As astronomers continue to survey the galaxy looking for Earth-like planets, they are beginning to realize how unique Earth really is. In fact, they are learning that most of the planets outside our own solar system resemble mini versions of Neptune.

That realization comes from two different and separate analyses of data collected by NASA’s space telescope. The telescope had looked for the last four years for planets outside our solar system that might be similar enough to Earth that astronomers could posit they might hold life.

Astronomers have then been narrowing the number down to any planets that are essential Earth’s twins, which would make them most likely to be habitable. The way Kepler was able to detect exoplanets was by watching how their host starts would dim as the planets passed in front of them.

This method revealed the exoplanets’ size to astronomers, but not their mass. The mass is the crucial feature to determine whether the planet is a rocky world like Earth or a gaseous like Neptune or Jupiter.

To determine mass, a research team led by Geoff Marcy of the University of California, Berkeley watched how the gravity of 42 planets tugged at their host starts. From that they were able to determine the masses of each planet.

The other study led by Yoram Lithwick from Northwestern University in Evanston, Ill., measured how 60 planets in multi-planet systems pulled on each other to determine their masses.

Using these disparate techniques, both teams were able to come to the same conclusions.  They found that worlds up to twice the size of Earth are dense and most likely rocky, like our own planet. Meanwhile, the planets that were between two and four times as big as Earth are actually lighter, which means they are either wetter or more gassy, like Neptune (which is four times the size of Earth). The astronomers determined that three-quarters of them were of the gassy type similar to Neptune.

“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.

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