M-dwarf stars make up over two-thirds of the stars in our Galaxy and frequently host Earth-sized planets. Their small size and cool temperatures offer a favourable planet-to-star contrast, making them prime targets for atmospheric characterisation of rocky exoplanets. Many evolution models also predict thick, Venus-like CO2 atmospheres for planets with Teq = 300K–1,500K. This “M-dwarf opportunity” sparked a number of JWST programmes, including two large surveys — the Hot Rocks Survey (GO 3730) and the 500-hour Director’s Discretionary Time Rocky Worlds programme — which use MIRI 15 μm photometry to search for atmospheric signatures through dayside thermal emission measurements. Despite these efforts, most targets observed so far show secondary eclipse depths inconsistent with substantial CO2 atmospheres.
LHS 1478 b stands out as a potentially critical exception. Initial observations revealed a shallow 15 μm eclipse suggesting CO2 absorption or a high albedo, making it perhaps the most promising atmospheric candidate identified to date in this regime. While correlated noise limited the original detection to <3σ, I will present first results from my follow-up campaign (JWST GO 7675, PI August), which confirm the eclipse signal in both spectroscopy (MIRI LRS) and photometry (MIRI F1500W).
In a second part of the talk, I will discuss physical processes shaping the secondary atmospheres of these planets. The habitable zone of M dwarfs lies close in, exposing terrestrial planets to strong XUV irradiation and tidal locking. Both phenomena make these worlds vulnerable to atmospheric collapse, a process wherein volatiles condense entirely on the permanent nightside. However, the ice reservoirs thus formed represent a stable volatile inventory that can be revaporised by meteorite impacts, potentially regenerating secondary atmospheres episodically. I will present a simple energy-budget framework to assess the viability of this mechanism, applied to Rocky Worlds targets.
Contact local & organisation : Pierre-Olivier LAGAGE
DAp