地球型惑星に水蒸気　存在を初めて確認

【パリＡＦＰ＝時事】ハビタブルゾーン（生命居住可能領域）内にある太陽系外惑星の大気中に水蒸気が存在することを初めて確認したとする論文が11日、英科学誌ネイチャー・アストロノミーに発表された。（写真は大気中に水の存在が確認されたスーパーアース「K2─18b」の想像図）
　論文によると、この惑星「Ｋ２─１８ｂ」は質量が地球の８倍、大きさが地球の２倍で、液体の水が存在できるハビタブルゾーン内で恒星の周りを公転している。
　Ｋ２─１８ｂは、米航空宇宙局（ＮＡＳＡ）のケプラー宇宙望遠鏡が発見した数百個の「スーパーアース（巨大地球型惑星）」の一つ。これまで４０００個余り見つかっている太陽系外惑星のうち、岩石でできた表面と水を含む大気の両方を持つことが確認されたのはこの惑星が初めて。
　論文の共同執筆者でロンドン大学ユニバーシティー・カレッジの天文学者ジョバンナ・ティネッティ氏はＡＦＰに対し、生命の痕跡を探す取り組みにおいて「この惑星は太陽系外で見つかっている中では最有力候補だ」と説明。「表面に海があるとの臆測はできないが、その可能性は十分ある」と述べた。
　同大のアンゲロス・チアラス氏率いる研究チームは、２０１６年と２０１７年にハッブル宇宙望遠鏡が収集した分光データを基に、Ｋ２─１８ｂの大気を通過した星の光をオープンソースのアルゴリズムを用いて分析。その結果、まぎれもない水蒸気の痕跡を確認した。
　水蒸気の正確な量は明らかではないが、コンピューターモデリングでは濃度が０．１～５０％であることが示された。一方、地球の大気中の水蒸気濃度は南極・北極の上空で０．２％、熱帯地方で最大４％。
　Ｋ２─１８ｂは、約１１０光年離れた天の川銀河内のしし座にある赤色矮星（わいせい）の周りを公転しており、有害な放射線を地球よりも多く浴びているとみられている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2019/09/12-10:06）

Water has been discovered for the first time in the atmosphere of an exoplanet with Earth-like temperatures that could support life as we know it, scientists revealed Wednesday.
Eight times the mass of Earth and twice as big, K2-18b orbits in its star's habitable zone at a distance -- neither too far nor too close -- where water can exist in liquid form, they reported in the journal Nature Astronomy.
This planet is the best candidate we have outside our solar system in the search for signs of life, co-author Giovanna Tinetti, an astronomer at University College London, told AFP.
We cannot assume that it has oceans on the surface but it is a real possibility.
Of the more than 4,000 exoplanets detected to date, this is the first known to combine a rocky surface and an atmosphere with water.
Most exoplanets with atmospheres are giant balls of gas, and the handful of rocky planets for which data is available seem to have no atmosphere at all.
Even if they did, most Earth-like planets are too far from their stars to have liquid water or so close that any H2O has evaporated.
Discovered in 2015, K2-18b is one of hundreds of so-called super-Earths -- planets with less than ten times the mass of ours -- spotted by NASA's Kepler spacecraft.
Future space missions are expected to detect hundreds more in the coming decades.
- 'Is Earth unique?' -
Finding water in a potentially habitable world other than Earth is incredibly exciting, said lead-author Angelos Tsiaras, also from UCL.
K2-18b is not 'Earth 2.0', he said. However, it brings us closer to answering the fundamental question: is the Earth unique?
Working with spectroscopic data captured in 2016 and 2017 by the Hubble Space Telescope, Tsiaras and his team used open-source algorithms to analyse the starlight filtered through K2-18b's atmosphere.
They found the unmistakable signature of water vapour. Exactly how much remains uncertain, but computer modelling suggested concentrations between 0.1 and 50 percent.
By comparison, the percentage of water vapour in Earth's atmosphere varies between 0.2 percent above the poles, and up to four percent in the tropics.
There was also evidence of hydrogen and helium as well. Nitrogen and methane may also be present but with current technology remain undetectable, the study said.
Further research will be able to determine the extent of cloud coverage and the percentage of water in the atmosphere.
- First of many -
Water is crucial in the search for life, in part because it carries oxygen.
Life as we know is based on water, said Tinetti.
K2-18b orbits a red dwarf star about 110 light years distant -- a million billion kilometres -- in the Leo constellation of the Milky Way, and is probably bombarded by more destructive radiation than Earth.
It is likely that this is the first of many discoveries of potentially habitable planets, said UCL astronomer Ingo Waldmann, also a co-author.
This is not only because super-Earths like K2-18b are the most common planets in our galaxy, but also because red dwarfs -- stars smaller than our Sun -- are the most common stars.
The new generation of space-based star gazing instruments led by the James Webb Space Telescope and the European Space Agency's ARIEL mission will be able to describe exoplanet atmospheres in far greater detail.
ARIEL, slated for a 2028 launch, will canvas some 1,000 planets, a large enough sampling to look for patterns and identify outliers.
Over 4,000 exoplanets have been detected but we don't know much about their composition and nature, said Tinetti. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution.