Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b

Abstract: Most known terrestrial planets orbit small stars with radii less than 60% that of the Sun. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point $-$ possibly indicative of atmospheric circulation. Here we report a phase curve measurement for the smaller, cooler planet LHS 3844b, a 1.3 Earth radius world in an 11-hour orbit around a small, nearby star. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of $1040\pm40$ kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are unstable to erosion by stellar wind. The data are well fitted by a bare rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.

• The paper confirms LHS 3844b lacks a thick atmosphere, using thermal phase curves that show a ~1040 K dayside temperature and negligible nightside heat transfer.
• The study employed Spitzer's IRAC to capture symmetric phase variations, dismissing the possibility of an atmosphere with >10 bar pressure and highlighting stellar wind erosion.
• Findings indicate a predominantly basaltic surface with a low Bond albedo, supporting theoretical models of atmospheric loss on hot terrestrial exoplanets orbiting M-dwarf stars.

Absence of a Thick Atmosphere on the Terrestrial Exoplanet LHS 3844b

The paper "Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b" addresses the atmospheric composition of LHS 3844b, a terrestrial exoplanet in close orbit around an M-dwarf star. This study holds significant implications for understanding atmospheric retention on terrestrial planets orbiting small stars, contributing to the broader discussion on planetary habitability and atmospheric evolution.

The authors utilized the Spitzer InfraRed Array Camera to monitor the thermal phase variation of LHS 3844b. Their analysis reveals symmetric phase variations with large amplitudes, indicating a dayside brightness temperature of approximately 1040 K and suggesting an absence of significant heat transfer to the nightside, which corresponds to a nightside temperature consistent with zero K at one standard deviation. These findings strongly imply that LHS 3844b lacks a substantial atmosphere capable of redistributing heat, thereby dismissing the presence of a thick atmosphere with more than 10 bar surface pressure with high confidence. The study further suggests that even less massive atmospheres are likely unstable due to stellar wind erosion.

The researchers also evaluated the planet's surface properties. The data were best aligned with a bare rock model exhibiting a low Bond albedo, less than 0.2 at two standard deviations. The spectral analysis implies a predominantly basaltic composition, similar to lunar and mercurial surfaces, potentially indicative of extrusive volcanism. These insights are grounded in the emission spectra and surface composition models compared against the observed data.

The paper posits that the absence of a thick atmosphere on LHS 3844b corroborates theoretical models predicting atmospheric loss on hot, terrestrial planets closely orbiting M-dwarfs. The findings underscore a notable vulnerability to both thermal and stellar erosion processes in such planetary systems, aligning with expectations from atmospheric evolution models.

Additionally, the study tested the tenability of thin atmospheres by examining oxygen and carbon dioxide mixtures across various pressures. The results suggest that hypothetical atmospheres, particularly those with high CO2 concentrations, would not withstand the observed thermal phase curve signatures.

Consideration of atmospheric escape mechanisms from stellar winds further supports the notion of a barren surface. The authors explore potential atmospheric erosion by rough estimates of hydrogen and oxygen loss rates over geologic timescales, further substantiating their conclusions on atmospheric instability.

The implications of this research extend to the search for habitable environments beyond our solar system. LHS 3844b serves as a paradigm for studying terrestrial exoplanets lacking protective atmospheres when in close proximity to cooler stars. Understanding the atmospheric properties of such planets aids in refining criteria for habitability assessments in future exoplanet surveys.

Future research opportunities involve probing lesser-irradiated terrestrial planets orbiting M-dwarfs, which may possess the capacity to retain atmospheres, thus becoming more suitable candidates for habitability studies. The anticipated observational capabilities of the James Webb Space Telescope and upcoming exoplanet missions will provide critical platforms for deeper exploration of atmospheric compositions and the evolutionary trajectories of exoplanets similar to LHS 3844b.