Category: Famous Exoplanets

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

GJ 699 b – Barnard’s star b

(Imaginary picture of Barnard’s star b – GJ 699 b   credit: Ryusuke Kuroki, Natsuki Hosono and Yosuke A. Yamashiki)

GJ 699 b (Barnard’s star b) is a Super Earth located about 6 light-years from our solar system, which orbits an M-type star, Barnard’s star, in about 233 days. It is located just on the snow line (the planetary orbit at which water freezes) of its host star, and the surface temperature is estimated to be about 105 Kelvin (minus 168℃). Since it was discovered by the radial velocity method, its mass has only been estimated but is thought to be about 3.2 times as heavy as the earth.

Barnard’s star is the closest star to Alpha Centauri, the star that is closest to our solar system. In fact, in the 1960’s American astronomer Pete van de Kamp thought he discovered a planet around Barnard’s star. Therefore, in the 1970s, it was common knowledge that there were planets orbiting the star, and many science fiction works based on these planets were born. However, this “discovery” was not confirmed by other telescopes, and it was later pointed out that it was a data error by the observation device, so it became a “phantom planet”.

The planet that was eventually discovered is different from van de Kamp’s “planet,” but it became a huge discovery all over the world because of how popular Barnard’s star has become. The planet  GJ 699 b was finally discovered as a result of continued long-term observation with multiple telescopes for more than 20 years, from June 1997 to November 2017. Since the very first exoplanet was discovered in October 1995, it is clear the search for a planet orbiting Barnard’s star set out soon after. 

From observation data to date, it seems that no planet larger than Earth orbits other M-type stars, and there is no planet larger than Earth in the habitable zone. So, unfortunately, it seems that there is no “second earth” orbiting the stars next to our solar system. However, there is a possibility of a different type of cool Super Earth that could inspire new science fiction.

Below is the analysis for GJ 669 b by ExoKyoto. The estimated temperature of the planet is 105 Kelvin (minus 168℃) according to a published paper, which is about the surface temperature of Jupiter’s moon Galileo, and it is thought to be an icy world in which liquid water cannot exist without an internal heat source. The average radiant energy from Barnard’s star is estimated to be 27.49 W / m2, which is less than Jupiter but about twice that of Saturn. However, since most of the rays are infrared rays (estimated visible light is 9.99%, infrared is 89.81%), and since much of the energy contributes directly to heat, it may be a little warmer than the albedo in the visible light region.

The planet radius has not been estimated because it is measured by the radial velocity method, but ExoKyoto estimated it to be about 1.37 times the radius of Earth (0.12 times that of Jupiter).

GJ 669 b has an elliptical orbit with an eccentricity of 0.32,  so it is located within 0.3 astronomical units from Barnard’ star at its closest distance, and about 0.5 astronomical units when moving away from it. Therefore, the planet might be subject to extreme seasons. However, the snowline (the position of the asteroid belt in the solar system) is located inside its orbit, so even if the maximum greenhouse effect limit by Kopparapu is used, it will be outside this limit (both are 0.13 astronomical units). It is considered to be a “cold” planet, much more than the previously described.

Furthermore, the host star has a low amount of activity, and radiation due to solar flares seem to be small, even considering the sufficient distance from its host star.

For more information on GJ 669 b, please visit the following database.

Size of GJ 699 b – Barnard’s star b

(Orbit of GJ 699 b – Barnard’s star b)

(Position of GJ699 b – Barnard’s star b)

(Position of GJ699 b – Barnard’s star b)

Kepler-62 System

Kepler-62 is a five-planet system about 1,200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring just two thirds the size of the sun and only one fifth as bright. At seven billion years old, the star is somewhat older than the sun.

Much like our solar system, Kepler-62 is home to two habitable zone worlds, Kepler-62f and Kepler-62e. Kepler-62f orbits every 267 days and is only 40 percent larger than Earth, making it the smallest known exoplanet in the habitable zone of another star. The other exoplanet in the habitable zone is Kepler-62e, which orbits every 122 days and is roughly 60 percent larger than Earth.

