Mass-radius relations for different compositions of Kepler-9d. For the rocky scenarios: a pure iron planet (dashed black), a mercury-like planet – 35% silicate mantle above a 65% iron core (thin black), an earth-like planet – 63% silicate mantle above a 33% iron core (thick black). For the gaseous compositions: an earth-like nucleus covered by an H-He envelope of 0.01% (dotted brown), 0.1% (dotted orange), and 1% (dotted red) by mass; or covered by a water envelope of 5% (solid purple) and 50% (solid dark blue) by mass, or a pure 100% water-vapor planet (solid light blue). The radius range for Kepler-9d (grey band) suggests the planet has no considerable H-He. A physically plausible upper limit for the mass is 16   M_⊕. Transiting exoplanets in a similar mass-radius range, as well as Uranus and Neptune, are shown for reference.