CoRoT-3b is a short-period transiting companion with roughly Jupiter masses but a radius close to Jupiter's. Its discovery combined a sequence of exoplanet transits with ground-based Doppler spectroscopy and additional checks. This measured both a large mass and a compact radius, placing it above the ordinary giant-planet population in mean mass density.[ref]
Transiting exoplanets from the CoRoT space mission VI by Magali Deleuil and others. Abstract: 34 transits, combined photometric and spectroscopic analysis, mass and radius
The object illustrates why a measured radius alone does not distinguish a giant planet from a brown dwarf: electron degeneracy pressure can give very different masses similar radii. Its mass also does not uniquely reveal its formation mechanism. The discovery paper explicitly left the formation-based interpretation open between a low-mass brown dwarf and an unusually massive planetary companion.
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