3つのうち1つは地球外生命体が存在する「ハビタブル惑星」が発見される

2016.05.23

地球から水瓶座方向に40光年離れた恒星系に、地球と同規模のサイズの惑星が3つ発見された。そのうち少なくとも一つは、生命の居住候補として優れており、また地球外生命体が存在する「ハビタブル惑星」の可能性があるという。

■木星とその衛星系に類似した「居住可能惑星」

今月初頭に科学誌ネイチャーで発表され、関係者の間で注目されている天体は、将来の人類の有力な“引越し先”になるかもしれない。

地球環境と類似する環境範囲内にあれば、人類の移住、生命の発生や進化もなし得るという仮説に基づく宇宙領域を「ハビタブルゾーン(生命居住可能地域)」というが、今回見つかったものは恒星から届く放射線量が地球の数倍で、恒星との距離が近すぎることも遠すぎることもなく、地表温度も適度かつ地表に水があって生命が存在し得る条件を備えていると推測されている。

3つの惑星が周回しているのはTrappist-1(2MASS J23062928-0502285)と呼ばれる矮星だ。赤褐色で表明温度が2600度と超低温(太陽の約半分)で質量は太陽の8%、大きさは8分の1ほどで、木星よりやや大きく、地球からはとても近いにもかかわらず肉眼でも望遠鏡でも見えないほどに暗い。この種の小さく暗い惑星は天の川付近ではよく見られるが、今回のTrappist-1はいままで同種の惑星が確認されていなかったエリアで発見された。

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発見したベルギー・リエージュ大学の研究チームは、チリのアタカマ砂漠にあるラ・シヤ天文台で、口径60cmの望遠鏡を用いて調査を行っている。地球から太陽までの距離と比較すると、3つの惑星とTrappist-1 は20〜100倍近く接近しているという。「この惑星系のスケールは太陽系というよりも、木星とその衛星系に類似している」そうで、3つすべての惑星がハビタブル惑星である可能性を視野に入れ、より大きな測定機器を準備することも検討しているという。

天文学者らによるこれまでの太陽系外惑星研究の主流は、光源の明るい星に集中していたが、リエージュ大学では逆に光の弱い60の矮星の探査に着手。

「小さな惑星の周囲は、現時点で我々が持つ技術を駆使して地球サイズの惑星における生命を検出できる唯一の場所」なのだという。他の明るい惑星ではその明るさゆえに生命の痕跡が埋もれてしまうため、Trappist-1のような暗い超低温矮星こそ検出可能で、着目すべきだとコメントしている。

■生命体がいる可能性が高い

研究チームは2015年9〜12月にかけての62夜にわたり、こうした恒星の光や明るさの変化を観測して定期的に光が遮られる日食のような現象を発見し、赤外線を検出できる望遠鏡を使って詳しく調べた。

観察により、内側に位置する2つの惑星の公転周期はそれぞれ1.5日と2.4日であることが判明。残る3つ目は4〜73日の間と、いまだ軌道がはっきりわからないというが、この惑星も生命体がいる可能性が高いと考えているという。

惑星らは常に同じ面がTrappist-1の方向に向き、片方は常に夜、もう片方は常に昼の状態であるらしい。共同研究者であるMITの天文学者ジュリアン・デ・ウィット氏は今後、生命の存在の可能性の重要な手がかりとなる大気成分など、より詳しい調査を行う予定だとしている。また各地の観測所と提携して惑星に水やメタン分子が存在するのかどうかの調査も計画しているとのこと。

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世界中の関係者が胸を踊らせているらしいこの大発見だが、地球から近いというこの40光年の距離、およそ380兆km(1光年=9.5兆kmで計算)。まったく想像もつかないような距離感だが、到達までに何百万年もかかるというから、正に天文学的数字である。

我々が生きているうちに人類はどこまで宇宙の謎に迫れるのか? 同時に我々はどこまで理解度を高められるのか? 個人的にも挑戦したいところである。

(文=Maria Rosa.S

参考:「Daily Mail」、ほか

参照元 : TOCANA


Are aliens living just 40 light-years away? Astronomers 'hit the jackpot' by finding THREE Earth-sized habitable worlds bathed in 'eerie red light' around a nearby star

