Category: Introduction to Exoplanets


(Kepler-452 b Ocean Planet Credit: Chise Hatsuoka, Habitable Research Unit SGH Moriyama High School)

Kepler-452b is located about 1,400 light-years away in the Cygnus constellation and is thought to be the first rocky planet discovered in the habitable zone. Its orbit is 384 days around its host star, Kepler-452, which is a G-type sun-like star around 60 billion years old. Since the planet is so similar to Earth, it has been called ‘Earth’s Cousin.’

(Kepler-452 b Credit: Fuka Takagi and Yosuke Yamashiki, generated using Planet Map Generator and OpenGL)

Kepler452b sub crowd
(Credit: Shione Fujita & SGH Moriyama High School)

Using ExoKyoto and the Weiss and Marcy method, the mass was estimated at about 4 times that of Earth, which is why the exoplanet is considered a ‘Super-Earth.’ Based on the assumption that it can easily have a large amount of water under high gravity and the fact that it is difficult to dissipate the acquired water, we imagine that this Ocean Planet is almost entirely occupied by oceans 30-50 km deep. If this is the case, the planet wouldn’t have continents but it is assumed to have only chains of islands like Hawaii.

However, since the host star is about 1.5 billion years older than our Sun, it is expected to radiate more energy than the Sun, and Kepler-452b probably receives more energy than the Earth. Therefore, the water on the surface might not exist due to the (runaway) greenhouse effect. Additionally, the planet Kepler-452b is about 1.5 billion years older than the Earth, so a detailed study of the surface environment could provide information on environmental changes that Earth might face in the future.

In any case, it is important to verify the existence or non-exitance of the planetary surface environment and ocean through detailed follow-up observations in the future.


The location of Kepler-452 shown on the ExoKyoto Stellar Screen Kepler452b_Stellar



For more information about Kepler-452b, please visit the ExoKyoto Database:

Journal Articles:

1.) Discovery and Validation of Kepler-452b: A 1.6-R⊕ Super Earth Exoplanet in the Habitable Zone of a G2 Star

2.) Climate and Habitability of Kepler 452b Simulated with a Fully Coupled Atmosphere–Ocean General Circulation Model

3.) Quantitative estimates of the surface habitability of Kepler-452b


WEB Articles:

1.) Kepler-452b: Earth’s Bigger, Older Cousin — Briefing Materials

2.) One Of The Most Earth-Like Worlds We’ve Found May Not Actually Exist

3.) Kepler 452 b: Inhabitable ‘Earth 2.0’ could be statistical mirage, study shows


Kepler-186f is an exoplanet that orbits the M-type main-sequence star, Kepler-186, which is located in the constellation Cygnus, about 492 light years from Earth.  With an orbit of about 130 days, Kepler-186f is the first near-Earth-sized planet discovered in the habitable zone outside our solar system, and it is the closest exoplanet to the Earth. Kepler-186f is and is one of five planets in the outer system. The  exoplanet orbits about 32.5 million miles (52.4 million kilometers) from the Sun.

(Credit: Natsuki Shirako & SGH Moriyama High School)

(Credit: Saaya Shimozaki & SGH Moriyama High School)

The host star, Kepler-186, is a red dwarf that has a red light. It has a mass of 0.478 and a surface temperature estimated at 3,788 K, about 2,000 degrees lower than our Sun’s. This means the energy of the light that reaches Kepler-186f is weak, composed fo many infrared rays. Because of this, the light that reaches the surface of the planet is unsuitable for energy absorption by green chlorophyll a, which is used by plants on Earth for photosynthesis.

If there was life on Kepler-186f, the plants might have black photosynthetic pigments that are more likely to absorb longer wavelengths of red light.

kepler186f 地表
(Credit: Saaya Shimozaki & SGH Moriyama High School)

Since Kepler-186f is close to its host star at about 0.36 AU (Earth-Sun distance = 1 AU), its rotation and orbit may be synchronized by tidal forces, which is due to the gravity from the host star. If this is the case, one surface of the planet would always be facing the host star, so this surface might be very dry, while the opposite side might be covered with ice. If this is the case, only the regions in between would be habitable.

