Temperate super-Earths/mini-Neptunes around M/K dwarfs Consist of 2 Populations Distinguished by Their Atmospheres

TL;DR

This study analyzes the atmospheres of 28 temperate super-Earths and mini-Neptunes around M/K dwarfs, revealing two distinct atmospheric populations with different spectral characteristics, which has implications for future atmospheric characterization.

Contribution

It identifies two distinct atmospheric populations among temperate small planets, based on transmission spectral slopes, using a uniform dataset from Kepler/K2 and Spitzer observations.

Findings

Distribution of spectral slopes is bimodal, indicating two atmospheric populations.

Approximately 20% of planets have atmospheres with detectable molecular features.

Most planets have hazy or cloudy atmospheres or lack detectable atmospheres.

Abstract

Studies of the atmospheres of hot Jupiters reveal a diversity of atmospheric composition and haze properties. Similar studies on individual smaller, temperate planets are rare due to the inherent difficulty of the observations and also to the average faintness of their host stars. To investigate their ensemble atmospheric properties, we construct a sample of 28 similar planets, all possess equilibrium temperature within 300-500K, have similar size (1-3 R_e), and orbit early M dwarfs and late K dwarfs with effective temperatures within a few hundred Kelvin of one another. In addition, NASA's Kepler/K2 and Spitzer missions gathered transit observations of each planet, producing an uniform transit data set both in wavelength and coarse planetary type. With the transits measured in Kepler's broad optical bandpass and Spitzer's 4.5 micron wavelength bandpass, we measure the transmission spectral slope, alpha, for the entire sample. While this measurement is too uncertain in nearly all cases to infer the properties of any individual planet, the distribution of alpha among several dozen similar planets encodes a key trend. We find that the distribution of alpha is not well-described by a single Gaussian distribution. Rather, a ratio of the Bayesian evidences between the likeliest 1-component and 2-component Gaussian models favors the latter by a ratio of 100:1. One Gaussian is centered around an average alpha=-1.3, indicating hazy/cloudy atmospheres or bare cores with atmosphere evaporated. A smaller but significant second population (20+\-10% of all) is necessary to model significantly higher alpha values, which indicate atmospheres with potentially detectable molecular features. We conclude that the atmospheres of small and temperate planets are far from uniformly flat, and that a subset are particularly favorable for follow-up observation from space-based platforms like HST and JWST.