Astrophile: The planets that formed by cell division

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Objects: Rocky planets KOI 55.01 and KOI 55.02
Size: 0.759 and 0.867 times Earth's diameter
Distance: 4000 light years from Earth

When the sun-like star KIC 05807616 ran out of hydrogen, its planets thought they were done for. The star began puffing off its outer layers, and over the next billion years, ballooned up into a red giant a million times its original volume.

Most of its planets were probably lost in the process, swallowed up by the star and never seen again. But one planet, a gas giant about five times the mass of Jupiter, held on for dear life. As the red giant engulfed it, it lost energy through friction, spiralling ever closer to the star's dense, million-degree core.

If it could just hang on until the star's outer layers swept back over it as they collapsed onto the star again, triggering helium fusion, maybe it would make it. But it spiralled too close, and got pulled apart by the star's gravity, splitting into daughter worlds in a process reminiscent of cell division.

"Some of [the pieces] did not survive, and fell into the star," says Noam Soker at the Technion ? Israel Institute of Technology in Haifa. "Some may have been ejected away, but two survived."

Those two pieces ? each a little smaller than Earth ? are still in orbit around the star, say Soker and Ealeal Bear, also at Technion ? Israel. They argue that this mitosis-like process better explains the properties of the two mini-Earths than the alternative explanation proposed by the planets' discoverers in December last year.

Strangely small

Last year, St?phane Charpinet at the University of Toulouse in France, and colleagues, found the apparent planets after scrutinising an old star about 4000 light years away (Nature, DOI: 10.1038/nature10631).

The star flared in brightness every 5.7 and 8.2 hours. The team could not match the odd oscillations up with any other source ? a nearby star, a starspot, or the star's own usual rumbling.

"I ended up concluding that what we are seeing are bodies orbiting around the star itself," Charpinet says. The variations in brightness came from the planets reflecting light from the star back towards Earth.

Charpinet and colleagues calculated that the planets were probably 0.759 and 0.867 times the diameter of Earth and 0.440 and 0.655 times Earth's mass. That was surprising because nothing that small had ever been found around a star that had gone through a red giant phase. A few large planets or objects midway in size between planets and stars ? called brown dwarfs ? had apparently survived this upheaval, but no one expected anything the size of the Earth to make it.

Synchronised orbits

Charpinet's team suggested that each of the planets had been a gas giant, Jupiter-sized or larger, and that they had lost their atmospheres when the swollen star engulfed them, and fallen towards the star as they lost energy through friction.

But that scenario did not make sense to Bear and Soker. The planets' orbits are strangely synchronised, with one planet circling the star about three times for every two orbits of the other. Bear and Soker say it is unlikely that the planets would have been able to maintain such an orderly dance after punishing journeys through the red giant.

Instead, they argue that the two planets are pieces of a single, massive world that came too close to the red giant and was pulled apart by the star's gravity. As they spiralled towards its core, they heated up the stellar gas they flew through, causing it to evaporate away. As a result, there was less gas in the star's outer layers, so when these layers collapsed down again after the star's red giant phase, the resulting helium-burning star was smaller than usual for this kind of star ? just what was observed for KIC 05807616.

"We see here a very interesting example of where planets can influence quite substantially the evolution of the parent star," Soker says.

Survivor: Planet edition

"The idea they're proposing sounds quite reasonable to me," Charpinet says. To distinguish between the two scenarios, Soker says all they would need to do is detect a third planet. That would tip the scales in favour of his cell-division model. "Three large planets spiralling inside the star, and all of them reach very close?" he says. Having three separate planets survive the red giant phase, migrate through its outer layers, and remain in orbit after the star re-collapsed would be very unlikely, he says. "Even two is not certain, but three for sure cannot work," he says.

Jason Nordhaus at the Rochester Institute of Technology in New York says the new explanation is interesting, but points out that the objects have not yet been confirmed as planets. They appeared as regular brightness fluctuations, suggesting they were reflecting their star's light, but they may simply be stellar pulsations, he says.

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