Author: exoplanetkyo

Proxima Centauri d

Proxima Centauri d is an exoplanet orbiting the star Proxima Centauri, located 4.2 light-years from our solar system.
The host star Proxima Centauri has an apparent magnitude of 11.1 and an absolute magnitude of 15.6.
It is a spectral type M5.5V star with about 0.1 times the mass of our sun and 0.1 times the radius, and the surface temperature is thought to reach 3050 degrees.
The exoplanet Proxima Centauri d orbits in about 5.2 days with an orbital radius of 0.03 astronomical units (4315898.6 km).

Proxima Centauri is about 4.2 light years away from the Earth and is considered the closest star to the Sun. The exoplanet that orbits it, Proxima Centauri d has a short orbital period of around 5.17 days. Proxima Centauri d orbits close to its host star. Hence, its blackbody temperature (assuming an albedo of 0.3) is calculated to be about 297 K. This is a temperature at which liquid water can exist. It is due to the low temperature of the host star, which is an M-type star. This means the exoplanet is located within the habitable zone.

The European Southern Observatory officially reported the discovery of Proxima Centauri d in February 2022. Previously, two other planets were confirmed to orbit Proxima Centauri, but although the star system has attracted attention, the discovery of this planet had not yet been reported. The radial velocity method was used to observe the planet (also called Doppler spectroscopy), which obtains information about the planet by observing the slight flicker of the host star during the planet’s orbit, which is affected by the planet’s mass. However, Proxima Centauri d’s mass is only about a quarter of the Earth, so its effect on the star is small. According to the observatory, the radial velocity of Proxima Centauri d was only about 40 cm per second. This observation required extreme precision, and took two years from the first detection of the planet’s existence to the time its discovery was reported.
The discovery of such a small planet is important because it could lead to the discovery of many more minor habitable planets that have not been discovered yet.

HD 40307 g

HD 40307 g was discovered using the radial velocity method, based on data observed by HARPS in 2012. The planet’s host star, HD 40307, is classified as a K type main-sequence star. The K-type star is slightly smaller than our Sun’s G-type star and is actually only 0.77 times the size of the Sun. Unfortunately, the star’s apparent magnitude is only 4.17, so it is not visible from Earth, but it is located 42 light-years away in the Pictor constellation. Six planets have been discovered orbiting HD 40307.

HD 40307 g, is the outermost of the six planets found orbiting the star. The planet orbits about 0.6 the distance between the Sun and Earth in about 197 days.
It is assumed to be about 7.1 times the mass of the Earth, which makes it a rocky planet. The radial velocity method measures the changes in the wavelength of light of a star to detect a planet. This is based on the Doppler Effect; the wavelength of light is shorter when the star is moving toward us and longer when the star is moving away. This means we can only detect the earthward motion, and the measured value versus the true value will be different depending on the observable angle. Therefore, only the lower limit is known.

この惑星の最大の特徴は、ハビタブルゾーンつまり水が液体で存在できる範囲にいることです。さらに、この星は大きさの分類上スーパーアースに分類されますが、初めて発見されたハビタブルゾーンにいるスーパーアース型の太陽系外惑星になります。くわえて、潮汐ロックがかかっている可能性が比較的低くなっています。潮汐ロックとは公転周期と自転周期が完全に一致して、主星に向けられる面がいつも同じになることです。比較的多くのハビタブル惑星が多く見つかっているより軽い恒星系では主星の温度が低いため主星と惑星の距離が近くなり潮汐ロックがかかっている可能性が高くなってしまいます。潮汐ロックがかかってしまうと主星に向いている面の気温が高くなり、逆に逆面では気温が低くなり生物が生きていくのにあまり適さない環境になってしまいます。HD 40307gは比較的その恐れが少なく生物が存在する希望がより高くなります。
(大山 航)


• M. Mayor, S. Udry, C. Lovis, F. Pepe, D. Queloz, W. Benz, J.-L. Bertaux, F. Bouchy, C. Mordasini, D. Segransan (2009). “The HARPS search for southern extra-solar planets. XIII. A planetary system with 3 Super-Earths (4.2,6.9&9.2Earth masses)”. Astronomy and Astrophysics 493 (2): 639-644
• Tuomi, Anglada-Escude, Gerlach, Jones, Reiners, Rivera, Vogt, Butler, Mikko, Guillem, Enrico, Hugh R. R., Ansgar, Eugenio J., Steven S., R. Paul (2012年). “Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307”

AU Mic b

AU Microscopii (hereinafter referred to as AU Mic) is a star located in the southern constellation Microscopium about 32.3 light-years (9.79 Parsecs) away from our solar system. AU Mic is a young red dwarf star that is classified as an M1 Ve. Its apparent magnitude is 8.7 and its temperature is 3730 K. It is a small star, at only 60% the radius of our sun, and it radiates only 9% of our sun’s light.

