Astronomers discover possible planet born from a dead star’s ashes

Snehalata Sahu/University of Warwick

In a cosmic first, scientists say they have detected a planet that may have formed from the burned remains of its dead host star.

The planet is likely a gas giant orbiting a white dwarf called HS 0209+0832 located about 270 light-years from Earth , according to the authors of a study published Monday in the journal Nature Astronomy. After analyzing observations from NASA’s Hubble Space Telescope and other instruments, the team found chemical evidence that suggests the planet could have condensed from material expelled by the star as it fizzled out of energy and collapsed into a white dwarf.

When a sunlike star dies, it first rapidly expands into a red giant and then sheds its outer layers, leaving behind a much smaller but dense, hot core — known as a white dwarf. Some of the first-generation planets, those that originally formed at the same time as the star, can survive this cataclysmic event if their orbit is wide enough. Astronomers have observed a handful of such first-generation survivors revolving around white dwarfs, but never before have they recorded a so-called second-generation planet — an entirely new world made from a dead star’s debris.

NASA/ESA/CSA/R. Crawford (STScI)

Planet found orbiting a dead star could preview what will happen to our solar system

The telltale sign that HS 0209+0832 might have birthed a new planet is unusual traces of heavy elements on the white dwarf’s surface that the study authors believe is planetary material raining down onto the leftover core. “This planetary material is very rich in an element called niobium,” said lead study author Jamie Williams, a doctoral student in the department of physics of the University of Warwick in England. “It’s the first time that this element is found in a white dwarf, and this implies that the planetary material is made from the ashes of the star as it died.”

Further analysis of observations from NASA’s Transiting Exoplanet Survey Satellite or TESS revealed a faint brightness signal repeating every 4.4 days — evidence that the researchers say is consistent with a giant planet orbiting the white dwarf.

“What’s interesting about planets orbiting close to white dwarfs is that because white dwarfs cool over time, their habitable zone is very stable. A second-generation planet could form and then be in the habitable zone for tens of billions of years,” Williams said.

Such a large period of stability could provide more ideal conditions for life, but more work is required to validate the second-generation planet discovery, he added. “It’s not a confirmed planet,” Williams said. “It’s only a candidate for now.”

After the red giant phase when a dying star quickly puffs up, it becomes a nuclear furnace that can produce various elements. However, once that fuel runs out and the star transforms into a white dwarf, the heavy elements — including nobium — sink downward rapidly, leaving only lighter elements such as hydrogen and helium on the surface. The researchers determined that the niobium detected on HS 0209+0832 must therefore come from the star’s surroundings. “We think these elements fell onto the white dwarf’s surface because the white dwarf is very hot and emitting loads of extreme ultraviolet radiation, which is stripping the atmosphere of a nearby planet,” Williams said.

The idea of second-generation planets has existed for at least as long as astronomers have known about exoplanets, said study coauthor David J. Wilson, a research associate at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.

Scientists detected the very first exoplanets around a pulsar — a type of rapidly rotating dead star — and because pulsars form as a result of massive explosions called supernovas, these planets are likely second-generation, as any original planets would have been destroyed by the immense blast, Wilson explained.

“Although white dwarfs don’t form via supernova, they are surrounded by debris,” he said via email. “You have the planets that were lucky enough to survive the star’s giant phases, the shattered remains of those that weren’t, and leftover gas and dust ejected by the star as it turned from a giant to a white dwarf. So it’s a compelling idea that all that stuff might coalesce into new planets.”

Astronomers still don’t know all the details about how first-generation planets form, which means the potential mechanisms behind this newly reported second-generation planet are even more uncertain. The planet-forming process might have derived from a collision between the dying star and a second celestial body, according to Williams. “There could have been another object close to the core of the star, maybe a star around 20% the mass of the sun, or a brown dwarf,” said Williams, referring to a class of cool objects that are an intermediate between a star and a planet.

If a second object fell into the star during its giant phase, it could have prevented all the dust and gas from spreading out into a massive cloud, making some of it spin in a disk. This disk would then have continued to rotate around the white dwarf, birthing a planet. “In a regular white dwarf there’s no disc, because the star’s material will just be ejected outwards during the red giant phase,” Williams added.

K. Miller, Caltech/IPAC

Astronomers discover never-before-seen two-faced star

The researchers plan to use Hubble and NASA’s Chandra X-ray Observatory to observe the system again over the next year, but they also requested time with the more powerful James Webb Space Telescope, which could provide more clues about the existence or features of the planet.

If confirmed, this distant world would belong to an entirely new planetary class and suggest that more second-generation planets could exist around white dwarfs, Williams said.

