Diligent sleuthing by astronomers has broken open a cold case in the data archive of NASA’s Hubble Space Telescope . In a study published Monday in Nature Astronomy, researchers report uncovering a surprising chemical clue that indicates the white dwarf star HS 0209+0832 may host a second-generation planet.
天文学家的辛勤调查揭开了美国宇航局哈勃太空望远镜数据档案中的一个悬案。在周一发表在《自然天文学》上的一项研究中，研究人员报告发现了一个令人惊讶的化学线索，表明白矮星 HS 0209+0832 可能拥有一颗第二代行星。
A white dwarf is the remnant core of a low-mass star that has burned through all its nuclear fuel and lost its outer envelope of gas and dust to space. A second-generation planet is a world that forms around the stellar remnant from its cast-off material.
白矮星是一颗低质量恒星的残余核心，该恒星已经烧尽了所有核燃料，并失去了气体和尘埃的外壳，进入太空。第二代行星是围绕其废弃物质的恒星残骸形成的世界。
“Rather than the white dwarf stage being a kind of epilogue to the story of a star and its planets, this research points to the systems we are familiar with only being the first chapter of a potentially much longer tale, with some new characters showing up. That’s a really exciting prospect to pursue,” said Jamie Williams, astronomer and lead author, a doctoral candidate at the University of Warwick in the United Kingdom.
“这项研究指出，白矮星阶段并不是恒星及其行星故事的尾声，而是指出我们所熟悉的系统只是一个可能更长的故事的第一章，其中一些新角色出现了。这是一个非常令人兴奋的前景，”天文学家、主要作者、英国华威大学的博士生杰米·威廉姆斯说。
Earth and the other planets in our solar system are first-generation planets, which form from material left over from a star’s birth.
地球和太阳系中的其他行星是第一代行星，它们由恒星诞生时留下的物质形成。
“What Hubble is showing us in this white dwarf system is something we haven’t seen before: a high abundance of the element niobium, the signature of which I was unfamiliar with when I first found it in the archival data,” Williams said.
威廉姆斯说：“哈勃在这个白矮星系统中向我们展示的是我们以前从未见过的东西：铌元素的丰度很高，当我第一次在档案数据中发现它时，我并不熟悉它的特征。”
When Hubble first observed the star in 1999, the data contained roughly 100 chemical features that could not be identified. Williams went back to those records armed with an updated chemical database and found that niobium matched many of the mystery features.
当哈勃在 1999 年首次观测这颗恒星时，数据包含大约 100 种无法识别的化学特征。威廉姆斯利用更新的化学数据库回顾了这些记录，发现铌与许多神秘特征相符。
Williams explained that, while niobium is found in our solar system and has multiple uses on Earth, including in jewelry and medical imaging devices, the amount Hubble found in the HS 0209+0832 system points to a planet forming not from a star’s birth, but from the material ejected as it dies.
威廉姆斯解释说，虽然铌在我们的太阳系中被发现，并且在地球上有多种用途，包括用于珠宝和医学成像设备，但哈勃在 HS 0209+0832 系统中发现的铌含量表明，一颗行星不是由恒星诞生时形成的，而是由恒星死亡时喷射出的物质形成的。
“Niobium and other elements heavier than iron are astronomically special because, unlike many common elements, they are not formed in the cores of stars by thermonuclear fusion,” said Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin – Madison and member of the research team. “Instead, these heavy elements can only be synthesized in the exotic conditions that briefly emerge inside dying stars. The presence of niobium is a signpost of these ‘death’ throes, and the expulsion of the dying star’s innards into space.”
“铌和其他比铁重的元素在天文学上很特殊，因为与许多常见元素不同，它们不是通过热核聚变在恒星核心形成的，”威斯康星大学麦迪逊分校理论天体物理学家、研究小组成员尼古拉斯·斯通说。 “相反，这些重元素只能在垂死恒星内部短暂出现的奇异条件下合成。铌的存在是这些‘死亡’阵痛的路标，也是垂死恒星内部被驱逐到太空的标志。”
Once the star ejected this chemically enriched material, the team theorizes that some of it coalesced into a gas giant planet. The remainder of the ejecta dispersed long ago, but the planet remains.
一旦恒星喷射出这种化学丰富的物质，研究小组就推测其中一些物质会合并成一颗气态巨行星。剩余的喷射物早已消散，但行星仍然存在。
“When Jamie asked me about niobium in relation to this study I was truly gobsmacked, as that element had not been reported in any other white dwarf analyzed to date. Once we realized it was there, everything fell into place,” said astronomer and study co-author Boris Gaensicke, also at the University of Warwick.
