The odd planet was discovered orbiting what's known as a millisecond pulsar—a tiny, fast-spinning corpse of a massive star that died in a supernova.
Astronomers estimate that the newfound planet is 34,175 miles (55,000 kilometers) across, or about five times Earth's diameter.
In addition, "we are very confident it has a density about 18 times that of water," said study leader Matthew Bailes, an astronomer at the Swinburne Centre for Astrophysics & Supercomputing in Melbourne, Australia.
"This means it can't be made of gases like hydrogen and helium like most stars but [must be made of] heavier elements like carbon and oxygen, making it most likely crystalline in nature, like a diamond."
Partner Gave Pulsar a Superfast Spin
The new millisecond pulsar, dubbed PSR J1719-1438, lies about 4,000 light-years away in the southern constellation Serpens. Bailes and his team found the star during a pulsar survey using the radio telescope at Australia's Parkes Observatory.
A pulsar is a type of stellar corpse that emits powerful beams of radio waves from its poles. If these beams sweep across Earth's field of view as the star rotates, radio telescopes on Earth can detect the star's regular pulses.
A millisecond pulsar is thought to form when the pulsar is siphoning material from a companion star. The action of eating matter speeds up the pulsar's spin to hundreds of rotations a second.
So far it seems millisecond pulsars are rare, with only about a hundred found in the last 30 years. The study team found PSR J1719-1438 by using supercomputers to comb through almost 200,000 gigabytes of data—enough to fill more than 23,500 standard DVDs.
The data show that the pulsar spins more than 10,000 times a minute. The astronomers also noticed that the star's radio pulses have an unusual modulation, which the team concluded must be due to the gravitational pull of a small orbiting object.
"Diamond" Planet a Stripped Dwarf?
About 70 percent of the known millisecond pulsars have orbital companions, but PSR J1719-1438 is only the second thought to have a planetary partner. That's probably because planets don't form around millisecond pulsars in the usual way, Bailes said.
Astronomers think planets are created from dusty disks of material swirling around newborn stars. As this material orbits the star, gravitational interactions cause clumps to form, and the clumps build mass as they sweep through the disk.
By contrast, the new study hints that pulsars can strip material away from their companions until all that's left of the consumed star is enough mass for a planet-like object.
The newfound "diamond" planet probably formed from a white dwarf star—the core of a dead sunlike star—that was being stripped of matter by the pulsar. The leftover object likely represents just 0.1 percent of the white dwarf's original mass, Bailes said.
Based on their data, the team calculates that the planet orbits the pulsar in just two hours and ten minutes at a distance of about 372,822 miles (600,000 kilometers).
More Pulsar Planets Out There
Bailes and his team would now like to know exactly how rare their discovery really is.
In all likelihood, this weird method of planet production requires special circumstances that rely on the white dwarf companion having a particular mass and chemical composition.
But even if the diamond planet is a result of a perfect storm of special circumstances, there should be more such worlds out there, Bailes said.
"The most exciting aspect to me is that we've only processed a small fraction of space so far," he said. "With the new supercomputers coming online, we should be in a strong position to possibly make many more discoveries like this one."
The diamond planet is described in this week's issue of the journal Science.