UNDERSTANDING OTHER WORLDS
How do we know what exoplanets are made of?
Size, mass and spectra constrain composition, but different interiors can fit the same observations.
Density is a clue
Combining mass and radius yields an average density. This can distinguish some dense rocky planets from low-density worlds with gaseous envelopes, but does not uniquely identify every layer.
Atmospheric fingerprints
Absorption features in spectra can constrain atmospheric molecules. Interpreting them requires models, error estimates and checks for contamination by the host star.
Models have limits
Interior and atmosphere models depend on assumptions. Even a precise radius is not a photograph of the surface, and a possible gas detection is not evidence of life on its own.