HD 209458 b

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