・Astronomers have spotted three Earth-sized planets orbiting a dwarf star
・The miniature planetary system is just 40 light-years away from Earth
・At least one of these worlds may be not too hot, not too cold, but 'just right'
・Scientists said they may have 'hit the jackpot' in the search for life

By RUSS SWAN FOR MAILONLINE
PUBLISHED: 15:00 GMT, 2 May 2016 | UPDATED: 09:58 GMT, 3 May 2016

They're not too hot, not too cold, and may provide just the right conditions for life to form. Astronomers have found three Earth-sized worlds within the so-called habitable 'Goldilocks' zone of a nearby star. The scientists believe at least one of these 'three bears' planets may be 'just right' for life to get going, giving the search for life elsewhere in the universe a significant boost.

The star is around 40 light-years away. By comparison Mars is, on average and based on its orbit, 12.5 light-minutes away. The international team of astronomers that found the planets said they are orbiting an ultracool brown dwarf star about one-eighth the size of our sun called 2MASS J23062928-0502285. Also known as Trappist-1, this dim and cool star fades slightly at regular intervals, indicating several objects are passing between the star and the Earth.

Detailed analysis has now shown that three planets with similar sizes to the Earth are present. Trappist-1 is much cooler and redder than the sun and barely larger than Jupiter. It lies in the constellation of Aquarius.
Such stars are both very common in the Milky Way and are very long-lived, but this is the first time planets have been found around one of them. Despite being so close to the Earth, this star is too dim and too red to be seen with the naked eye or even visually with a large amateur telescope.



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The international team of astronomers that found the planets said they are orbiting an ultracool brown dwarf star about one-eighth the size of our sun called 2MASS J23062928-0502285, also known as Trappist-1. This artist's impression shows how the star looks from the surface of one of the newly-found planets

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Trappist-1 is much cooler and redder than the sun and barely larger than Jupiter. Such stars are common in the Milky Way but this is the first time planets have been found around one. The trio of exoplanets is up to 100 times closer to their star than the Earth is to the sun (illustrated) and all three could be habitable

According to Michael Gillon of the University of Liege, Belgium, the planetary system looks more like moons orbiting Jupiter than our conventional idea of a solar system. 'The planets are 20 to 100 times closer to their star than Earth is to the sun,' he explained. 'This is more similar in scale to the system of Jupiter's moons than the solar system.' The team used a modest 60cm telescope, based in Chile, to find the planets. The astronomers believe all three may be habitable, and they are now turning larger instruments on these new worlds to find out more. While other exoplanet searches have concentrated on bright stars, Gillon and his team set up the Trappist survey specifically to scan the 60 nearest dwarf stars. 'Systems around these tiny stars are the only places where we could detect life on Earth-sized exoplanets with our current technology' he said. 'So this is where we should start to look.'

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In September, Trappist-1 was seen to fade slightly at regular intervals, showing that several objects were passing in front of it. The current position of the star (bottom) is shown by a red cross. Its relative position in 1953 (top) and 1998 (middle) is indicated by a red circle to show its orbit compared to its current position

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In their report, the astronomers said all three planets are likely tidally locked to their dim star, with hemispheres of permanent night and day. Other exoplanet searches have concentrated on bright stars while the latest study specifically sscans our 60 nearest dwarf stars. Stock image

In September 2015, Trappist-1 - was seen to fade slightly at regular intervals, showing that several objects were passing in front of it.
Further observations confirmed these were Earth-sized planets. The two innermost planets orbit in 1.5 and 2.4 days, while the third is less certain and might be anything between four and 73 days. The team believes this third 'Daddy Bear' planet is the most likely to be habitable out of the three. In their report, published today in the journal Nature, the astronomers said all three planets are likely tidally locked to their dim star, with hemispheres of permanent night and day. For the inner two, this might restrict any life-friendly sweet spots to the 'twilight zone'. But on the third, the whole planet may be able to support life. Because this solar system is relatively close, it should be possible to detect whether the planets have an atmosphere, and even to detect signs of life. 'We can study their atmosphere and composition, and within our generation, assess if they are actually inhabited,' added co-author Julien de Wit, an astronomer at MIT. He added: 'All of these things are achievable, and within reach now. This is a jackpot for the field.'



参照元 : Daily Mail