A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds (1907.00449v1)

Abstract: With no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes likely represent the most common outcome of planet formation. Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune. Here, we analyze a combined Hubble/Spitzer Space Telescope dataset of 12 transits and 20 eclipses of the sub-Neptune GJ 3470 b, whose mass of 12.6 $M_\oplus$ places it near the half-way point between previously studied exo-Neptunes (22-23 $M_\oplus$) and exoplanets known to have rocky densities (7 $M_\oplus$). Obtained over many years, our data set provides a robust detection of water absorption (>5$\sigma$) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogen-dominated atmosphere similar to a gas giant, but strongly depleted in methane gas. The low, near-solar metallicity (O/H=0.2-18) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is destroyed much more efficiently than previously predicted, suggesting that the CH$_4$/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2-3 $\mu$m characteristic of Mie scattering, which enables narrow constraints on the cloud particle size and makes GJ 3470b a keystone target for mid-IR characterization with JWST.

• The paper reveals that GJ 3470b’s atmosphere is hydrogen-dominated with low metallicity and exhibits methane levels at least three orders of magnitude lower than expected.
• It utilizes extensive Hubble and Spitzer time-series spectroscopy across 0.55–5.0 µm to construct robust transmission and thermal emission spectra.
• The study identifies Mie-scattering clouds with narrowly constrained particle sizes and suggests non-equilibrium processes, prompting a reassessment of exoplanet formation models.

Analysis of GJ 3470b's Atmosphere: A Methane-Depleted Sub-Neptune

This paper presents a detailed atmospheric characterization of GJ 3470b, a sub-Neptune exoplanet, utilizing a comprehensive dataset obtained through the Hubble and Spitzer Space Telescopes. The focus is on understanding the atmospheric structure, metallicity, and composition of this exoplanet with a mass of 12.6 M⊕, an object residing between Earth and Neptune in terms of size and mass.

Key Findings and Methodology

GJ 3470b's atmosphere is observed to be hydrogen-dominated with low metallicity, possessing strikingly low methane levels, a result that deviates from the conventional expectations for such exoplanets. The research employs time-series spectroscopy from twelve Hubble and twenty Spitzer observations, covering visible to infrared wavelengths (0.55 to 5.0 µm), to construct a robust transmission and thermal emission spectrum.

Notable findings include:

• Water Absorption: A statistically significant detection of water absorption at 1.4 µm (5.2σ), concurrent with high clouds in the atmosphere.
• Methane Depletion: Quantitative analysis reveals a methane abundance at least three orders of magnitude lower than anticipated for a solar-composition atmosphere.
• Mie-Scattering Clouds: Observational evidence points to Mie scattering due to cloud particles, achieving a narrow constraint on particle size at 0.60±0.06 µm.

The research further elucidates the spectroscopic measurements by applying atmospheric retrieval and photochemical models, suggesting that the unexpectedly low methane levels could result from unknown catalytic processes or interior heating potentially induced by tidal forces.

Implications and Future Research

The findings contribute significantly to the understanding of atmospheric processes in sub-Neptune exoplanets, indicating the necessity to reassess chemical equilibrium models, particularly the CH4/CO transition. The low metallicity suggests a formation scenario involving direct accretion of primordial gas, diverging from expectations of high metallicity typically linked to core accretion models beyond the ice line.

Prospects for JWST Observations

The paper underscores GJ 3470b as a compelling target for upcoming JWST observations. Given the transparency beyond 3 µm due to Mie-scattering particles, GJ 3470b presents an excellent occasion to explore mid-infrared atmospheric characteristics potentially revealing further insights into the equilibrium and non-equilibrium chemistry of sub-Neptunes.

In conclusion, the detailed atmospheric characterization of GJ 3470b challenges current theoretical models, highlighting the complexity of sub-Neptune atmospheres and urging refined models considering non-classical methane depletion pathways. Future work should focus on leveraging more precise instruments like the JWST to expand the exoplanetary atmospheric understanding and refine formation theories, particularly for small planets in close-in orbits around low-mass stars.