The size of Kepler-62f is known, but its mass and composition are not. However, based on previous exoplanet discoveries of similar size that are rocky, scientists are able to determine its mass by association.

The two habitable zone exoplanets orbiting Kepler-62 have three interior companions, two larger than Earth and one about the size of Mars. Kepler-62b, Kepler-62c, and Kepler-62d orbit every five, 12, and 18 days, respectively, making them very hot and inhospitable for life as we know it.


[Imaginary Picture of Kepler-62 e: Credit Fuka Takagi and Yosuke A. Yamashiki]

Journal Articles:

1.) Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone

2.) Mercury-T: A new code to study tidally evolving multi-planet systems. Applications to Kepler-62

3.) Water-Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f

WEB Articles

1.) The Math: What Life On Kepler-62e Would Be Like

2.) Kepler-62e

3.) Water Planets in the Habitable Zone: A Closer Look at Kepler 62e and 62f

Pi Mensae c

<Image of Pi Mensae c Exokyoto system automated selection, © 2018 ExoplanetKyoto- Ryusuke Kuroki, Yosuke A. Yamashiki and Natsuki Hosono>

Pi Mensae c orbits around a somewhat large and bright G-type star Pi Mensae, and is categorized as a Super Earth, with a radius of about two times that of Earth. This planet was first reported in arXiv in September 2018, the first exoplanet discovered by TESS, the Transiting Exoplanet Survey Satellite. Furthermore, another orbiting planet, Pi Mensae b was discovered in 2001 and was found to have a mass 10 times that of Jupiter.

<From the right: (1) our sun and Pi Mensae (HD 39091) (2) Jupiter (3) Earth and Pi Mensae comparison>

Pi Mensae c is thought to have a small mass relative to its size and be less dense than the Earth. Therefore, it is presumed to be a planet with a lot of water, or have a thick atmosphere. Also, since the distance from the host star is only about 1/50 of the distance from the sun to Mercury, water is thought to evaporate at very high temperatures on its surface.

<pi Mensae c (HD 39091 c) orbit, and the runaway greenhouse effect line (in green)>

Pi Mensae c is about 60 light-years away from the earth and the apparent magnitude is 5.67, so if you are in a very dark environment you can see it with the naked eye. If you are ever traveling in the southern hemisphere and see this star, you can tell everyone that “a Super Earth revolves around that star!”

For more information on Pi Mensae c, please see the ExoKyoto database:

<Pi Mensae b>  Super Jupiter with an eliptical orbit.

<Imaginary image of pi Mensae b Exokyoto system automated selection, – © 2018 ExoplanetKyoto – Ryusuke Kuroki, Yosuke A. Yamashiki and Natsuki Hosono>

Two exoplanets have been discovered orbiting the main sequence star Pi Mensae (HD 39091), which is said to have a radius of 2.1 times the sun. The first, Pi Mensae b (HD39091b) was discovered in 2001. This planet is thought to have a mass of 10.02 times that of Jupiter, and it has an elliptical orbit of around 1-5 astronomical units from its host star. This orbit goes from just inside the Venus-equivalent orbit, to the outside of the habitable zone and takes about 2093 days. The radius of Pi Mensa b has yet to be accurately observed.

For more information on Pi Mensae b, please see the ExoKyoto database:

<Elliptical orbit of Pi Mensae b (HD 39091 b)(purple line) and the habitable zone according to Kopparapu et al. 2013>

Mensa comes from the Latin meaning “table.” In Japanese, the Mensa constellation is “table-san” which translates to “table mountain.” Table Mountain (Mons Mensae in Latin), al plateau-like mountain overlooking the city in Cape Town, South Africa, which is a characteristic topography around the Cape of Good Hope. It is one of the 14 (12) constellations in the southern sky discovered by the French astronomer Abbé Nicolas-Louis de Lacaille.

A photo of Table Mountain for reference.

<Mons Mensae – Table Mountain, photo taken by Yosuke A. Yamashiki in 2000, © 2018 ExoplanetKyoto>>