A Jupiter-size planet that escaped its star’s death

一颗逃脱恒星死亡命运的木星大小行星

WD 1856 b is the only confirmed case of a planet that survived the death of a Sun-like star. It’s a Jupiter-size world orbiting a white dwarf—the burned-out remnant of a Sun-like star. Now, a team of astronomers has used the James Webb Space Telescope to take a closer look at this planet for the first time, and what they found makes an already strange system even stranger. WD 1856 b 是目前唯一被证实的、在类太阳恒星死亡后幸存下来的行星案例。它是一颗围绕白矮星（类太阳恒星燃尽后的残骸）运行的木星大小的世界。现在，一个天文学家团队首次利用詹姆斯·韦伯空间望远镜（JWST）对这颗行星进行了深入观测，而他们的发现让这个本就奇特的系统变得更加诡异。

A feeding frenzy

疯狂的探索

WD 1856 b was an accidental discovery. Astronomers pointed the TESS observatory at a sample of roughly 2,000 white dwarfs in 2020. These stars are the remains of a Sun-like star that have already gone through a red-giant phase, leaving behind an Earth-size body that’s primarily composed of elements like carbon and oxygen. The TESS team was searching for small objects like comets or asteroids that might transit across the face of these dead stars. What they found in the WD 1856 system was a gas giant. WD 1856 b 的发现纯属偶然。2020 年，天文学家将 TESS 望远镜对准了约 2,000 颗白矮星样本。这些恒星是类太阳恒星经历红巨星阶段后的残骸，留下的核心大小与地球相当，主要由碳和氧等元素组成。TESS 团队当时正在寻找可能穿过这些死星表面的小天体，如彗星或小行星。然而，他们在 WD 1856 系统中发现的却是一颗气态巨行星。

“As soon as they looked at it, they said, okay, that’s weird,” said Christopher O’Connor, a theoretical astrophysicist at Cornell University and co-author of the recent Nature study on WD 1856 b. The white dwarf is about seven times smaller than the gas giant circling around it. Its brightness should be dropping to nearly nothing each time the planet crosses in front of it, but instead it’s dipping by about half. O’Connor thinks the reason is a grazing transit, where only the edge of the planetary disk clips the face of the star. “That’s a very unlikely viewing angle,” he said, “but it’s the only way to explain what we actually see.” “他们一看到它，就说：好吧，这太奇怪了，”康奈尔大学理论天体物理学家、近期发表在《自然》杂志上关于 WD 1856 b 研究的合著者克里斯托弗·奥康纳（Christopher O’Connor）说道。这颗白矮星比环绕它的气态巨行星小约七倍。按理说，每当行星从恒星前方经过时，恒星的亮度应该几乎降至零，但实际上亮度只下降了一半左右。奥康纳认为原因是“掠食式凌日”（grazing transit），即只有行星圆盘的边缘掠过了恒星表面。“这是一个极不可能的观测角度，”他说，“但这是解释我们所见现象的唯一方式。”

What’s more, the planet orbits at about 0.02 AU from the white dwarf, which goes against our ideas of how the death of a star should reshape its system. “When the star expands to become a red giant, it consumes the inner planets,” O’Connor explains. Then, in the process of shrinking down to a white dwarf, it loses about half of its original mass, which means its gravitational pull becomes weaker. “The outer planets, like gas giants, should migrate outward by about a factor of two,” O’Connor said. WD 1856 b, though, apparently did not migrate outward. It got closer. 此外，这颗行星距离白矮星仅约 0.02 个天文单位（AU），这违背了我们关于恒星死亡如何重塑其系统的认知。“当恒星膨胀成为红巨星时，它会吞噬内侧行星，”奥康纳解释道。随后，在收缩成白矮星的过程中，它会损失约一半的原始质量，这意味着其引力会变弱。“外侧行星，比如气态巨行星，应该会向外迁移约两倍的距离，”奥康纳说。然而，WD 1856 b 显然没有向外迁移，反而靠得更近了。

The discovery immediately has the science community buzzing. “It sent theoretical astrophysicists into a feeding frenzy,” O’Connor said. “When you find something that’s totally bizarre, totally in the wrong place, totally unexpected from any previous way of thinking about things—that’s the Universe inviting us to get creative.” First, though, scientists needed more data to get creative with, so O’Connor’s team booked time on the James Webb Space Telescope to take a closer look at what was going on in the WD 1856 system. 这一发现立即在科学界引起了轰动。“这让理论天体物理学家们陷入了疯狂，”奥康纳说。“当你发现一些完全怪异、位置完全错误、且完全超出以往任何思维定式的东西时，这就是宇宙在邀请我们发挥创造力。”不过，科学家们首先需要更多数据来激发创造力，因此奥康纳的团队预定了詹姆斯·韦伯空间望远镜的观测时间，以更深入地了解 WD 1856 系统中究竟发生了什么。

