LP 791-18 d

LP 791-18 d is an exoplanet orbiting the star LP 791-18, located 86.4 light-years (parsecs) from the Solar System, and was announced in 2023. The star LP 791-18 has an apparent magnitude of 16.9 and an absolute magnitude of 14.8. This star has 0.1 times the mass of the Sun, 0.2 times the Sun’s radius, a surface temperature of 2,960 K, and a spectral type of M6V.

A planet roughly the same size as Earth. And there’s even a possibility of volcanic activity and an atmosphere!?

The Earth-sized exoplanet LP 791-18 d is a planet orbiting the red dwarf star LP 791-18, located approximately 90 light-years from the Solar System in the direction of the constellation Copernicus. Planets b and c have previously been discovered in this star system. The newly discovered planet d is located in an orbit between planets b and c, orbiting the star with an orbital period of 2.75 days. Its radius is estimated to be approximately 1.03 times that of Earth, making it very similar in size. Additionally, Planet d’s mass is comparable to that of Earth. Planet b has a radius about 1.2 times that of Earth and an orbital period of approximately 0.94 days, while Planet c has a radius about 2.5 times that of Earth, a mass about nine times that of Earth, and an orbital period of approximately 4.99 days.

Planet d is located near the inner boundary of the habitable zone and is attracting attention as a planet of interest for research into the origins of life, as it has the potential to retain an atmosphere. The planet’s orbit is slightly elliptical due to gravitational pull from the large, massive Planet c, which orbits in the adjacent outer orbit. As it orbits along this elliptical path, Planet D is subjected to tidal forces from the star, causing it to deform slightly. This deformation may generate internal friction within the planet, heating it and triggering active volcanic activity on its surface—a mechanism similar to that which heats Io, Jupiter’s moon, which exhibits the most active volcanic activity in the solar system. Future observations of the planet’s atmosphere may yield important discoveries regarding how crustal activity affects the planetary atmosphere.

Like Earth’s Moon, Planet d has a synchronous rotation period due to tidal forces, meaning it always keeps the same face toward the star LP 791-18. Consequently, the day side is extremely hot, reaching 300–400 K, and it is highly likely that water has evaporated there. On the other hand, since the night side is thought to be sufficiently cool, if volcanic activity is occurring, Planet D may have an atmosphere, and water vapor could condense in the atmosphere on the night side, potentially resulting in the presence of liquid water.

Furthermore, Planet D’s active volcanic activity may play a role in releasing substances into the atmosphere that would otherwise be trapped within the planet’s crust. These substances include carbon, which is essential for life. If we can successfully detect the composition of this planet’s atmosphere, it will be possible to conduct a detailed investigation into the effects of the planet’s crustal activity on its atmosphere. This could lead to research on the origins of life and is significant from the perspective of astrobiology.

The findings of this study were published in the British scientific journal *Nature* on May 17, 2023 (BST). An international research team, including Professor Noriyasu Narita (Visiting Professor at the Center for Astrobiology, National Institutes of Natural Sciences) from the Graduate School of Arts and Sciences at the University of Tokyo, Specially Appointed Assistant Professor Akihiko Fukui, and Specially Appointed Researcher Mayuko Mori, utilized NASA’s Transiting Exoplanet Survey Satellite (TESS), NASA’s Spitzer Space Telescope, and numerous ground-based telescopes—including the multi-color simultaneous imaging cameras MuSCAT and MuSCAT2, developed by researchers at the University of Tokyo and the Center for Astrobiology.

〈References〉Article Information

・東京大学 大学院総合文化研究科・教養学部：火山活動の可能性がある地球サイズの惑星を発見 ー 潮汐力により加熱された系外惑星 LP 791-18d

・Spitzer Space Telescope：NASA’s Spitzer, TESS Find Potentially Volcano-Covered Earth-Size World

・NASA JPL

・IAC：Astronomers find Earth-sized world potentially covered in volcanoes

・Nature：A temperate Earth-sized planet with tidal heating transiting an M6 star 論文: 2023年5月17日

（文責：日置）

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HD 104985 b

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HD 104985 b is an exoplanet orbiting the star HD 104985, located 316.7 light-years (parsecs) from the Solar System, and was announced in 2003. The star HD 104985 has an apparent magnitude of 5.8 and an absolute magnitude of 0.9. This star has 1.6 times the mass of the Sun, a radius 10.9 times that of the Sun, a surface temperature of 4,786 K, and a spectral type of G9 III. In this star’s planetary system, HD 104985 b orbits the star with an orbital period of 199.5 days and a semi-major axis of 0.95 astronomical units (142,117,977.2 km).

