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<p>Recent transit observations of K2-18 b and TOI-270 d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (equilibrium temperature <i>T</i><sub>eq</sub> = 250–400 K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO<sub>2</sub> abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here we present the JWST NIRSpec/PRISM 0.7–5.4-μm transmission spectrum of a third temperate sub-Neptune, the 2.4 <i>R</i><sub>⊕</sub> planet LP 791-18 c (<i>T</i><sub>eq</sub> = 355 K), which is even more favourable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite the radius, mass and equilibrium temperature of LP 791-18 c being between those of K2-18 b and TOI-270 d, we find a drastically different transmission spectrum. Although we also detect methane on LP 791-18 c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO<sub>2</sub> absorption. Overall, we infer a deep metal-enriched atmosphere (246–415 times solar) for LP 791-18 c, with a CO<sub>2</sub>-to-CH<sub>4</sub> ratio smaller than 0.07 (at 2<i>σ</i>), indicating less H<sub>2</sub>O in the deep envelope of LP 791-18 c and implying a relatively dry formation inside the water-ice line. These results show that sub-Neptunes that are near analogues in density and temperature can show drastically different aerosols and envelope chemistry and are intrinsically diverse beyond a simple temperature dependence.</p>

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Diversity in the haziness and chemistry of temperate sub-Neptunes

  • 期刊论文

  • 发表日期：

    2025年12月12日

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  • Pierre-Alexis Roy,
  • Björn Benneke,
  • Marylou Fournier-Tondreau,
  • Louis-Philippe Coulombe,
  • Caroline Piaulet-Ghorayeb,
  • David Lafrenière,
  • Romain Allart,
  • Nicolas B. Cowan,
  • Lisa Dang,
  • Doug Johnstone,
  • Adam B. Langeveld,
  • Stefan Pelletier,
  • Michael Radica,
  • Jake Taylor,
  • Loïc Albert,
  • René Doyon,
  • Laura Flagg,
  • Ray Jayawardhana,
  • Ryan J. MacDonald,
  • Jake D. Turner

摘要

Recent transit observations of K2-18 b and TOI-270 d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (equilibrium temperature Teq = 250–400 K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO2 abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here we present the JWST NIRSpec/PRISM 0.7–5.4-μm transmission spectrum of a third temperate sub-Neptune, the 2.4 R⊕ planet LP 791-18 c (Teq = 355 K), which is even more favourable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite the radius, mass and equilibrium temperature of LP 791-18 c being between those of K2-18 b and TOI-270 d, we find a drastically different transmission spectrum. Although we also detect methane on LP 791-18 c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO2 absorption. Overall, we infer a deep metal-enriched atmosphere (246–415 times solar) for LP 791-18 c, with a CO2-to-CH4 ratio smaller than 0.07 (at 2σ), indicating less H2O in the deep envelope of LP 791-18 c and implying a relatively dry formation inside the water-ice line. These results show that sub-Neptunes that are near analogues in density and temperature can show drastically different aerosols and envelope chemistry and are intrinsically diverse beyond a simple temperature dependence.