The discovery would also have implications for our own cosmic neighborhood, Wilson said. “The sun will eventually become a white dwarf, so we’re also looking at the future of the solar system here — maybe the Sun will get a new planet someday!”

The new discovery of a potential second-generation planet is incredibly exciting, said Sarah Casewell, a lecturer at the School of Physics and Astronomy of the University of Leicester, England, who was not involved with the study.

“We know of a large number of white dwarfs that are polluted by planetary material which is similar in composition to rocks within our solar system,” she wrote in an email. “However, in this case, the white dwarf is polluted by incredibly unusual material and the composition of this material is similar to atoms we see being created as stars end their lives.”

More than 95% of all stars in the universe will eventually become white dwarfs, and astronomers have already cataloged hundreds of thousands of them. Finding a planet that has formed after the white dwarf would mean that second-generation planets around other white dwarfs are possible — and that after our sun dies, new planets could emerge in our own solar system, Casewell said.

NASA/ESA/CSA/STScI/Ralf Crawford (STScI)

Radio signal detected for the first time from a planet outside our solar system

Previous observations of white dwarfs have shown that some planetary systems survive a star’s self-destruction, but the recent findings open the possibility of the genesis of new planets, said Amy Bonsor, an associate professor at the University of Cambridge in England, who also did not participate in the work. “This paradigm shift allows us to consider whether in the future we could characterise or investigate the potential habitability of these second-generation planetary systems,” she wrote in an email.

This discovery would also add more evidence to research that has shown planetary systems are varied and complex, even after the host star dies, according to Susan Mullally, a mission scientist at the Space Telescope Science Institute, the science operations center for Hubble.

“Some planets may survive the death of the star, others may be tossed into space or eaten by the star. If second-generation planets are possible, then many more white dwarf stars will have planets than we may otherwise expect,” Mullally, who was not part of the study, wrote in an email. “This evidence of a second-generation exoplanet indicates there may be more to the afterlives of exoplanets than previously expected.”

Snehalata Sahu/华威大学

科学家称，他们首次探测到一颗行星，这颗行星可能是由其死亡宿主恒星燃烧后的残骸形成的。

根据周一发表在《自然·天文学》杂志上的一项研究，这颗行星很可能是一颗围绕着一颗名为HS 0209+0832的白矮星运行的气态巨行星，该白矮星距离地球约270光年。研究团队分析了来自美国宇航局哈勃太空望远镜和其他仪器的观测数据后，发现了化学证据，表明这颗行星可能是由这颗恒星在能量耗尽并坍缩成白矮星时喷射出的物质凝聚而成的。

当一颗类似太阳的恒星死亡时，它首先会迅速膨胀成一颗红巨星，然后抛射出外层物质，留下一个体积小得多但密度极高、温度极高的核心——即白矮星。一些第一代行星，也就是与恒星同时形成的行星，如果它们的轨道足够宽，就能在这场灾难性的事件中幸存下来。天文学家已经观测到一些围绕白矮星运行的第一代幸存行星，但此前从未记录到所谓的第二代行星——一个由死亡恒星残骸形成的全新世界。

NASA/ESA/CSA/R. Crawford（STScI）

发现的围绕一颗死亡恒星运行的行星或许预示着我们太阳系的未来走向

HS 0209+0832可能孕育了一颗新行星的蛛丝马迹，是这颗白矮星表面存在异常的重元素痕迹。研究人员认为，这些痕迹是行星物质降落到其残存核心上形成的。该研究的主要作者、英国华威大学物理系博士生杰米·威廉姆斯表示：“这些行星物质富含铌元素。这是首次在白矮星中发现铌元素，这意味着这些行星物质是由这颗恒星死亡时的残骸形成的。”

对美国宇航局凌日系外行星巡天卫星（TESS）观测结果的进一步分析显示，每隔 4.4 天会重复出现一个微弱的亮度信号——研究人员表示，这一证据与一颗围绕白矮星运行的巨行星相符。

威廉姆斯说：“围绕白矮星运行的行星的有趣之处在于，由于白矮星会随着时间推移而冷却，它们的宜居带非常稳定。第二代行星可以形成，然后在宜居带内存在数百亿年。”

他补充说，如此长的稳定期可能为生命提供更理想的条件，但还需要更多工作来验证第二代行星的发现。“它还不是一颗已确认的行星，”威廉姆斯说，“目前它只是一个候选行星。”

在红巨星阶段，一颗垂死的恒星会迅速膨胀，变成一个能够产生各种元素的核反应堆。然而，一旦燃料耗尽，恒星演变成白矮星，包括铌在内的重元素就会迅速下沉，只留下氢和氦等较轻的元素留在表面。研究人员由此推断，在HS 0209+0832上探测到的铌必定来自这颗恒星周围的环境。“我们认为这些元素之所以落到白矮星表面，是因为白矮星温度极高，并释放出大量的极紫外线辐射，这些辐射正在剥离附近行星的大气层，”威廉姆斯说道。