Eight minutes of light

八分钟的光芒

The JWST observations were done on April 27, 2023, and captured a single transit that lasted just eight minutes. The viewing angle and the unusual size mismatch between the star and its planet posed an immediate technical problem. Standard exoplanet transmission spectroscopy assumes a smaller planet is entirely silhouetted against the face of a much larger star, which was not the case here. To get around it, the team developed new equations to express the transmission spectrum as the time-varying area of the planet overlapping the star’s disk. Then, they modified POSEIDON, software for reconstructing exoplanets’ atmospheres based on JWST data to account for the grazing transit geometry (the software had been developed by Ryan MacDonald, the lead author of the study). JWST 的观测于 2023 年 4 月 27 日进行，捕捉到了一次仅持续八分钟的凌日过程。观测角度以及恒星与行星之间异常的尺寸差异带来了直接的技术难题。标准的系外行星透射光谱法假设较小的行星完全处于较大恒星表面的剪影中，但这里的情况并非如此。为了解决这个问题，团队开发了新的方程，将透射光谱表示为行星与恒星圆盘重叠的随时间变化的面积。随后，他们修改了用于根据 JWST 数据重建系外行星大气的软件 POSEIDON，以适应这种掠食式凌日的几何结构（该软件由本研究的第一作者瑞安·麦克唐纳开发）。

When the scientists were done crunching numbers, WD 1856 b’s atmosphere proved somewhat surprising. It turned out the planet is shrouded in aerosol hazes, and its atmosphere contains methane. It is also far hotter than the team expected. WD 1856 b apparently emits roughly 25 times more energy into space than it receives from its cooling host star. Even though its star, according to O’Connor, has been dead for about 6 billion years, the planet is glowing. This extraordinary temperature, O’Connor argues, tells us a lot about WD 1856 b’s history. 当科学家们完成数据计算后，WD 1856 b 的大气层结果令人惊讶。事实证明，这颗行星被气溶胶霾所笼罩，且大气中含有甲烷。它也比团队预期的要热得多。WD 1856 b 向太空辐射的能量大约是它从正在冷却的宿主恒星接收到的能量的 25 倍。尽管据奥康纳称，它的恒星已经死亡了约 60 亿年，但这颗行星仍在发光。奥康纳认为，这种异常的温度揭示了许多关于 WD 1856 b 历史的信息。

Running hot

持续高温

“We expected this planet to be roughly as hot as Jupiter, but it wasn’t,” O’Connor said. At about 0.02 AU from a white dwarf that has been cooling for 6 billion years, WD 1856 b should be somewhere between 150 and 200 Kelvin, close to the temperature of Jupiter’s cloud tops. Instead, it is around 400 Kelvin. “Whatever is causing this planet to glow, it must be an internally derived heat rather than just re-radiating energy from the star,” O’Connor said. The planet, according to the team, cannot be radiating warmth left over from its formation. Something must have heated it at some point. “我们原以为这颗行星的温度大约和木星相当，但事实并非如此，”奥康纳说。在距离一颗已经冷却了 60 亿年的白矮星约 0.02 个天文单位的地方，WD 1856 b 的温度应该在 150 到 200 开尔文之间，接近木星云顶的温度。然而，它却在 400 开尔文左右。“无论是什么导致这颗行星发光，那一定是一种内部产生的热量，而不仅仅是重新辐射恒星的能量，”奥康纳说。团队认为，这颗行星不可能是在辐射其形成时残留的热量。一定有什么东西在某个时刻加热了它。

Working backward through planetary cooling models, the team managed to estimate when it happened. Doing so, the scientists figured out the most probable reason why WD 1856 b got so close to its star. The team initially came up with two competing scenarios to explain how WD 1856 b ended up in its current orbit. The first is a common-envelope model, in which the planet was originally in a close orbit and survived being engulfed when its star expanded into a red giant, emerging from the stellar envelope hot and tight against the remnant core. In the second, a high-eccentricity migration model, the planet started farther out, had its orbit destabilized by gravitational interactions with companion objects (WD 1856 has two distant stellar companions) and then spiraled inward over billions of years through a sequence of highly eccentric plunges. 通过行星冷却模型进行反向推算，团队设法估算出了这一事件发生的时间。通过这种方式，科学家们找出了 WD 1856 b 如此靠近其恒星的最可能原因。团队最初提出了两种相互竞争的方案来解释 WD 1856 b 是如何进入当前轨道的。第一种是“共同包层模型”（common-envelope model），即行星最初处于近轨道，在恒星膨胀为红巨星时幸免于被吞噬，并从恒星包层中以高温状态紧贴残骸核心出现。第二种是“高偏心率迁移模型”（high-eccentricity migration model），即行星最初位于更远的地方，因与伴星（WD 1856 有两颗遥远的伴星）的引力相互作用导致轨道不稳定，随后在数十亿年间通过一系列高偏心率的坠落逐渐向内盘旋。