Aerosols and hydrocarbons in the atmosphere of a white dwarf planet.
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- Record sourced from PubMed, PMID 42387166.
- Also identified by DOI 10.1038/s41586-026-10514-7.
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Abstract
Most stars, including our Sun, will one day evolve into red giants and, subsequently, white dwarfs. Several planet candidates have recently been identified orbiting white dwarfs<sup>1-4</sup>, demonstrating that planets can survive the stellar post-main-sequence stage intact. Little is known about the atmospheric composition of post-main-sequence planets, with the most evolved transiting planets with atmospheric detections so far orbiting subgiants<sup>5,6</sup>. Here we report an atmospheric detection for the white dwarf planet WD 1856 b, achieved through transmission spectroscopy with the James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) PRISM. Our 0.5-5.0-μm spectrum reveals the presence of hydrocarbons (odds ratio of 167:1-5,377:1, with CH<sub>4</sub> preferred at 17:1-30:1), aerosols (2 × 10<sup>5</sup>:1-2 × 10<sup>6</sup>:1) and thermal emission from the planetary nightside (2 × 10<sup>63</sup>:1-2 × 10<sup>73</sup>:1). Our spectral analysis constrains the mass of WD 1856 b to 4.3-10.9 M<sub>J</sub>, finds a carbon-enriched atmosphere (with a CH<sub>4</sub> abundance of approximately 7%) and an effective temperature exceeding the expected planetary equilibrium temperature (390-412 K versus 160 K). On the basis of cooling models, these results indicate that WD 1856 b underwent a migration-related reheating event 3.0-5.5 Gyr into the white dwarf phase, consistent with post-main-sequence tidal evolution to the present-day 0.02-AU circular orbit. Our results provide a window into the ultimate fate of giant planets orbiting stars with masses similar to our Sun.