Astronomers have discovered the densest extrasolar planet yet: A Jupiter-mass remnant of a carbon- and oxygen-rich star that measures no more than 55,000 kilometers across. The odd world, whose density is on average at least 23 grams per cubic centimeter (or about twice that of lead), is probably crystalline and possibly largely composed of ultradense diamond. The planet orbits its parent sun—a pulsar, or a rapidly-spinning neutron star that emits an intense beam of radio waves, dubbed PSR J1719-1438—once every 2 hours and 10 minutes, the researchers report online today in Science. The planet’s orbit measures about 600,000 kilometers across, only 50% more than the average distance from Earth to the moon. How such a dense planet formed is unclear, the researchers say, but it’s probably the crystalline vestige of a white dwarf star whose atmosphere was stripped away by the parent pulsar. Most of the pulsars that spin faster than once per 20 milliseconds are part of a binary star system, but about 30% have no companions whatsoever, the scientists note. Only one other rapidly-spinning pulsar is known to be orbited by Earth-mass planets—a sign that exotic planets such as this megadiamond are, like their Earthly counterparts, rare indeed.

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