该研究的合著者、科罗拉多大学博尔德分校大气与空间物理实验室的研究员大卫·J·威尔逊表示，第二代行星的概念至少与天文学家了解系外行星的时间一样长。

科学家们探测到了围绕脉冲星（一种快速旋转的死亡恒星）的第一批系外行星。威尔逊解释说，由于脉冲星是由被称为超新星的巨大爆炸形成的，这些行星很可能是第二代行星，因为任何原始行星都会被巨大的爆炸摧毁。

“虽然白矮星并非由超新星爆发形成，但它们周围却环绕着大量碎片，”他通过电子邮件表示。“这些碎片包括那些幸运地在恒星巨星阶段幸存下来的行星、那些未能幸存的行星破碎后的残骸，以及恒星从巨星转变为白矮星时喷射出的剩余气体和尘埃。因此，所有这些物质有可能凝聚成新的行星，这是一个很有说服力的想法。”

天文学家至今仍未完全了解第一代行星的形成过程，这意味着这颗新发现的第二代行星背后的潜在机制更加扑朔迷离。威廉姆斯认为，这颗行星的形成过程可能源于垂死恒星与另一颗天体的碰撞。“在恒星核心附近可能存在另一个天体，也许是一颗质量约为太阳20%的恒星，或者是一颗褐矮星，”威廉姆斯说道。褐矮星是一类介于恒星和行星之间的低温天体。

如果第二个天体在恒星巨星阶段落入其中，它可能会阻止所有尘埃和气体扩散成巨大的云团，使其中一部分物质围绕白矮星旋转形成一个圆盘。这个圆盘会继续围绕白矮星旋转，最终孕育出一颗行星。“普通的白矮星没有圆盘，因为恒星的物质会在红巨星阶段被抛射出去，”威廉姆斯补充道。

K. Miller，加州理工学院/IPAC

天文学家发现一颗前所未见的双面星

研究人员计划在未来一年内使用哈勃望远镜和美国宇航局的钱德拉X射线天文台再次观测该系统，但他们也请求使用功能更强大的詹姆斯·韦伯太空望远镜进行观测，这可能会提供更多关于该行星是否存在或特征的线索。

威廉姆斯说，如果得到证实，这个遥远的世界将属于一个全新的行星类型，并表明白矮星周围可能存在更多的第二代行星。

威尔逊说，这一发现也将对我们所在的宇宙邻域产生影响。“太阳最终会变成一颗白矮星，所以我们也在展望太阳系的未来——也许有一天太阳会拥有一颗新的行星！”

英国莱斯特大学物理与天文学院讲师莎拉·卡斯韦尔（Sarah Casewell）表示，发现一颗潜在的第二代行星令人无比兴奋，她本人并未参与这项研究。

她在邮件中写道：“我们知道有很多白矮星都受到行星物质的污染，这些物质的成分与我们太阳系内的岩石相似。然而，就这颗白矮星而言，它受到的污染物质极其特殊，而且这种物质的成分与我们观测到的恒星生命终结时产生的原子相似。”

宇宙中超过95%的恒星最终都会变成白矮星，天文学家已经记录了数十万颗白矮星。卡斯韦尔说，如果能发现一颗在白矮星之后形成的行星，就意味着其他白矮星周围存在第二代行星——也就是说，在我们太阳死亡之后，我们自己的太阳系中也可能出现新的行星。

NASA/ESA/CSA/STScI/Ralf Crawford (STScI)

首次探测到来自太阳系外行星的无线电信号

此前对白矮星的观测表明，一些行星系统能够在恒星自燃后幸存下来，但最近的发现开启了新行星诞生的可能性。英国剑桥大学副教授艾米·邦索尔（Amy Bonsor）表示，她并未参与这项研究。她在电子邮件中写道：“这种范式转变使我们能够思考，未来是否能够对这些第二代行星系统进行表征或研究其潜在的宜居性。”

据哈勃太空望远镜科学运行中心——太空望远镜科学研究所的任务科学家苏珊·穆拉利称，这一发现也将为研究提供更多证据，这些研究表明，即使在宿主恒星死亡之后，行星系统仍然是多样化和复杂的。

“有些行星或许能在恒星死亡后幸存下来，有些则可能被抛入太空或被恒星吞噬。如果第二代行星确实存在，那么白矮星拥有行星的数量将远超我们的预期，”穆拉利（他并未参与这项研究）在一封电子邮件中写道。“第二代系外行星的发现表明，系外行星的‘死后生命’可能比我们之前预想的更加丰富多彩。”