Secondary Atmosphere
A secondary atmosphere is an atmosphere that builds up from outgassing of a planet’s interior or surface chemistry after the primordial H/He envelope has been lost to stellar radiation or escape processes.
Composition drivers
Secondary-atmosphere composition is primarily set by the planetary interior’s oxygen fugacity:
- Low fugacity → reducing, CO/H₂-rich (e.g., 55-cancri-e, src-55-cancri-e-hydrogen-atmosphere-2026-06)
- High fugacity → oxidizing, CO₂/SO₂-rich (e.g., l-98-59-d, src-l-98-59-d-sulphur-magma-world-2026-05; Venus analog)
JWST context
JWST has confirmed secondary atmospheres on some lava planets (55 Cnc e) while finding no detectable atmosphere on others (e.g., src-jwst-lhs-3844b-bare-rock-2026). The diversity in outcomes constrains secondary-atmosphere formation efficiency as a function of interior composition, stellar environment, and outgassing history.
Biosignature implications
Distinguishing a secondary atmosphere rich in CO₂ or SO₂ from a biogenic atmosphere is a key challenge for biosignatures interpretation. Abiotic outgassing can mimic some biosignature gases, requiring multi-gas context to discriminate (false-positive-biosignatures).