Kepler discovers the first extrasolar rocky planet!

Artist's impression of Kepler 10-b (Credit: NASA/Kepler Mission/Dana Berry)

Full story at
Star and planet stats at: (Kepler ID is 11904151)
Discovery paper at:

Big news today – Kepler has discovered its first extrasolar rocky planet, around a sun-like star about 564 lightyears away! Full stats, including the transit lightcurve can be found here. Interestingly the star may be very old – the age estimate is around 11.9 billion years – about as old as stars can get in our galaxy!

It’s a roaster though – it orbits very close to its star, at less than 0.02 AU (that’s about 20 times closer than Mercury’s orbit around the sun!), orbiting its star once every 20 hours! It’s certainly tidelocked at that distance (its orbit is circular too because of tidal evolution), which means its dayside temperature is around 2000K, so that has to be molten. The surface on the darkside on the other hand is probably solid since there’s no way for heat to be transported there – you wouldn’t be able to get a planet-covering atmosphere that close to the star. That said, maybe there could be an atmosphere of vapourised rock covering the dayside, which could rain out silicates on the darkside near the terminator – imagine that!

It’s a hefty planet too – 1.4 earth radii, about 4.6 times more massive than Earth, and with a bulk density of about 8800 kg/m³ (similar to the density of iron). While that seems to imply that the planet is made out of iron, that’s not likely to be true because the increased density could (in part at least) be due to self-compression .

Solids such as rock and metal are compressible to a certain extent, which means that their density increases when they’re put under pressure. Extremely high pressures can be found planetary interiors as a result of all the overlying planetary material, which means that the density of iron and rock in the interior is much higher than that of iron and rock on the surface. That increases the average density of the planet, which will be more noticeable in a superearth like this one where the pressure at the outer boundary of the inner core is likely to be over 1000 Gigapascals (about 10 million atmospheres – over five times higher than at the inner/outer core boundary on Earth).

If the planet was somehow actually made entirely of iron – which would be very unlikely – its density would actually be much higher than 8800 kg/m³ as a result of self-compression. So I suspect that the planet is actually rocky but has a larger metallic core than Earth has, and self-compression has also significantly increased the density of its interior – together, these give the planet its high bulk density.

Either way, it’s very exciting – both because it shows that we can detect earth-sized planets around other stars, and because it means that there are planets that small out there. This bodes very well for our chances of finding an earth-sized rocky planet in a star’s habitable zone (and who knows, maybe we’ll find one on Planethunters!)

2 Responses to “Kepler discovers the first extrasolar rocky planet!”

  • What I found very interesting is that a planet like this orbits so close to a star that is quite similar to our sun.

    Now if there are lots of metal on a world like this, and worlds like this are common, then it would change the business for the belters in Traveller. 😉

  • They’d have to mine the darkside though (which would be [i]extremely[/i] cold given it never sees the sun, unless there is some heat transport mechanism that I’m missing). Also the 2.26g surface gravity would be somewhat problematic too!

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