白色矮星を公転する巨大惑星、８２光年先で初めて発見

【ロンドンＡＦＰ＝時事】白色矮星（わいせい）となった恒星の近くを原形を保ったまま周回する系外惑星が初めて発見された。研究結果が16日、発表された。見つかった惑星「WD　1856b」は木星ほどの大きさがあり、研究者らは、この巨大惑星の運命をたどることにより、約50億年を経て太陽が白色矮星になる時の太陽系の姿を予想することができるとしている。（写真は系外惑星探査衛星「TESS」の想像図。米航空宇宙局提供。資料画像）
　太陽のような恒星は、内部の水素が燃え尽きると「死の段階」に入る。最初に大きく膨張して光り輝く赤色巨星となり、近くにある惑星は膨張する赤色巨星にのみ込まれてしまう。
　その後、赤色巨星は崩壊して縮小し、燃え尽きた中心核のみが残る。これが白色矮星だ。熱エネルギーの残りでかすかな光を放つこの超高密度の星の燃えさしは、数十億年かけてゆっくりと光を失っていく。
　今回の発見では、縮小した主星「ＷＤ　１８５６＋５３４」の前を１．４日ごとに通過する惑星ＷＤ　１８５６ｂの様子を米航空宇宙局（ＮＡＳＡ）の系外惑星探査衛星「ＴＥＳＳ（テス）」が捉えた。惑星ＷＤ　１８５６ｂは、縮小した主星の約１０倍の大きさを誇るという。
　主星が赤色巨星期に入ると、近くに位置する惑星が残存する可能性はほぼなくなるとこれまでは考えられていた。太陽系では、太陽が赤色巨星になると、水星や金星、そしておそらく地球ものみ込まれてしまうことが予想されている。
　英科学誌ネイチャーで発表された研究でチームを率いた米ウィスコンシン大学マディソン校のアンドリュー・バンダーバーグ助教は、「当初は主星からはるか遠くの軌道を周回していたが、主星が白色矮星となった後に何らかの形で内側に移動してきたことを研究結果は示唆している」とＷＤ　１８５６ｂについて説明する。
　研究では、さまざまなシナリオでシミュレーションを行った。その結果、ＷＤ　１５８６ｂは他の惑星との相互作用によって、主星に近接する軌道へと投入された可能性があると分かった。
　今回の研究には参加していない英シェフィールド大学のスティーブン・パーソンズ氏は、研究の解説記事の中で、「今後この恒星系内で新たな惑星が検出されるというような非常に興味深い展望をもたらしている」と今回の発見について話す。
　また、白色矮星ＷＤ　１８５６＋５３４は地球からわずか８２光年の距離にあるため、他の惑星がＷＤ　１８５６＋５３４に及ぼす重力効果を宇宙望遠鏡による探査で検出できる可能性についても触れた。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/09/17-15:23）

Astronomers have discovered a planet the size of Jupiter closely orbiting the smouldering remains of a dead star, the first time that an intact exoplanet has been discovered travelling around a white dwarf, according to research published Wednesday.
Researchers said the fate of this giant planet, called WD 1586 b, offers a potential vision of our own Solar System when the Sun eventually ages into a white dwarf in around five billion years.
When it has burned through its stores of hydrogen, a star like the Sun enters its death-throes, first swelling enormously into an incandescent red giant that scorches and engulfs nearby planets.
Then it collapses, reducing it to its burnt-out core.
This is the white dwarf, an extremely dense stellar ember glowing faintly with leftover thermal energy and slowly fading over billions of years.
Previous research has suggested some white dwarfs can retain more distant remnants of their planetary systems.
But until now no intact planets had been detected in orbit around one of the dead stars.
The discovery came as something of a surprise, said Andrew Vanderburg, assistant professor at the University of Wisconsin-Madison, who led the research published in Nature.
A previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid.
The planet is around ten times bigger than its shrunken parent star, known as WD 1856+534.
It was detected sweeping past the white dwarf every 1.4 days using NASA's Transiting Exoplanet Survey Satellite (TESS).
Siyi Xu, an assistant astronomer at the US National Science Foundation's Gemini Observatory, said that because there was no discernible debris from the planet around the star, researchers concluded it was intact.
We've had indirect evidence that planets exist around white dwarfs and it's amazing to finally find a planet like this, said Xu, in a statement from NSF's NOIRLab.
The discovery suggests that planets can end up in or near the white dwarf's habitable zone, and potentially be hospitable to life even after their star has died, the statement said.
- 'Tantalising' -
What remains a mystery however is how the planet got so close to the white dwarf.
It is thought that the red giant phase makes it unlikely that nearby planets will survive -- when the same process happens to our Sun, Venus and Mercury would be expected to be engulfed and possibly the Earth as well.
Our discovery suggests that WD 1856b must have originally orbited far away from the star, and then somehow journeyed inwards after the star became a white dwarf, said Vanderburg.
Now that we know that planets can survive the journey without being broken up by the white dwarf's gravity, we can look for other, smaller planets.
After simulating various scenarios, the authors suggest that WD 1586b might have been thrown into a close orbit due to interactions with other planets.
In an independent commentary on the discovery, Steven Parsons of Sheffield University, said the discovery offers the tantalizing prospect of detecting additional planets in this system in the future.
Current estimates are that around one-third of all Sun-like stars host planetary systems, while the Milky Way contains around ten billion of these stars, he said.
The white dwarf WD 1856+534 is just 82 light years from Earth, so Parsons said gravitational effects of other planets on the white dwarf would potentially be detectable by space observatory missions.
This system therefore opens up an entirely new field of exoplanetary research, he added.