ハッブル、踊る影の中に隠された生まれたばかりの惑星発見

見えない新生惑星が若い星の周りにほこりを起こしている

私たちの宇宙はとても気まぐれで、時々呼吸をするのが大好きです。 2017年、天文学者たちは、近くの若い星TW Hydraeを囲んでいるほこりやガスの円盤を横切る巨大な影を見て驚きました。 シャドウは、外側ディスクの平面に対してわずかに傾いたほこりやガスの内側ディスクによって落ちます。 影はシステムが地球に向かうように傾いているので、天文学者は影が時計の周りを動く手のように円盤の周りを散らすときに円盤の鳥瞰図を提供するので、明確に見ることができます。

ただし、時計には異なる速度で回転する2つの針（時と分）があります。 そしてTW Hydraeも同様です。 天文学者は、2つの外部ディスクに対して傾斜した別の内部ディスクに現れる2番目の影を見つけるためにハッブルを使用しました。 したがって、互いにわずかに傾斜した少なくとも3つのネストされたディスクは、システムがますます複雑に見えます。 ディスクは、星の周りの目に見えない惑星のためのプロキシです。 各惑星は星の近くの物質を重力によって引き付けており、惑星がなければ完璧に平らなパンケーキ状のディスクを曲げます。 私たちの太陽系の惑星は互いに数度傾いた軌道面を持っているので、これは驚くべきことではありません。 TW Hydraeは天文学者に、私たちの太陽系が形成されている間にどのように見えるかを見るためのリングサイドスポットを提供します。

数年間隔で撮影されたハッブル宇宙望遠鏡の比較画像は、若い星TW Hydraeを囲むガスとほこりの円盤を横切って反時計回りに動く2つの不気味な影を発見しました。 ディスクは地球に向かって傾斜しており、天文学者に星の周りで何が起こるのかについての鳥瞰図を提供します。 2016年に撮影された左の画像は、1つの影だけを示しています。 [A] 11時位置に。 この影は、外側のディスクにわずかに傾いて、星の光を遮断する内側のディスクによって落ちます。 左の図は、別のネストされたディスクに表示された2番目の影を示しています。 [C] 2021年の写真のように7時位置にあります。 元の内部ディスクが表示されます。 [B] この後で見る。 影は時計針のように異なる速度で星の周りを回転します。 彼らはほこりを軌道に引き寄せた2つの目に見えない惑星の証です。 これはお互いに少し傾いています。 これは、宇宙望遠鏡イメージングスペクトロメーターで撮影した可視光線の写真です。 詳細を改善するために人工色が追加されました。 出典：NASA、ESA、STScI、John Debes（ESA用AURA / STScI）、Joseph DePasquale（STScI）

ハッブル宇宙望遠鏡は、惑星形成ディスクの周りの影の演劇に従います。

若い星TW Hydraeは科学者たちがそれを観察する「影人形劇」をしています。[{” attribute=””>NASA’s Hubble Space Telescope.

In 2017, astronomers reported discovering a shadow sweeping across the face of a vast pancake-shaped gas-and-dust disk surrounding the red dwarf star. The shadow isn’t from a planet, but from an inner disk slightly inclined relative to the much larger outer disk – causing it to cast a shadow. One explanation is that an unseen planet’s gravity is pulling dust and gas into the planet’s inclined orbit.

Now, a second shadow – playing a game of peek-a-boo – has emerged in just a few years between observations stored in Hubble’s MAST archive. This could be from yet another disk nestled inside the system. The two disks are likely evidence of a pair of planets under construction.

TW Hydrae is less than 10 million years old and resides about 200 light-years away. In its infancy, our solar system may have resembled the TW Hydrae system, some 4.6 billion years ago. Because the TW Hydrae system is tilted nearly face-on to our view from Earth, it is an optimum target for getting a bull’s-eye-view of a planetary construction yard. 

The second shadow was discovered in observations obtained on June 6, 2021, as part of a multi-year program designed to track the shadows in circumstellar disks. John Debes of AURA/STScI for the European Space Agency at the Space Telescope Science Institute in Baltimore, Maryland, compared the TW Hydrae disk to Hubble observations made several years ago.

“We found out that the shadow had done something completely different,” said Debes, who is principal investigator and lead author of the study published in The Astrophysical Journal. “When I first looked at the data, I thought something had gone wrong with the observation because it wasn’t what I was expecting. I was flummoxed at first, and all my collaborators were like: what is going on? We really had to scratch our heads and it took us a while to actually figure out an explanation.” 

The best solution the team came up with is that there are two misaligned disks casting shadows. They were so close to each other in the earlier observation they were missed. Over time they’ve now separated and split into two shadows. “We’ve never really seen this before on a protoplanetary disk. It makes the system much more complex than we originally thought,” he said.

The simplest explanation is that the misaligned disks are likely caused by the gravitational pull of two planets in slightly different orbital planes. Hubble is piecing together a holistic view of the architecture of the system.

The disks may be proxies for planets that are lapping each other as they whirl around the star. It’s sort of like spinning two vinyl phonograph records at slightly different speeds. Sometimes labels will match up but then one gets ahead of the other.

“It does suggest that the two planets have to be fairly close to each other. If one was moving much faster than the other, this would have been noticed in earlier observations. It’s like two race cars that are close to each other, but one slowly overtakes and laps the other,” said Debes.

The suspected planets are located in a region roughly the distance of Jupiter from our Sun. And, the shadows complete one rotation around the star about every 15 years – the orbital period that would be expected at that distance from the star. 

Also, these two inner disks are inclined about five to seven degrees relative to the plane of the outer disk. This is comparable to the range of orbital inclinations inside our solar system. “This is right in line with typical solar system style architecture,” said Debes. 

The outer disk that the shadows are falling on may extend as far as several times the radius of our solar system’s Kuiper belt. This larger disk has a curious gap at twice Pluto’s average distance from the Sun. This might be evidence for a third planet in the system.

Any inner planets would be difficult to detect because their light would be lost in the glare of the star. Also, dust in the system would dim their reflected light. ESA’s Gaia space observatory may be able to measure a wobble in the star if Jupiter-mass planets are tugging on it, but this would take years given the long orbital periods.

The TW Hydrae data are from Hubble’s Space Telescope Imaging Spectrograph. The James Webb Space Telescope’s infrared vision may also be able to show the shadows in more detail.

Reference: “The Surprising Evolution of the Shadow on the TW Hya Disk” by John Debes, Rebecca Nealon, Richard Alexander, Alycia J. Weinberger, Schuyler Grace Wolff, Dean Hines, Joel Kastner, Hannah Jang-Condell, Christophe Pinte, Peter Plavchan and Laurent Pueyo, 4 May 2023, The Astrophysical Journal.
DOI: 10.3847/1538-4357/acbdf1

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.