“当杰米向我询问铌与这项研究的关系时，我真的大吃一惊，因为迄今为止在任何其他白矮星分析中都没有报道过这种元素。一旦我们意识到它的存在，一切就都水到渠成了，”同样来自华威大学的天文学家、研究合著者鲍里斯·盖恩斯克(Boris Gaensicke)说。
The research team confirmed the Hubble observations with data from NASA’s retired FUSE (Far Ultraviolet Spectroscopic Explorer) mission, which also showed strong signatures of niobium in the HS 0209+0832 system.
研究小组利用美国宇航局退役的 FUSE（远紫外光谱探测器）任务的数据证实了哈勃的观测结果，该任务还显示了 HS 0209+0832 系统中铌的强烈特征。
NASA’s TESS (Transiting Exoplanet Survey Satellite) also observed the white dwarf for four months, allowing it to detect periodic brightness variations that indicate that a planet orbits at a distance of about 3.7 million miles (6 million kilometers), much closer than Mercury orbits the Sun.
NASA 的 TESS（凌日系外行星勘测卫星）也对这颗白矮星进行了四个月的观测，使其能够探测到周期性的亮度变化，这些变化表明一颗行星的轨道距离约为 370 万英里（600 万公里），比水星绕太阳运行的距离要近得多。
The research team estimates the candidate planet is a gas giant about the size of Jupiter that is rapidly losing atmosphere. Because the white dwarf star is relatively new, it is still very hot and likely blasting this planet with energy that is stripping its outer material. This could result in the planet having a comet-like tail of material that would form a disk around the white dwarf star and fall back on to its surface, leading to Hubble detecting the niobium when studying the star. Despite this mass loss, Williams said that the planet is likely not a temporary blip on the cosmic radar.
研究小组估计，候选行星是一颗与木星大小相当的气态巨行星，正在迅速失去大气层。由于白矮星相对较新，因此它仍然非常热，并且可能会用能量爆炸这颗行星，从而剥离其外部物质。这可能会导致这颗行星拥有类似彗星的物质尾部，这些物质会在白矮星周围形成一个圆盘，然后落回其表面，从而导致哈勃在研究这颗恒星时检测到铌。威廉姆斯表示，尽管存在质量损失，这颗行星可能并不是宇宙雷达上的暂时现象。
“If the second-generation planet is there, I think it is likely to survive. Eventually the white dwarf will cool and then maintain a consistent temperature, with the planet in its stable habitable zone for millions of years,” Williams said.
威廉姆斯说：“如果第二代行星在那里，我认为它很可能会幸存下来。最终，白矮星会冷却，然后保持恒定的温度，使行星处于稳定的宜居带数百万年。”
Williams added that there is still a lot of work to do to understand these types of systems — how second-generation planets form, how common or rare they are, and how they evolve in orbit around a “dead” star. He’ll use Hubble to explore these questions for the next several years, hoping to build up substantial data and statistics about these new types of celestial bodies.
威廉姆斯补充说，要了解这些类型的系统还有很多工作要做——第二代行星是如何形成的，它们有多常见或罕见，以及它们如何在围绕“死亡”恒星的轨道上演化。他将在接下来的几年中利用哈勃望远镜探索这些问题，希望建立有关这些新型天体的大量数据和统计数据。
“I think this research is an important example of the fact that scientific discovery is not a straight path,” Gaensicke said. “It often needs that magical moment when people discuss big questions on their minds and realize that together they can find unexpected answers.”
“我认为这项研究是一个重要的例子，说明科学发现并不是一条笔直的道路，”盖恩斯克说。 “当人们讨论心中的重大问题并意识到共同努力可以找到意想不到的答案时，往往需要这样一个神奇的时刻。”
The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
哈勃太空望远镜已经运行了三十多年，并不断做出突破性的发现，塑造我们对宇宙的基本理解。哈勃望远镜是 NASA 和 ESA（欧洲航天局）之间的国际合作项目。美国宇航局位于马里兰州格林贝尔特的戈达德太空飞行中心负责管理望远镜和任务操作。洛克希德·马丁航天公司还支持戈达德的任务运营。位于巴尔的摩的太空望远镜科学研究所由天文学研究大学协会运营，为美国宇航局进行哈勃科学操作。