同じく 2013 年にはチリの Very Large Telescope による波長分解能の高い詳細な観測から、惑星自体の H2O, CO ガス輝線放射が検出されたことでも話題になりました。そのほかこの年には、NASA のチャンドラと欧州の XMM ニュートンという2つの X 線天文衛星でこの天体を観測することにより、太陽系外の恒星の手前をその星を公転する系外惑星が横切る「トランジット」が、X 線で初めて検出されています。
(Position in Stellar Map of star HD 189733 and its Exoplanet HD 189733 b)
(Zoomed position in Stellar Map of star HD 189733 and its Exoplanet HD 189733 b (zoom level 3))
TRAPPISTとは、TRAnsiting Planets and PlanetesImals Small Telescopesの略で、ベルギー・リエージュ大学(http://www.ulg.ac.be/cms/c_5000/accueil)の天文地学海洋専攻(AGO)のプロジェクトでチリのESO La Silla 天文台 とモロッコのOukaïmden 天文台(2016.10.6開始)に設置された望遠鏡ネットワークであり、このTRAPPIST-1は2016年にLa Silla天文台で発見され、地球よりわずかに大きな惑星が３つ、このクラスの赤色矮星の周りに初めて発見されました1) 。さて、特にこのTRAPPIST-1系のハビタブルゾーンにあると言われた3番目の惑星TRAPPIST-1dのトランジット観測による周期と軌道が確定せず、ハビタブルゾーンの惑星発見のニュースはキャンセルされるかと心配されていました。ところがそれがさらなる大発見につながったのです。
また、Kopparapu et al.2013によると、ハビタブルゾーンにある惑星は、d, e, f ,g となり、先ほどのcは内側境界の中に位置してしまいます。TRAPPIST-I dはしかしながらRecent Venusの内側に位置はしますが、暴走温室限界線の内側にあるので、そのままでは海洋は存在できませんが、潮汐ロックされている場合境界領域(terminator)に狭い海が存在しうるとも考えられます。TRAPPIST-I gはしかしながら、外側境界最大温室限界付近のため、十分な温室効果ガスがある場合のみ居住可能だと考えられます。
(Kopparapu et al. 2013 によるハビタブルゾーンとTRAPPIST-1b,c,d,e,f,g,hの軌道位置,赤線がRecent Venus境界線、緑色が薄い色からそれぞれ火星・地球・スーパーアースサイズの暴走温室限界線,その外側の薄青色が最大温室効果限界線(Maximum G), その外側の青が初期火星線(Early Mars)。この判定によるとTRAPPIST-1 e, f, gがハビタブルゾーンとなる)
(Position in Stellar Map of star TRAPPIST-1 and its Exoplanet TRAPPIST-1 b,c,d,e,f,g,h)
(Zoomed pos.in Stellar Map of star TRAPPIST-1 and its Exoplanet TRAPPIST-1 b,c,d,e,f,g,h)
1)Michaël Gillon, Emmanuël Jehin, Susan M. Lederer, Laetitia Delrez, Julien de Wit, Artem Burdanov, Valérie Van Grootel, Adam J. Burgasser, Amaury H. M. J. Triaud, Cyrielle Opitom, Brice-Olivier Demory, Devendra K. Sahu, Daniella Bardalez Gagliuffi, Pierre Magain & Didier Queloz. Temperate Earth-sized planets transiting a nearby ultracool dwarf star, Nature 533, 221–224 (12 May 2016) doi:10.1038/nature17448, Received 11 January 2016 Accepted 18 February 2016 Published online 02 May 2016 http://www.nature.com/nature/journal/v533/n7602/full/nature17448.html
2)Michaël Gillon, Amaury H. M. J. Triaud, brice-Olivier Demory, Emmanuël Jehin1, Eric Agol, Katherine M. Deck, Susan M. Lederer, Julien de Wit, Artem burdanov, James G. Ingalls, Emeline bolmont, Jeremy Leconte, Sean N. Raymond, franck Selsis, Martin Turbet, Khalid barkaoui, Adam burgasser, Matthew R. burleigh, Sean J. Carey, Aleksander Chaushev, Chris M. Copperwheat, Laetitia Delrez, Catarina S. fernandes, Daniel L. Holdsworth, Enrico J. Kotze, Valérie Van Grootel, yaseen Almleaky, Zouhair benkhaldoun, Pierre Magain & Didier Queloz. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature 542, 456–460 (23 February 2017) doi:10.1038/nature21360.
(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.
The location of Kepler-452 shown on the ExoKyoto Stellar Screen
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.