Also, because wind moves from higher temperatures to lower temperatures, on the surface of the planet, strong winds are expected to blow in a certain direction from the day side to the night side, which could mean that any plants growing on the surface are always being pushed in the same direction.

This planet is sometimes called “Earth 2.0.” It is a habitable planet that is about the same size as Earth. However, since its host star is very different from our Sun, it is probably best to think of it as a “different” Earth-type planet.



(Kepler 186f in the ExoKyoto Stellar Window)

For more information about Kepler-186f, please visit:

Journal Articles

1.) An Earth-Sized Planet in the Habitable Zone of a Cool Star



WEB Articles

1.) Kepler-186f, the First Earth-size Planet in the Habitable Zone

2.) 5 Things to Know About Alien Planet Kepler-186f, ‘Earth’s Cousin’

3.) Kepler 186f


Kepler-16b orbits the binary stars, a K-type Kepler-16A and an M-type Kepler-16B in the constellation Cygnus. Kepler-16b is the first object discovered orbiting a binary star. This means that two “suns” are visible from the surface of the planet. Its orbit is located at the outer edge of the habitable zone of the brighter star, Kepler-16A.

Kepler-16b has a radius of 8.5 times that of Earth and a mass of 105 times that of Earth. It is close to the size Saturn from our solar system, and is considered a gas giant. If it has a sufficient atmosphere, there is a possibility for life to occur.

kep-16b main1jpg
(Credit: Shione Fujita & SGH Moriyama High School)

From this imaginary satellite, if you look into the sky, you will see Kepler-16b (the Saturn-like planet with rings) and two suns (Kepler-16(AB)) shining in the distance. It is interesting to think that life could be occurring on a “habitable moon,” which is unknown in our solar system, The landscape would be completely different than on Earth.

When Kepler-16b was first introduced at a research conference in the United States, the audience gave it a standing ovation. People thought that Luke Skywalker’s hometown of Tatooine, from the movie “Star Wars,” had finally been discovered! It was such a wonderful moment where science fiction caught up with reality.

Circumpolar planets have been discovered since then, and various studies are now being actively investigated, including the possibility of different ways of forming planets from single stars, and calculating more complex habitable zones. Since there are more binary stars in the universe than single stars, it is important to understand these binary planets.


(Kepler-16b in the ExoKyoto Stellar Window)

For more information about Kepler-16b, please visit:

HD 209458 b

HD 209458 b (also known as Osiris) was the first exoplanet in the world to be observed by the transit method in 1999. It orbits the star V376 in the  Pegasus constellation, and is named after the ancient Egyptian god Osiris. It is a “Hot Jupiter” that orbits very close its host star, and its surface temperature is thought to be over 1,000°C. Due to the star’s gravity, one surface of the planet is constantly facing the host star.

(Image Credit: Shione Fujita, Habitable Research Group, SGH Moriyama High School)

(Image Credit: Ryusuke Kuroki, Yosuke Yamashiki & Natsuki Hosono)

HD 209458 b was also the first exoplanet to have a confirmed atmosphere. By observing the light from the star that passes through the planet’s atmosphere at each wavelength, the molecules that make up the planet’s atmospheric composition can be understood.  Due to these observations, it was determined that the lower part of HD 209458 b’s atmosphere was found to contain sodium, while the upper layer contains hydrogen and carbon molecules. This also suggests that HD 209458 b is very different from the planets we know in our solar system, with a hot, violently ejected atmosphere with a comet-like trail and floating clouds of minerals.

Currently, the only way to understand the characteristics of exoplanet atmospheres is to observe the light from the host star that passes through the atmospheres of these planets. Because many stars can be observed at once analyzed automatically using this method, the transit method is a very powerful observation tool.

HD 209458 b’s transit can be observed with a relatively small 30cm diameter telescope. If people can analyze the data themselves and obtain a light curve that shows the existence of exoplanets, the distant universe of “exoplanets” will feel closer. We hope everyone will give it a try.



