Some extrasolar planets may be made substantially from carbon compounds,
including diamond, according to a report presented this week at the
conference on extrasolar planets in Aspen, Colorado. Earth, Mars and
Venus are “silicate planets” consisting mostly of silicon-oxygen
compounds. Astrophysicists are proposing that some stars in our galaxy
may host “carbon planets” instead.

“Carbon planets could form in much the same way as do certain meteorites in
our solar system, the carbonaceous chondrites,” said Dr. Marc J. Kuchner
of Princeton University, making the report in Aspen together with Dr. Sara
Seager of the Carnegie Institute of Washington. “These meteorites contain
large quantities of carbon compounds such as carbides, organics, and
graphite, and even the occasional tiny diamond.” Imagine such a meteorite
the size of a planet, and you are picturing a carbon planet.

Planets like the Earth are thought to condense from disks of gas orbiting
young stars. In gas with extra carbon or too little oxygen, carbon
compounds like carbides and graphite condense out instead of silicates,
possibly explaining the origin of carbonaceous chondrites and suggesting
the possibility of carbon planets. Any condensed graphite would change
into diamond under the high pressures inside the carbon planets,
potentially forming diamond layers inside the planets many miles thick.

Some of the already known low- and intermediate-mass extrasolar planets
may be carbon planets, which should easily survive at high temperatures
near a star if they have the mass of Neptune. Carbon planets would
probably consist mostly of carbides, thought they may have iron cores and
substanial atmospheres. Carbides are a kind of ceramic used to line the
cylinders of motorcycle engines among other things.

The planets orbiting the pulsar PSR 1257+12 are good candidates for carbon
planets; they may have formed from the disruption of a star that produced
carbon as it aged. So are planets located near the center of the Galaxy,
where stars are more carbon-rich than the sun, on average. Slowly, the
galaxy as a whole is becoming more carbon-rich; in the future, all planets
formed may be carbon planets.

“There’s no reason to think that extrasolar planets will be just like the
planets in the solar system.” says Kuchner. “The possibilities are

Kuchner added, “NASA’s future Terrestrial Planet Finder (TPF) mission may
be able to spot these planets.” The spectra of these planets should lack
water, and instead reveal carbon monoxide, methane, and possibly
long-chain carbon compounds synthesized photochemically in their
atmospheres. The surfaces of carbon planets may be covered with a layer
of long-chain carbon compounds–in other words, something like crude oil
or tar.

The first TPF telescope, an optical telescope several times the size of
the Hubble Space Telescope is scheduled to launch in 2015. The TPF
missions are designed to search for planets like the Earth and determine
whether they might be suitable for life.