical surveys have discovered nearly 6,000 exoplanets, including many habitable planets, which may harbor liquid water on their surfaces. The search for life on such planets is one of the most significant scientific endeavors of this century, with direct imaging observation projects currently under development.

On Earth-like planets, the characteristic reflectance spectrum of terrestrial vegetation, known as “vegetation red edge”, is considered as a key biosignature. However, ocean planets, with most of their surfaces covered by water, are unlikely to support terrestrial vegetation. To broaden the scope of life detection on ocean planets, this study examined the characteristics of reflectance spectra from floating plants and tested their detectability.

Results

The study investigated the reflectance spectra of floating plants across different scales, from individual leaves in laboratory settings to large-scale observation via satellite remote sensing of lake vegetation.

Although floating leaves exhibit considerable morphological variation among species, their general trend reveals a pronounced red edge, often comparable to or even exceeding that of terrestrial plants. This enhancement is attributed to air gaps in sponge tissue that provide buoyancy and specialized epidermal structures that offer water repellency. While floating leaves show slightly reduced reflectance when wet, they still display a more distinct red edge than submerged water plants (Figure 1).

However, on a larger scale, the red edge signature of floating vegetation weakens due to lower vegetation density and reduced leaf overlap on the water surface. Landscape-scale analyses using satellite remote sensing (Sentinel-2; ESA) with the Normalized Difference Vegetation Index (NDVI) flourishes in summer and disappears in winter, causing the NDVI to be relatively low when averaged over the year. Nevertheless, the fluctuation between minimum and maximum NDVI values is more pronounced for floating vegetation compared to forests. To further investigate this pattern, a large-scale survey of 148 lakes and marshes across Japan was conducted. The study revealed a characteristic seasonal NDVI variation, shifting from negative values in winter to positive values in summer (Figure 2). Importantly, while water suppresses the reflectance of floating vegetation, its own reflectance is even lower and remains stable. It enhances the detectability of seasonal NDVI fluctuations, which remain robust against atmospheric and cloud interference, suggesting that this method could be promising for detecting life on habitable exoplanets in the future.

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The results of the project solicitation (AB0606) have been released at the University of Tokyo!

Research Overview:

A research group led by Associate Professor Yohei Suzuki of the Graduate School of Science at the University of Tokyo and members of the International Commission on Space and Astronautical Research (COSPAR) Working Group for the Development of Safety Assessment Protocols for Mars Return Sample (SSAP) have improved the safety assessment system released by the Working Group in 2022. The reason for the improvement is that there is a very high possibility of detecting traces of Martian life at the site of contact between rock and water, but the formation of clay interferes with the acquisition of signals of life traces. Therefore, the research group tested various analytical methods using clay-containing areas of basalt on Earth, where the group had found microbial life. As a result, they succeeded in simultaneous detection of clay and microorganisms by infrared irradiation. In the future, the applicability of analytical methods will be evaluated using Earth rocks similar to the Mars return sample, which is expected to dramatically improve the technology for detecting Martian life.

（Quoted from arelease by the University of Tokyo）

Please refer to the University of Tokyo releasefor details.

Publication(
Journal：International Journal of Astrobiology

Title：Submicron-scale detection of microbes and smectite from the interior of a Mars-analogue basalt sample by opticalphotothermal infrared spectroscopy

Authors：Yohey Suzuki*, Frank E. Brenker, Tim Brooks, Mihaela Glamoclija, Heather V. Graham, Thomas L. Kieft, Francis M. McCubbin, Mark A. Sephton and Mark A. van Zuilen
(*Projects adopted by the public)

DOI：10.1017/S1473550425000011

AB022001 薮田ひかる	広島大学	太陽系の起源と進化の体系的理解をめざすマルチスケール小天体科学
AB022002 癸生川陽子	横浜国立大学	宇宙における有機物の形成・進化および生命の移動・居住可能性に関するアストロバイオロジー宇宙実験研究拠点
AB022003 赤沼哲史	早稲田大学	タンパク質の起源に纏わる「鶏と卵のパラドックス」の解決による地球と宇宙での生命誕生場の推定
AB022004 古川善博	東北大学	初期火星における生命関連有機分子の生成に関する研究
AB022005 亀田真吾	立教大学	強紫外線輻射を受ける地球型惑星のハビタビリティ
AB022006 河原創	東京大学	データ科学手法で迫る新世代の太陽系外惑星探査

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