51 Pegasi b

1995 年に人類史上初めて、スイスのミシェル・マイヨール（Michel Mayor）らにより発見された最初の太陽系外惑星です。マイヨールとディディエル・クゥエロツらは当時最新鋭の高分散分光器ELODIEを備えたフランスのオート・プロヴァンス天文台（Observatoire de Haute-Provence: OHP）にて、視線速度法によりペガスス座51番星を観測し、木星質量の惑星が太陽系の水星軌道の内側を自転周期 4.2 日で公転していることを Nature 誌に発表しました (i)。この功績により、両氏は2019年ノーベル物理学賞を受賞しました。

（Imaginary Picture of 51 Pegasi b as original “Hot Jupiter” Credit：Yosuke Yamashiki, Ryusuke Kuroki & Natsuki Hosono)

（Hot Jupiter 51 Pegasi b orbiting around its host star 51 Pegasi, Credit：Yosuke Yamashiki, Ryusuke Kuroki & Natsuki Hosono)

ペガサスに騎乗したギリシャ神話の英雄ベレロポンにちなんでベレロフォン（Bellerophon）と呼ばれることもあるこの系外惑星（51 Pegasi b）は、その灼熱の推定環境にちなんでホットジュピター（灼熱の木星）と分類されました。その後、視線速度法により、数々のホットジュピターが発見されています。例えば同じペガサス座のオサイリス（Osiris: HD 209458 b）などは公転周期がわずか 3.5 日で主星のまわりを公転し、ハッブル望遠鏡により 2001 年に大気中に酸素と炭素が含まれていることが観測された初めての系外惑星です。

太陽系の形成過程の標準モデルでは、ガス惑星は中心星から遠くはなれた場所（~5AU）で形成されるとされていたため、このような中心星近傍（<0.05AU）に存在する巨大ガス惑星の形成過程は多くの議論を呼びました。その後、恒星から遠く離れて形成された巨大ガス惑星が軌道変遷により水星軌道の内側にまで移動してきた可能性が最も高いとされています。

なお、ミシェル・マイヨール氏は第 31 回京都賞を受賞され、同年にはノーベル物理学賞候補にもノミネートされました。そして、2019年には宇宙論のJames Peebles, そして共同発見者のディディエル・クゥエロツ(Didier Queloz)とともにノーベル物理学賞を受賞しました。

(i) Michel Mayor & Didier Queloz. 1995. A Jupiter-mass companion to a solar-type star. Nature 378(23): 355-359.

マイヨールとクゥエロツは、高分解能分光計（高分散分光器）ELODIEを備えたフランスのオート・プロヴァンス天文台にてペガスス座51番星（51 Pegasi）の視線速度を測定し、木星質量の惑星が太陽系の水星軌道の内側（0.05AU）を公転周期 4.2 日で公転していることを発見し、また系外惑星（51Pegasi b）が、小さな赤色矮星からガスが流れ出た残骸であるという可能性と同時に、元々恒星から遠く離れて（~5AU）形成された木星質量のガス惑星が内側に移動してきたと考えられる可能性を示しています。特に恒星 51 Pegasi の推定寿命が G 型星の寿命に近い 100 億年と推定されたこともあり、惑星軌道の変遷によりガス惑星が内側に移動した可能性があることが発表後議論されました。また、この視線速度の変化が大質量星のパルサーに由来するものではないことを明らかにし、系外惑星発見の揺るぎないデータと論述を示しました。またフィレンツェにおける研究発表を通じて、ハーバード・スミソニアン天体物理センターを含む他の天文グループにより視線速度変化の周期が 4.2 日であるという独立調査がなされ、その信頼性が確認されました。

（文責：山敷庸亮）

（ExoKyoto Stellar Window を用いて表示した 51 Peg b の位置）

51 Pegasi b についての詳しい情報はこちら。
http://www.exoplanetkyoto.org/exohtml/51_Peg_b.html

Kepler-452b

(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)

（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.

（文責：藤田汐音・佐々木貴教）

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

For more information about Kepler-452b, please visit the ExoKyoto Database:
http://www.exoplanetkyoto.org/exohtml/Kepler-452_b.html

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

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