Author: exoplanetkyo

TOI-700 d

TOI-700 is a red dwarf located in the Dorado constellation, about 101 light-years from Earth. Its surface temperature is 3480K, it is a spectral H2V type star, and its mass and radius are roughly four times that of our sun. The name of the star, TOI, stands for Tess Objects of Interest and refers to a catalog of celestial bodies that have shown the existence of orbiting planets using the TESS exoplanet search satellite. Objects listed in the TOI catalog are also observed with methods other than the transit method, such as Doppler spectroscopy and direct imaging. TOI-700 has also been referred to as Gaia DR2 5284517766615492736 because it was observed by the Space Telescope Gaiain 2013, launched by the ESA.

TOI-700 was observed with the transit method, and three planets were discovered orbiting around it. TOI-700d, which is the furthest of the planets at 0.16 AU, is a rocky planet 1.2 times the size of Earth, it is also likely to be within the area where water can exist in a liquid state (habitable zone). TESS has discovered a number of exoplanets since its launch in 2018, but this is the first Earth-sized planet to be found in the habitable zone.

According to the ExoKyoto Spectral Module, TOI-700d receives an estimated 85.72% of infrared light from its star, 13.97% is visible, and 0.30% is ultraviolet.

TOI-700d has a radius of about 1.19 that of the Earth, and its mass has not been measured yet, but it is estimated to be around 2.26 Earth’s mass using ExoKyoto’s mass estimation module. Its orbit is just outside the runaway greenhouse limit of Kopparapu et al. 2013 at 37 days. The planet could be tidally locked and it is possible that one side could be covered in plants.

For more information, visit our database page


(Image credit: ESA/Hubble, M. Kornmesser)

K2-18 b was discovered in 2015 orbiting around its host star K2-18. The star is an M-type dwarf and it is located about 110 light years from Earth. 

The exoplanet K2-18 b, located in the habitable zone, has a mass about 9 times that of Earth, which means it’s either an icy giant like Neptune or a rocky world with a thick, hydrogen-rich atmosphere. It is also very close to its host star, so it orbits at around 0.1429 AU and it takes 32.9 days to complete one orbit. 

Although it orbits close to the star, the exoplanet is still in the habitable zone, which means it can support liquid water on its surface. In fact, in September of 2019, researchers published two independent studies showing that there is a possibility of water vapor on K2-18 b. This was discovered by observations made with the Hubble Space Telescope. Some early models predict that K2-18b’s effective temperature falls somewhere between -100 and 116 degrees Fahrenheit, and if it is about as reflective as Earth, its equilibrium temperature would be roughly the same as our home planet.

“This is the only planet right now that we know outside the solar system that has the correct temperature to support water, it has an atmosphere, and it has water in it—making this planet the best candidate for habitability that we know right now,” University College London astronomer Angelos Tsiaras, a coauthor of one of the two studies, said during a press conference.

 Image via Alex Boersma/iREx.

This is the first time that water vapor has been identified in the atmosphere of a non-gas-giant exoplanet in the habitable zone of its star, but soon after the announcement, many planetary scientists critiqued how the discovery was covered in the media.

In fact, just a few days later, Scientific American posted a harsh critique of the media coverage, claiming that K2-18 b is not at all habitable:

“The crux of the issue is the size of the planet. K2-18b is about 2.7 times the size of Earth—so large that the planet must have a massive, extended atmosphere, with a significant fraction of light hydrogen gas. An atmosphere like this makes K2-18b much more akin to Neptune than it is to Earth.

In a hydrogen-rich atmosphere, the temperature and pressure increase the deeper you go. By the time the rocky core is reached, the pressure is expected to be thousands of times higher than the surface of Earth, and the temperature can approach 5,000 degrees Fahrenheit.

These conditions are bad news for the formation of complex molecules, such as DNA, which aren’t stable at high temperatures and pressures. Even if we’re very open-minded about the conditions required for life to evolve, there’s broad consensus that complex molecules of some kind are necessary, to ensure that enough information is provided for replication to occur. Complex molecules cannot form on the deep surface of K2-18b.”

Although the planet may not habitable, it is still a huge discovery with great implications of finding more exoplanets that can hold water. The search for another habitable world outside of our solar system, or life on another planet goes on.

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