The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to large uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary processes provide a route to understanding planetary interiors. Planets found in and near the typically barren hot-Neptune ‘desert’ 1 , 2 (a region in mass–radius space that contains few planets) have proved to be particularly valuable in this regard. These planets include HD149026b 3 , which is thought to have an unusually massive core, and recent discoveries such as LTT9779b 4 and NGTS-4b 5 , on which photoevaporation has removed a substantial part of their outer atmospheres. Here we report observations of the planet TOI-849b, which has a radius smaller than Neptune’s but an anomalously large mass of $$39.1{\,}_{-2.6}^{+2.7}$$ 39.1 − 2.6 + 2.7 Earth masses and a density of $$5.2{\,}_{-0.8}^{+0.7}$$ 5.2 − 0.8 + 0.7 grams per cubic centimetre, similar to Earth’s. Interior-structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than $${3.9}_{-0.9}^{+0.8}$$ 3.9 − 0.9 + 0.8 per cent of the total planetary mass. The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation 6 . Although photoevaporation rates cannot account for the mass loss required to reduce a Jupiter-like gas giant, they can remove a small (a few Earth masses) hydrogen and helium envelope on timescales of several billion years, implying that any remaining atmosphere on TOI-849b is likely to be enriched by water or other volatiles from the planetary interior. We conclude that TOI-849b is the remnant core of a giant planet. Observations of TOI-849b reveal a radius smaller than Neptune’s but a large mass of about 40 Earth masses, indicating that the planet is the remnant core of a gas giant.

中文翻译：

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热海王星沙漠中的残余行星核心

巨行星的内部仍然知之甚少。即使对于太阳系中的行星，由于观测困难，行星核心的性质也存在很大的不确定性。经历了罕见进化过程的系外行星为理解行星内部提供了一条途径。在通常贫瘠的热海王星“沙漠”1、2（质量半径空间中包含很少行星的区域）中及其附近发现的行星已被证明在这方面特别有价值。这些行星包括 HD149026b 3 ，它被认为具有异常巨大的核心，以及最近的发现，例如 LTT9779b 4 和 NGTS-4b 5 ，在这些行星上，光蒸发已经去除了大部分外层大气。在这里，我们报告了对 TOI-849b 行星的观测，它的半径小于海王星的半径，但质量异常大，为 39 美元。1{\,}_{-2.6}^{+2.7}$$ 39.1 − 2.6 + 2.7 地球质量和 $$5.2{\,}_{-0.8}^{+0.7}$$ 5.2 − 0.8 +每立方厘米 0.7 克，与地球相似。内部结构模型表明，任何由纯氢和氦组成的气态包层不超过 $${3.9}_{-0.9}^{+0.8}$$ 3.9 − 0.9 + 行星总质量的 0.8%。这颗行星可能在通过热自破坏或巨行星碰撞而经历极端质量损失之前是一颗气态巨行星，或者它可能避免了大量的气体吸积，可能是通过间隙打开或晚期形成 6 。虽然光蒸发率不能解释减少类似木星的气态巨行星所需的质量损失，但它们可以在数十亿年的时间尺度上去除一小部分（几个地球质量）氢和氦包层，这意味着 TOI-849b 上的任何剩余大气都可能富含水或来自行星内部的其他挥发物。我们得出结论，TOI-849b 是一颗巨行星的残核。对 TOI-849b 的观测显示，其半径小于海王星的半径，但质量约为地球质量的 40 倍，表明这颗行星是一颗气态巨行星的残余核心。