Gliese 876 and its Three Planets

The image on this page represents the star and its three detected planets at their relative sizes. The star's diameter is reported as 30% of Sol's, or 260,000 miles (418,000 km; see the Extrasolar Planets Encyclopaedia). The sizes of the three planets are not precisely understood, but their masses are reasonably well constrained at 7.5 MEARTH, 0.62 MJUP, and 1.93 MJUP, in order of distance from the host star (see Exoplanets.org). For the inner planet, Diana Valencia and colleagues have calculated three alternative models of composition and structure, representing three possible diameters (Valencia et al. 2007). As a volatile-rich "Ocean Planet" its diameter would be about 14,900 miles (24,000 km); as a "Super Earth," with a composition similar to that of our own planet, its diameter would be about 13,400 miles (21,600 km); and as a volatile-poor "Super Mercury," its diameter would be about 11,500 miles (18,600 km).

This image uses the value of 260,000 miles for the diameter of the host star, and assumes values of 13,000 miles, 88,000 miles, and 89,000 miles for the diameters of the three planets, in order of increasing distance from the star (see Fortney et al. 2007, "Planetary radii across five orders of magnitude in mass and stellar insolation"). The result is a dramatic visual demonstration of the fact that any giant planets in orbit around a red dwarf are likely to have diameters that are a significant fraction of the host star's. This contrasts with the situation in the Solar System, where the diameter of the largest planet, Jupiter, is only 10% of the diameter of the Sun.