An international team led by scientists from the Astrobiology Center in Japan, the University of Tokyo, the National Astronomical Observatory of Japan, and Tokyo Institute of Technology has successfully discovered a new extrasolar planet named Gliese 12 b through a collaboration between NASA’s TESS campaign and a strategic survey program (SSP) of the Subaru Telescope. Gliese 12 b has a size similar to Earth and Venus, and is orbiting around its host star, Gliese 12, with a period of 12.8 days. Despite its close proximity to its host star, the amount of radiation Gliese 12 b receives is comparable to that of Venus, because the host star is much cooler than the Sun. The planet may still retain a certain amount of atmosphere, making it one of the most suitable targets out of all of the planets discovered so far to investigate the atmosphere of a planet like Venus. It remains an open question why the surface environment of Venus – a sibling of Earth – became so harsh for life compared to that of Earth. In the near future, NASA’s JWST and extremely large telescopes, such as TMT, will be used to characterize the atmosphere of Gliese 12 b in detail, greatly improving our understanding of the conditions necessary for habitability.
Figure 1: Artist’s conception of the newly discovered planet Gliese 12 b, which is orbiting a red dwarf star located 40 light-years away. This artist’s conception assumes that the planet retains a tenuous atmosphere. Future follow-up observations will clarify what kind of atmosphere the planet actually retains. (Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)) [Youtube]
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