New Scientist | Astrophile | Lisa Grossman | 2013 Aug 21
Object: KOI 1843.03
Year: 4.25 hours
When Joe lived back on Earth, it felt like the years were flying by. Then he went to work in the mines on the iron planet, and discovered how wrong he'd been. One side of the small world enjoyed perpetual night, and in his heat-resistant spacesuit it was just cool enough to stand and drill into the ground. There was neither sunrise nor sunset to mark the passage of time, yet he knew the iron planet whirled around its star so fast that every time he lay down for his 8 Earth-hours of shut-eye, he woke up two years later.
If Joe the space miner existed, that would be his life on KOI 1843.03, a potential planet found by NASA's Kepler space telescope. This odd world appears to have the shortest known year, completing an orbit around its star in just 4 hours, 15 minutes.
Because it is so close to the star, one side of the planet most likely always faces the star and feels more of its gravitational pull, creating tidal forces that squish and stretch the tiny body. And to withstand the star's gravitational wrath, the planet must be made almost entirely of iron.
"When you're so close to the star, tidal interactions become so strong that they can rip apart the surface of a planet and destroy it," says Roberto Sanchis-Ojeda, a graduate student at the Massachusetts Institute of Technology. "The only way the planet can hold on to its own matter is by having a really high density."
Cannonball planet
Kepler spent four years seeking planets that cross in front of their stars, or transit, and block a little bit of starlight at regular intervals. While most astronomers were trying to find worlds far enough away from their stars to be habitable, Sanchis-Ojeda and colleagues wanted to find out how short a planet's year could be. They sifted through the Kepler data looking for planets whose days last only a few hours. These worlds would have transited thousands of times during Kepler's watch, making them easy to spot.
"Until now with Kepler, we never talked about orbital periods of 4, 5, 6 hours," says Sanchis-Ojeda. His team found about 20 candidate planets that orbit their stars in less than half an Earth day. The one with the shortest year is KOI 1843.03, a planet about 0.6 times the size of Earth that orbits a sun-like star. But while Earth travels around the sun at about 30 kilometres per second, this planet zooms around its star at 250 kilometres per second.
To hold itself together, the planet must be at least 70 per cent iron with a thin mantle of silicates, the team calculates. The cannonball planet could even be made entirely of iron.
Night-time glow
KOI 1843.03 is not a confirmed planet yet – a pair of stars that orbit each other behind the target star can create the same signal, for instance. But the team is fairly confident that it's real, says Sanchis-Ojeda, in part because the same star has two more possible planets at greater distances, so more planets are likely to have formed.
The jury is still out on how such closely orbiting planets are created. They could have been born as rocky worlds further out in the planetary system and migrated inwards due to gravitational interactions with other planets. Or they could be the cores of once-massive planets that migrated and whose outer layers were blasted away by stellar radiation.
"The bottom line is, it's a puzzle. We don't know how you could end up with this kind of planet," says Dimitar Sasselov of Harvard University, who was not involved in the new work. If the planets migrated into position, they should still be spiralling in towards their stars and would be quickly demolished. So it is surprising to see so many of them.
"The fact that there is more than one of these basically tells you there may be something else going on," says Sasselov.
He adds that KOI 1843.03 may be so close to its host that it is actually inside the star's upper atmosphere, or corona. Both sides of the planet may then be bombarded with radiation, he says: "So you may be sitting on the night side and still get a very high dose of these particles hitting you directly. These particles are so energetic they'll irradiate the surface and probably make it glow."
Let's hope Joe's mining suit is made of tough stuff.
The Roche limit for close-orbiting planets: Minimum density, composition constraints,
and application to the 4.2-hour planet KOI 1843.03 - Saul Rappaport et al
- Astrophysical Journal Letters 773(1) L15 (2013 Aug 10) DOI: 10.1088/2041-8205/773/1/L15
arXiv.org > astro-ph > arXiv:1307.4080 > 15 Jul 2013
<< Previous Astrophile