Category: Introduction to Exoplanets

Radiation Exoplanet

Kepler-283 cにおける1年に1度発生しうる中心星のスーパーフレア・CMEによって引き起こされる惑星大気(N2+O2を仮定)での空気シャワーによる被ばく量推定

Proxima Centauri bにおける1年に1度発生しうる中心星のスーパーフレア・CMEによって引き起こされる惑星大気(N2+O2を仮定)での空気シャワーによる被ばく量推定
Ross-128 bにおける1年に1度発生しうる中心星のスーパーフレア・CMEによって引き起こされる惑星大気(N2+O2を仮定)での空気シャワーによる被ばく量推定

Teegarden b, c

Imaginary picture of tidally locked Teegarden c
Imaginary Picture, Miu Shimizu

An international team led by the University of Göttingen (Germany) with participation by researchers from the Instituto de Astrofísica de Canarias (IAC) have discovered, using the CARMENES high-resolution spectrograph at the Calar Alto Observatory (Almería), two new Earth-like planets around one of the closest stars within our galactic neighborhood.

The Teegarden star is only 12.5 light-years away. It is a red dwarf in the direction of the constellation of Aries. Its surface temperature is 2,700 degrees C, and its mass is only one-tenth that of the sun. Even though it is so near, its faintness impeded its discovery until 2003. Two planets have been discovered orbiting the ultra-cool M dwarf that are thought to be temperate, rocky planets. They have orbital periods of 4.9 and 11.4 days, and both planets are likely to be within the Habitable Zone as well as tidally locked. If they’re confirmed, both of the newly spotted worlds are nearly identical to Earth in mass, and both planets are in orbits that could allow liquid water to trickle and puddle on their surfaces. This surface liquid water that is thought to be present on both planets, makes for a wide range of atmospheric properties, which makes them attractive targets for bio-signature searches. They are among the most Earth-like exoplanets yet discovered.


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>>

Kepler-283 c

Kepler-283 c is located 1741.0 light-years (534 parsecs) from our solar system and was discovered in 2014 orbiting it’s host star Kepler-283. The apparent magnitude of the host star is 13.9 and the absolute magnitude is 5.3. Kepler-283 is about 0.8 the mass of our sun and has about 0.6 the radius. Its surface temperature is 4351 K and is a spectral K5 type star. The planet Kepler-283 c orbits the star every 92.7days, it’s orbital radius is 0.34 SEAU. 

According to an announcement from NASA, this planet is located in the habitable zone. ExoKyoto has come up with the same conclusion. Kepler-283 c is the only habitable planet in which superflares from the host star (Kepler-283) were observed during observation with the Kepler Space Telescope. Based on this, the estimated radiation exposure dose was calculated for various cases, taking into account the frequency of flares, but the doses were level and none caused problems (Yamashiki et al. 2019 ApJ).

A second planet, Kepler-283 b, has also been found in the Kepler-283 solar system, which is a Hot Jupiter located inside the habitable zone.

For more information on Kepler-283 c, please visit the ExoKyoto database page below.