Identifying Atmospheres on Rocky Exoplanets through Inferred High Albedo

• Published in: The Astrophysical journal Vol. 886; no. 2; pp. 141 - 151
• Main Authors: Mansfield, Megan, Kite, Edwin S., Hu, Renyu, Koll, Daniel D. B., Malik, Matej, Bean, Jacob L., Kempton, Eliza M.-R.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.12.2019; IOP Publishing
• Subjects: Albedo; Astrophysics; Atmosphere; Atmospheric pressure; Cloud albedo; Clouds; Dwarf stars; Extrasolar planets; Irradiation; James Webb Space Telescope; Mathematical analysis; Object recognition; Planetary atmospheres; Planetary composition; Planetary surfaces; planets and satellites: atmospheres; planets and satellites: surfaces; planets and satellites: terrestrial planets; Red dwarf stars; Space telescopes; Terrestrial planets; Thermal emission
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab4c90

The upcoming launch of the James Webb Space Telescope 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 Tsub = 410-1250 K. This range corresponds to equilibrium temperatures of Teq = 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.