Identifying Atmospheres on Rocky Exoplanets through Inferred High Albedo

Abstract

The upcoming launch of the James Webb Space Telescope (JWST ) means that we will soon have the capability to characterize the atmospheres of rocky exoplanets. However, it is still unknown whether such planets orbiting close to M dwarf stars can retain their atmospheres, or whether high-energy irradiation from the star will strip the gaseous envelopes from these objects. We present a new method to detect an atmosphere on a synchronously rotating rocky exoplanet around a K/M dwarf, by using thermal emission during secondary eclipse to infer a high dayside albedo that could only be explained by bright clouds. Based on calculations for plausible surface conditions, we conclude that a high albedo could be unambiguously interpreted as a signal of an atmosphere for planets with substellar temperatures of T sub = 410-1250 K. This range corresponds to equilibrium temperatures of T eq = 300-880 K. We compare the inferred albedos of eight possible planet surface compositions to cloud albedo calculations. We determine that a layer of clouds with optical depths greater than τ = 0.5 -7, would have high enough albedos to be distinguishable from a bare rock surface. This method of detecting an atmosphere on a rocky planet is complementary to existing methods for detecting atmospheres, because it provides a way to detect atmospheres with pressures below 1 bar (e.g. Mars), which are too tenuous to transport significant heat but thick enough to host high-albedo clouds.

TRAPPIST-1: Global results of theSpitzerExploration Science Program Red Worlds

“…Thanks to their small size, mass, and luminosity combined with their relatively large infrared brightness, the nearest ultracool dwarf stars (spectral types M7 and later) represent promising targets for the detailed study of potentially habitable transiting exoplanets with upcoming giant telescopes such as JWST or ELTs (Kaltenegger & Traub 2009;Malik et al 2019;Mansfield et al 2019;Koll 2019). This fact motivated the development of the new ground-based transit survey called the Search for habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS).…”

Disentangling atmospheric compositions of K2-18 b with next generation facilities

“…In general, clouds increase the planet albedo by reflecting visible light. Mansfield et al [113] has shown that cloud reflection could be distinguished from surface reflection by an increased albedo in the case of rocky planets. While this could be an interesting option, they only investigated surfaces for temperatures higher than 410K.…”

Experimental Determination of Mantle Solidi and Melt Compositions for Two Likely Rocky Exoplanet Compositions

“…At present, knowledge of the specific characteristics of rocky exoplanets is largely limited to their host star composition, amount of stellar irradiance, orbital radius, mass, and planetary radius (atmospheric thickness is included in this measurement). There are methods to detect or infer the presence of an atmosphere (Koll et al, 2019;Kreidberg, 2018;Kreidberg et al, 2019;Mansfield et al, 2019) and spectroscopic characterization of exoplanet atmospheres will soon be possible via missions such as NASA's James Webb Space Telescope (JWST), ESA's Characterising Exoplanet Satellite (CHEOPS), and ESA's Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), and ground-based efforts such as the European Extremely Large Telescope's (ELT) Midinfrared ELT Imager and Spectrograph (METIS). Unfortunately, measurements of surface spectra are unlikely to be returned until missions such as NASA's Large UV/Optical/IR Surveyor (LUVOIR), so it behooves the community to experimentally investigate exoplanet melt compositions; the link between melt composition, outgassed volcanic volatiles, and atmosphere composition may allow for the deduction of geochemical properties of the solid interior based on future exoplanet atmospheric data (e.g., Bower et al, 2019;Spaargaren et al, 2020).…”