August 14, 2025

The SPACE Program. I. The featureless spectrum of HD86226 c challenges sub-Neptune atmosphere trends

Key Points

A featureless spectrum in HD86226 c suggests no typical H/He-dominated atmosphere, and points to metal enrichment instead.
The transmission spectrum showed a constant transit depth of 418±14 ppm, with only 0.01 scale heights for a H/He-dominated atmosphere.
Using Hubble Space Telescope data, near-infrared and ultraviolet observations evaluated atmospheric characteristics for HD86226 c.
These findings indicate HD86226 c does not follow existing aerosol trends, emphasizing the need for more observations with the James Webb Space Telescope.

Abstract

Sub-Neptune exoplanets are the most abundant type of planet known today. As they do not have a Solar System counterpart, many open questions exist about their composition and formation. Previous spectroscopic studies have ruled out aerosol-free hydrogen-helium dominated atmospheres for many characterized sub-Neptunes but are inconclusive about their exact atmospheric compositions. Here we characterize the hot (T_ sub-Neptune HD, 86226, c (R=2. 2 mearth), which orbits its G-type host star on a four-day orbit. The planet is located in a special part of the sub-Neptune parameter space: Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We used Hubble Space Telescope data taken with WFC3 and STIS from the Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared (1. 1--1. 7 meter) transmission spectroscopy and ultraviolet characterization of the host star. We report a featureless transmission spectrum that is consistent within 0. 4, σ with a constant transit depth of 418±14, ppm. The amplitude of this spectrum is only 0. 01 scale heights for a H/He-dominated atmosphere, excluding a cloud-free solar-metallicity atmosphere on HD, 86226, c with a confidence of 6. 5, σ. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to metal enrichment M H >2. 3 (3, σ confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO₃), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we performed a detailed investigation of cloud formation in high metallicity, high-temperature atmospheres. Our results highlight that HD, 86226, c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the James Webb Space Telescope could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species that is otherwise atypical for sub-Neptunes.

AI에게 질문

Bookmark