[HD 104985 b Overview]

HD 104985 is a 6th-magnitude star (apparent magnitude) in the constellation Camelopardalis, located approximately 317 light-years from Earth. HD 104985 b is a planet with an orbit slightly less than one astronomical unit from this star. HD 104985 b has a radius nearly identical to Jupiter’s and a mass 8.3 times that of Jupiter. To draw an analogy with our solar system, HD 104985 b is a Jupiter-sized planet located between the orbits of Venus and Earth. Although it orbits at a distance roughly equivalent to Earth’s, the central star is extremely massive, so the environment is believed to be scorching hot.

[The First Exoplanet Detected in Japan: Demonstrating Japan’s Unique Contribution to the World]

HD 104985 b was detected using the radial velocity method with the 188-cm reflecting telescope owned by the Okayama Astrophysical Observatory and was announced in 2003 by Fumie Sato (then affiliated with the National Astronomical Observatory of Japan; now at Tokyo Institute of Technology) and his colleagues. This was the first detection of an exoplanet in Japan and attracted significant attention both domestically and internationally. Since the world’s first observation of an exoplanet in 1995, a fierce “planet-hunting race” had been underway worldwide, and Japan had now joined the fray.

While previous observations had targeted stars similar to the Sun, Sato and his team began searching for planets around giant stars—stars that have evolved and expanded significantly in size. In fact, HD 104985 has a radius 10.6 times that of the Sun and was one of the giant stars Sato and his team had identified as a candidate for observation. After two years of persistent observation, they successfully proved that exoplanets exist even around giant stars, demonstrating Japan’s unique contribution to exoplanet research.

[The Okayama Astrophysical Observatory’s 188-cm Reflecting Telescope: A Telescope That Supported Astronomical Observations in Japan for Half a Century]

The Okayama Astrophysical Observatory, one of the projects of the National Astronomical Observatory of Japan, began operations in Asakuchi City, Okayama Prefecture, in 1962. Until 2018, when operations as a project came to an end, it served as an outstanding optical and infrared astronomical observatory for approximately 56 years, utilized by many researchers. The observatory’s largest telescope is the 188-cm reflector, which has contributed to numerous significant discoveries. Its contributions to the discovery of exoplanets have been particularly notable; including joint observations with other telescopes, it has contributed to the discovery of 58 new exoplanets to date.

(For details on the 188cm telescope’s overview and achievements, please refer to this page.)

188-cm reflector telescope（https://www.nao.ac.jp/research/telescope/188cm.html）

After the project concluded, the telescopes at the Okayama Astrophysical Observatory were transferred to researchers from the participating universities to serve as their dedicated instruments. Currently, the 188-cm reflecting telescope is out of service due to a dome malfunction. Let’s look forward to the day when restoration work is complete and we can once again see the 188-cm reflecting telescope back in action.

[Kyoto University Okayama Astronomical Observatory’s Seimei Telescope: One of the Largest Telescopes in East Asia]

Following in the footsteps of the 188-cm telescope, a new telescope began operations in Okayama in 2019. This is the Seimei Telescope, owned by Kyoto University. With a primary mirror consisting of an 18-element composite mirror with a diameter of 3.8 meters, it is the largest telescope in East Asia. (There are differing opinions as to whether it is the “largest” or “one of the largest.” Please refer here for details.)

Seimei Telescope (Photo by the author)

The name “Seimei Telescope” is derived from Abe no Seimei, a Onmyōji (master of divination) from the Heian period. Abe no Seimei, who conducted astronomical observations throughout the country, is said to have established a residence for astronomical observation near the summit of Mt. Abe, located northwest of the current Okayama Astronomical Observatory. The telescope was named “Seimei Telescope” in honor of Abe no Seimei, a pioneer in astronomical research with ties to Okayama.

The Seimei Telescope is also used for the search for and observation of exoplanets, and a new high-dispersion spectrograph called GAOES-RV will begin operations in the second half of fiscal year 2023. A high-dispersion spectrograph is a device that separates the light collected by the telescope into its component wavelengths for detection; it is indispensable for observing exoplanets using the radial velocity method. With improved performance compared to previous high-dispersion spectrographs, GAOES-RV is expected to enable the observation of exoplanets around fainter stars.

(For more information on GAOES-RV, please click here.)

From Japan’s first detection of an exoplanet to the latest observational equipment, we’ve traced the history of exoplanet exploration. While the allure of exoplanets is endless, the telescopes and instruments used to discover them hold a unique charm of their own. If this has piqued your interest even a little, why not turn your attention not only to the star-studded night sky but also to the “big eyes” set up on the ground?

（文責：渡邊新）

Click here for detailed information on HD 104985 b

http://www.exoplanetkyoto.org/exohtml/HD_104985_bJP.html