The most interesting thing about AU Mic is the debris disk found around it, which is circumstellar disk of dust that orbits the star. This disk was found and then confirmed in 2003 by Paul Kalas and collaborators using the University of Hawaii 2.2-m telescope on Mauna Kea, Hawaii. The disk was detected from about 35 to 210 astronomical units from the star, a region where dust lifetimes exceed the present stellar age. The total amount of dust that makes up the disk is thought to be at least 6 lunar masses.

Within the debris disk, a planet was recently discovered. AU Mic b orbits its host star in about 8.46 days at a distance of 0.07 astronomical units. It has a radius 0.4 of Jupiter and a mass of about 0.18 of Jupiter. The fact that a planet exists within the debris disk offers scientists a chance to study planet formation and evolution.

(Ling Cassandra)

Imaginary Picture of AU Mic b (Ryusuke Kuroki, Yosuke A. Yamashiki)
Size of AU Mic & AU Mic b in comparison with our Solar System
Habitable zone calculated based on Kopparapu et al.(2013) around the star AU Mic

For more information on AU Mic, please visit the ExoKyoto database:

Imaginary Picture of AU Mic b by Miu Shimizu

Kepler-1649 c

Kepler-1649 c is an Earth-sized exoplanet discovered by re-analyzing data from the Kepler space telescope. Its size is around 1.06 times the radius of the earth, and the mass is 1.21 times the mass, estimated by ExoKyoto. The host star, Kepler-1649, is an M5V type red-dwarf with a surface temperature of 3240K. Its radius is estimated to be about 25% of the sun and its mass is about 21.9%. The exoplanet orbits the red dwarf, at around 0.0855 astronomical units (1,280,000 km) which takes about 19.5 days. The estimated black body temperature of Kepler-1649 c is 245.39K assuming an albedo of 0.3, which is almost the same as the earth (255K). If the atmospheric pressure and components are similar to the earth, it is very probable that its environment also resembles the Earth

However, the exoplanet is most likely tidally locked, due to it revolving around a red dwarf star. Also, the light seen from the surface of the planet would be significantly different from sunlight on Earth, composed of about 90% infrared rays and only 8.87% visible light component (according to Exo Kyoto).

Ultraviolet radiation, including extreme ultraviolet radiation, is estimated to be about 0.17%, which requires more detailed observation.

Kepler-1649 c’s orbit and habitable zone according to Kopparapu et al.2013.
Size comparison according to ExoKyoto

Kepler-1649’s estimated spectra using the ExoKyoto spectrum module

For more detailed information on Kepler-1649 please visit to the following database page


WASP-76b is an exoplanet that revolves around the star WASP-76 about 640 light years away in the Pisces constellation.

The star WASP-76 has an apparent magnitude of 9.5, an absolute magnitude of 4.1, and is a spectral F7 type star with a surface temperature of 6250K.

The star has a mass about 1.5 times that of the sun and a radius about 1.7 times that of the sun.

WASP-76b is the only planet orbiting WASP-76 and it is classified as a hot jupiter.

The exoplanet was discovered by the transit method in 2013.

Its mass is about 0.92 times that of Jupiter, but the powerful radiation from the main star expands the atmosphere and the radius is about 1.83 times that of Jupiter, making it a low-density planet.

The largest radius of its orbit is about 0.033 AU (5 million km) and its orbit lasts around 1.8 days.

Its temperature is 2700K on the day-side and 1800K on the night-side; it is tidally locked. 

In 2020, observations at the Paranal Observatory with the Very Large Telescope (VLT) revealed that the day-side atmosphere was rich in iron vapor.

A strong wind blows on the surface because the temperature difference between the day-side and the night-side is close to 1000K. The wind and the rotation carry iron vapor from the day-side to the night-side, which causes cooling. It is thought that this causes condensation, which becomes rain. 



TOI-1338b is a binary planet, orbiting around the binary star system TOI-1338. It was discovered by a high school student doing a NASA internship program.

TOI-1338 is a binary star located in the Pictor constellation, about 1300 light-years away from Earth. The main star has a mass of about 1.1 times that of our sun, while the companion star has a mass about ⅓ of the sun. It takes about 15 days for them to orbit around each other. The main star has a radius about three times that of the sun, and a surface temperature of 5723 K, which is similar to the sun. It is a spectral G4 type star, with an apparent magnitude of 11.5, and an absolute magnitude of 3.5. The name, TOI, is an acronym for “TESS Object of Interest” which refers to the stars and planets discovered by the exoplanet exploration satellite TESS. With the discovery of TOI-1338b, TOI-1338 became the first binary star that has a binary planet discovered by TESS.

TOI-1338b is a unique planet that has a mass about 7 times that of the Earth, and was discovered by the transit method in 2020. Because the star is binary, the transit cycle is irregular at about 93-95 days. Since the orbit almost coincides with the orbit of the binary star, the planet always has a solar eclipse. According to ExoKyoto’s spectrum module, the light on the planet is 44.51% visible light, 47.80% infrared, and 7.66% ultraviolet.