  We suggest that some extrasolar planets <~ 60 Earth masses will form
substantially from silicon carbide and other carbon compounds. Pulsar planets
and low-mass white dwarf planets are especially good candidate members of this
new class of planets, but these objects could also conceivably form around
stars like the Sun. This planet-formation pathway requires only a factor of two
local enhancement of the protoplanetary disk's C/O ratio above solar, a
condition that pileups of carbonaceous grains may create in ordinary
protoplanetary disks. Hot, Neptune-mass carbon planets should show a
significant paucity of water vapor in their spectra compared to hot planets
with solar abundances. Cooler, less massive carbon planets may show
hydrocarbon-rich spectra and tar-covered surfaces. The high sublimation
temperatures of diamond, SiC, and other carbon compounds could protect these
planets from carbon depletion at high temperatures.
