Planet KOI-314c, imagined here, is the lightest planet to have both its mass and physical size measured. It weighs the same as Earth, but is 60 per cent wider, meaning it must have a very thick atmosphere. It orbits a dim, red dwarf star. KOI-314c interacts gravitationally with another planet, KOI-314b (in the background), allowing us to measure the masses of both worlds (Image: C. Pulliam/D. Aguilar/CfA)

Earth is an extreme world. Of the thousands of confirmed or candidate planets astronomers have discovered in our galaxy, the most common type is a world unlike anything in our solar system: an enigmatic ball of either rock or gas that is bigger than Earth, but smaller than Neptune.

That’s according to two independent analyses of data from NASA’s Kepler space telescope, which spent four years hunting for small planets beyond our solar system. The result narrows down the number of known worlds that can be true Earth twins – and which therefore might host tech-savvy land-dwellers akin to humans.

Kepler discovered planets by watching their host stars’ light dim slightly when those planets crossed in front of them. This method reveals a planet’s size, but not its mass – and astronomers need to know both to calculate a planet’s density – a clue to composition. Knowing what a planet is made of is crucial to determining whether it’s a solid, rocky world like Earth, or a puffy ball of gas more like Neptune or Saturn.


Weighing worlds

The researchers turned to other methods to weigh tens of small planets in Kepler’s cache. A team led by Kepler team member Geoff Marcy at the University of California, Berkeley, found the masses of 42 small exoplanets by watching how their gravity tugged their host stars to and fro. An independent group led by Yoram Lithwick at Northwestern University in Evanston, Illinois, weighed 60 planets in multi-planet systems by measuring how they pulled on each other.

Both groups found a clear pattern: worlds up to twice the size of Earth are dense and probably rocky, resembling our own planet. Those between two and four times Earth’s width are lighter, so are either wetter or gassy – more like versions of Neptune, which is itself four times Earth’s width.

Looking across Kepler’s entire haul of detections, three-quarters of the worlds it has discovered are of this gassy variety, a planetary type that is not found at all among our eight planets.

“Mini-Neptunes dominate the inventory of 3000-plus planets discovered by Kepler,” Marcy says. He and Lithwick presented their results Monday at a meeting of the American Astronomical Society (AAS) in Washington DC.

Bucking the trend

The findings fit nicely with theories of planet formation, which suggest that planets above a certain size cannot be made of mostly rock. The more dense material you pile on to a rocky planet, the more it shrinks under its own gravity.

“The theorists have shown very clearly that the gravity of the planet causes it to compress, so you essentially never get planets that are rocky and larger than twice the size of the Earth,” Marcy says. The pattern also implies that the smallest worlds should not be light and covered in gas.

But one freshly discovered exoplanet candidate is already bucking the trend. Also during the AAS meeting, David Kipping at the Harvard-Smithsonian Center for Astrophysics in Massachusetts presented preliminary data on a planet called KOI-314c (see picture, above). It has the same mass as Earth but is nearly twice the size, which makes it the lightest known exoplanet in Kepler’s catalogue.

Its unusual density hints that the small planet has a rocky core with a gassy atmosphere perhaps hundreds of kilometres thick. “It is nothing like the Earth, because it clearly has a very extended atmosphere,” says Kipping. “That suggests you can’t just draw a line in the sand at two Earth masses. There is a more blurry divide between the two regimes.”

Marcy questions how such a world could have survived. KOI-314c is also very close to its host star, with a year that lasts just 23 Earth days. In such a tight embrace, radiation from the star should quickly strip away its gases, making this potential planet an oddity.

Lucky Earth

Even if the dividing lines are fuzzy, the data still mean that fewer worlds than previously thought have rocky surfaces, limiting our current pool of candidates for an Earth twin.

That may not be bad news for life in general, but it could mean that advanced civilisations are rare. Complex creatures could still evolve and thrive on mini-Neptunes, although they would have better odds on the smallest ones with only a layer of water over the rock. Bigger ones with thick gassy envelopes would have daunting surface pressure, says Marcy. And chances are slim that mini-Neptune life would be very advanced.

“Earth is lucky. If the oceans were two or three times thicker, we’d have no technological life, because how could you build computers in a water world? How could you build a violin, or how could Rembrandt have ever painted in a water world?” Marcy says. “I think there are a lot of planets that have a little more water than the Earth covering the continents. Then you could have microbial life, fish, maybe marine mammals or even birds. But smart, articulate, machine-oriented critters like us humans on a water world? I think it’s unlikely.”