「浮遊惑星」新たに７個確認 宇宙望遠鏡「ユークリッド」

【パリＡＦＰ＝時事】欧州宇宙機関（ＥＳＡ）が打ち上げた宇宙望遠鏡「ユークリッド」の観測データから、新たに７個の「浮遊惑星」が確認された。ＥＳＡは先週、昨年７月に打ち上げたユークリッドの科学的成果を初めて発表。無数の銀河や星の誕生領域、天の川銀河と同じ渦巻き銀河などの画像が公開された。（写真は、地球程度の質量の氷に覆われた浮遊惑星の想像図。米航空宇宙局〈ＮＡＳＡ〉作成）
　新たに確認された浮遊惑星は巨大ガス惑星で、その質量は木星の４倍以上。約１５００光年の距離にある地球に最も近い星形成領域、オリオン大星雲で見つかった。ユークリッドはまた、過去に検出された浮遊惑星数十個の存在も確認している。
　地球は太陽という恒星の周りを回る惑星だ。だが浮遊惑星は地球とは異なる。恒星の周りを周回することはなく、１日や１年といった周期もない永遠の闇の中にある。
　スペインの天文学者エドゥアルド・マルティン氏は、恒星の光を反射しない浮遊惑星を見つけるのは、「干し草の山から針を見つける」ようなものだとＡＦＰに語った。
　それでも今回ユークリッドが発見したような、若くて高温な浮遊惑星は比較的見つけやすいのだという。
　一部の研究は、浮遊惑星は恒星の２０倍ほどの数が存在しているとしている。これに従えば、地球のある天の川銀河だけでも数兆個の浮遊惑星がある計算になる。
　２０２７年に打ち上げが予定されている米航空宇宙局（ＮＡＳＡ）のローマン宇宙望遠鏡は、さらに多くの浮遊惑星を発見するものと期待されている。
　■生命存在の可能性は？
　浮遊惑星は地球外生命体を探すかぎともなり得る。
　恒星からの熱が存在しない浮遊惑星では、極寒の世界が広がり、表面は凍り付いていると考えられている。そのため、生命を支えるエネルギーが存在するのであれば、それは天体内部から発生しているということになる。
　これは、太陽の光が届かない、地球の熱水噴出孔周辺の状況と似ている。こうした考えについて、英ロンドン大学クイーンメアリー校の天文学者ギャビン・コールマン氏は、最良な条件が整っている場合でも、浮遊惑星の極度に孤立した環境では、細菌や微生物といった生命を支えることしかできないだろうと予想する。
　それでも浮遊惑星の世界は奇妙で「畏怖と神秘の感情」をかき立てるとコールマン氏は言う。
　「私たちは皆、空に太陽があることを当たり前として育ってきたので、地平線の上に他の星が見えず、ただ宇宙空間を漂っている星には興味をそそられる」と同氏はＡＦＰに語った。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2024/05/31-18:19）

The Euclid space telescope has discovered seven more rogue planets, shining a light on the dark and lonely worlds floating freely through the universe untethered to any star.
Without being bound to a star, as the Earth is to the Sun, there are no days or years on these planets, which languish in perpetual night.
Yet scientists believe there is a chance they could be able to host life -- and estimate there may be trillions dotted throughout the Milky Way.
Last week the European Space Agency released the Euclid telescope's first scientific results since the mission launched in July.
Among the discoveries were seven new free-floating planets, gas giants at least four times the mass of Jupiter.
They were spotted in the Orion Nebula, the nearest star-forming region to Earth, roughly 1,500 light years away.
Euclid also confirmed the existence of dozens of other previously detected rogue planets.
Spanish astronomer Eduardo Martin, the lead author of a pre-print study published on arXiv.org Friday, said this was likely just the tip of the iceberg.
Because they do not reflect the light of a star, spotting rogue planets is like finding a needle in a haystack, Martin told AFP.
Younger planets, such as those discovered by Euclid, are hotter, making them a little easier to see.
- 'Awe and mystery' -
Some research has suggested there are around 20 rogue planets for every star, which could put their number in the trillions in our home galaxy alone.
Given there are thought to be hundreds of billions of galaxies across the universe, the potential number of free-floating worlds becomes difficult to fathom.
When NASA's Roman space telescope launches in 2027 it is expected find many more rogue planets, possibly offering clarity about how many could be out there.
Gavin Coleman, an astronomer at the Queen Mary University of London who was not involved in the Euclid research, said these strange worlds often evoked feelings of awe and mystery.
We've all grown up with the Sun in the sky, and so to think of a planet just drifting throughout space with no star on their horizon is fascinating, he told AFP.
But not all rogue planets wander alone. Four of the more than 20 confirmed by Euclid are believed to be binaries -- two planets orbiting each other in a single system.
- Could they host life? -
If rogue planets are habitable, they could be a key target in humanity's search for extraterrestrial life.
Some of our closest neighbours are likely rogue planets, Martin said.
Lacking heat from a nearby star, free-floating planets are believed to be cold, with frozen surfaces.
That means any life-supporting energy would have to come from inside the planet.
Most of Neptune's energy comes from within, Coleman pointed out.
And geothermal vents allow animals to survive on Earth that have never seen the Sun's rays.
But even under the best conditions, this extreme isolation would likely be able to support only bacterial and microbial life, Coleman said.
- Advantage of being alone -
Rogue planets could be thought of as traversing a lonely path through the cosmos.
But being around a star has its downsides, said study co-author Christopher Conselice, professor of extragalactic astronomy at the UK's University of Manchester.
One particular downside comes to mind.
Once the Sun becomes a red giant -- in an estimated 7.6 billion years -- it will greatly expand, swallowing the Earth.
Rogue planets do not have to worry about eventually being destroyed by a star. These things will last forever, Conselice told AFP.
If you don't mind the cold temperatures you could survive on these planets for eternity.
The Euclid study also offered clues to how rogue planets are created, Conselice said.
Some could be formed in the outer part of a solar system before getting detached from their star and floating away.
But the study indicates that many rogue planets may be created as a natural byproduct of the star-formation process, he said.
This suggests a really close connection between stars and planets and how they form, he said.
There's no firm answers yet, he added.