An international team has successfully discovered a new extrasolar planet named Gliese 12b through a collaboration between a strategic survey program (SSP) of the Subaru Telescope and NASA’s TESS campaign. Gliese 12b 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 12b 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. In the near future, James Webb Space Telescope (JWST) and extremely large telescopes, such as TMT, will be used to characterize the atmosphere of Gliese 12b in detail.
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.
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 via the transit technique 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. The analysis of the data taken with TESS and the MuSCAT series determined the orbital period of Gliese 12b to be 12.8 days and the radius to be 0.96 Earth radii. Furthermore, the astronomers constrained the mass of Gliese 12b to be less than 3.9 Earth masses by combining the Doppler velocity measurements taken with IRD and those with CARMENES on the CAHA 3.5 m telescope.
“We’ve found the nearest, transiting, temperate, Earth-size world located to date,” said Masayuki Kuzuhara, a project assistant professor at the Astrobiology Center in Tokyo, who co-led the research team with Akihiko Fukui, a project assistant professor at the University of Tokyo. “Although we don’t yet know whether it possesses an atmosphere, we’ve been thinking of it as an exo-Venus, with similar size and energy received from its star as our planetary neighbor in the solar system.”
What kind of planet is Gliese 12b? 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 –– the amount of light a planet receives from the host star –– Gliese 12b 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.
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-1c, is very similar to Gliese 12b and Venus in size (1.1 Earth radii) and insolation (2.2 times Earth’s insolation). However, recent observations by the JWST revealed that the atmosphere of TRAPPIST-1c 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 dissipated due to the interaction of these high-energy particles with the atmosphere 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 12b and its host star is more than 4 times larger than that between TRAPPIST-1c and its host. Accordingly, the effect of high-energy particles on Gliese 12b is much weaker than that on TRAPPIST-1c, making it possible that Gliese 12b might retain a certain amount of atmosphere compared with TRAPPIST-1c.
Given that Gliese 12 is a neighbor of the Sun, Gliese 12b 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 12b and comparing it with those of Venus and TRAPPIST-1c, scientists will be able to reveal how the atmospheres of terrestrial planets vary depending on the radiation environments around the host stars.
“We plan to use future observations to determine its formation origin. Follow-up observations with SJTU’s JUST telescope are also expected,” said Prof. Masahiro Ogihara at Tsung-Dao Lee Institute, Shanghai Jiao Tong University, who is one of the core members of IRD-SSP project.
This research was published May 23 in The Astrophysical Journal Letters.
Contact:
Masahiro Ogihara
Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China
Email: ogihara@sjtu.edu.cn
Gliese 12 b, which orbits a cool, red dwarf star located just 40 light-years away, promises to tell astronomers more about how planets close to their stars retain or lose their atmospheres. In this artist’s concept, Gliese 12 b is shown retaining a thin atmosphere.
Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
Gliese 12 b’s estimated size may be as large as Earth or slightly smaller — comparable to Venus in our solar system. This artist’s concept compares Earth with different possible Gliese 12 b interpretations, from no atmosphere to a thick Venus-like one. Follow-up observations with James Webb Space Telescope will help determine just how much atmosphere the planet retains as well as its composition.
Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
Paper link: https://iopscience.iop.org/article/10.3847/2041-8213/ad3642