哈勃发现隐藏在舞动阴影中的新生行星

看不见的新生行星在年轻恒星周围激起尘埃

我们的世界变化无常，有时喜欢玩捉迷藏。 2017 年，天文学家惊讶地看到一个巨大的阴影吞没了附近年轻恒星 TW Hydrae 周围的尘埃和气体盘。 阴影是由略微倾斜于外盘平面的尘埃和气体内盘投下的。 之所以能清楚地看到阴影，是因为该系统在地球上面对面倾斜，让天文学家可以鸟瞰圆盘，因为阴影像指针在时钟上移动一样在圆盘周围飞来飞去。

但是手表有两根指针（小时和分钟），它们以不同的速度扫动。 事实证明，TW Hydrae 也是。 天文学家利用哈勃望远镜发现了从另一个内盘出现的第二个阴影，向两个外盘倾斜。 因此，系统看起来越来越复杂，至少有三个重叠的圆盘彼此略微倾斜。 这些圆盘是恒星周围不可见行星的代表。 每颗行星都利用其引力将恒星附近的物质拉动，并使如果没有行星则完全平坦的薄饼状圆盘变形。 这并不奇怪，因为我们太阳系中的行星的轨道平面的倾角彼此相差几度。 TW Hydrae 为天文学家提供了一个观察我们的太阳系在其形成时期可能是什么样子的座位。

哈勃太空望远镜拍摄的相隔数年的对比图像显示，两个怪异的阴影逆时针移动穿过年轻恒星 TW Hydrae 周围的气体和尘埃盘。 这些圆盘在地球上面对面倾斜，从而使天文学家可以鸟瞰恒星周围发生的事情。 左图摄于 2016 年，只有一个影子 [A] 现在是 11:00。 这个阴影是由略微倾斜于外盘并阻挡星光的内盘投下的。 左图显示了从另一个中间盘出现的第二个阴影 [C] 7:00 a.m.，2021年拍摄。原装内盘标示 [B] 在随后的节目中。 阴影以不同的速率围绕恒星旋转，如顺时针方向。 它们是两颗看不见的行星将尘埃吸入轨道的证据。 这使他们稍微靠在一起。 这是一张用太空望远镜的成像光谱仪拍摄的可见光图像。 添加了合成颜色以增强细节。 图片来源：NASA、ESA、STScI、John Debes (AURA/STScI for ESA)、Joseph DePasquale (STScI)

哈勃太空望远镜跟随围绕行星形成盘的皮影戏

年轻的明星 TW Hydrae 在科学家们的注视下玩“皮影戏”[{” 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.