Kepler-186f

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.

（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.

（文責：下崎紗綾・佐々木貴教）

（Kepler 186f in the ExoKyoto Stellar Window）

For more information about Kepler-186f, please visit:
http://www.exoplanetkyoto.org/exohtml/Kepler-186_f.html

Journal Articles

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

2.) FORMATION, TIDAL EVOLUTION AND HABITABILITY OF THE KEPLER-186 SYSTEM

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

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.

（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.

（文責：藤田汐音・佐々木貴教）

（Kepler-16b in the ExoKyoto Stellar Window）

For more information about Kepler-16b, please visit:
http://www.exoplanetkyoto.org/exohtml/Kepler-16_(AB)_b.html

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.

（文責：芝池諭人）

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

For more information about HD 209458 b, please visit the following:
http://www.exoplanetkyoto.org/exohtml/HD_209458_b.html

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）

<発見直後に描いてもらったイラスト>

（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。

しかし、もしもこの惑星上に知的生命が住んでいたとしたら、電磁波を使えば数年程度で交信を行うことが可能です。人類にとって初めての地球外生命体との「コンタクト」が、近い将来実現されるかもしれません（！？）

（文責：佐々木貴教）

（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。
http://www.exoplanetkyoto.org/exohtml/Proxima_Cen_b.html

ジャーナル記事

1.) THE HABITABILITY OF PROXIMA CENTAURI B I: EVOLUTIONARY SCENARIOS

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

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

WEB記事

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