(HD 209458 b’s location using the ExoKyoto Stellar Window)

For more information about HD 209458 b, please visit the following:

HD 149026 b

HD 149026 b is an exoplanet in the Hercules constellation that orbits the star HD 149026. It was discovered in 2005 by Bunei Sato’s team using the radial velocity method with observations from the Subaru Telescope and the Keck Observatory.

(Imaginary Picture of HD149026b, credit Yosuke Yamashiki, Ryusuke Kuroki & Netsuki Hosono)


HD 149026 b transits its host star, which allowed researchers to measure its observed radius. Its mass was found to be 0.36 that of Jupiter and its radius is 0.75 that of Jupiter, making it quite dense for its size (1.6 g/cm3; more than twice as dense as Saturn). Therefore, it is presumed to be a hot gas giant with a massive central core that has 67 times the mass of Earth.

In fact, it is difficult to believe that a gas giant with such a large central core would be possible under the general planet formation theory. This is because when a celestial body grows to be about ten times the size of Earth during planet formation, it is thought to capture the gas from the surrounding protoplanetary disk using its gravitational force, and quickly grow into a gas planet. Once it develops into a gas planet, it would unlikely be able to capture large amounts of solid material inside it, so how it was able to form such a massive central core before it became a gas giant is a great mystery. The question about how HD 149026 b was formed is still being actively debated.

In 2015, the International Astronomical Union asked the public to vote on names for some exoplanet systems. As a result of this vote, the host star, HD 149026, will be called “Ogma” and the planet HD 149026 b will be called “Smertrios.”

(Written by Sasaki Takanori)

(Translated by Cassandra Ling)


(HD149026 using ExoKyoto’s Stellar Window)

For more information about HD 149026 b, please visit:

Proxima Centauri b

Proxima Centauri b は、赤色矮星である Proxima Centauri の周りのハビタブルゾーンを回る、地球サイズの系外惑星です。半径は地球よりもわずかに大きいだけであるため、地球と似た岩石惑星である可能性が高いと考えられます。2016年8月にヨーロッパ南天天文台により、視線速度法を用いて発見されました。地球からわずか 4.22 光年の位置にあり、現在までに発見されている系外惑星の中で、太陽系から最も近い惑星です。

(クレジット:Miu Shimizu & SGH Moriyama High School)


(クレジット:Shione Fujita & SGH Moriyama High School)

中心星の Proxima Centauri は、M 型矮星とよばれる太陽よりも小さく暗い恒星であるため、ハビタブルゾーンの位置は太陽型星と比べてはるかに中心星の近くにきます。そのためこの惑星も、中心星からわずか 750万km の位置を周回しています(ちなみに地球は太陽から 1億5,000万km のいちを周回しています)。なお、ハビタブルゾーンが中心星の近くにあるばあい、その中に位置する惑星は中心星に対して「潮汐ロック」されている可能性が高く、中心星に対して常に同じ面を向けていることが予想されます。

また中心星の Proxima Centauri は、星全体がフレアを起こすことで急激に増光する「フレア星」であることがわかっています。Proxima Centauri b は、中心星との距離が 0.05 AU(地球-太陽間の距離 = 1AU)と非常に近いため、フレアが起きた際には、高エネルギー粒子や X 線などが大量に地表に降り注ぐことになります。残念ながら、生命が存在するには厳しい環境かもしれません。



(Imaginary Picture of Proxima Centauri b, credit, Fuka Takagi & Yosuke Yamashiki, created using Planet Generator and OpenGL)



(ExoKyoto Stellar Window を用いて天球上に表示した Proxima Cen b)

Proxima Centauri b についての詳しい情報はこちら。



2.) The habitability of Proxima Centauri b II. Possible climates and observability

3.) A terrestrial planet candidate in a temperate orbit around Proxima Centauri



1.) The habitability of Proxima Centauri b

2.) An Earth-like Atmosphere May Not Survive Proxima b’s Orbit

3.) Superflare Blasts Proxima b, the Nearest Exoplanet, Dimming Hopes of Life