NASA Kepler Discovers Three Different 'Flavors' of Planets--From Rocky to Gassy

Planets come in two different categories in our solar system. There are rocky terrestrial planets, like Earth, and then large gas giants, like Jupiter and Neptune. But we're missing a third category--planets that are about one to four times the size of Earth. Now, NASA's Kepler has revealed that these types of planets are extremely common around other stars.

Kepler finds exoplanets through a technique called the transit method. This occurs when a star dims slightly as a planet passes in front of it from our point of view. By seeing how much starlight it blocks, we can learn how large the planet is. In order to learn more about the planet and understand its composition, though, its mass has to be determined, which is a little harder.

"We were particularly interested in probing the planetary regime smaller than four times the size of Earth, because it includes three-fourths of the planets found by Kepler," said Lars A. Buchhave, one of the researchers, in a news release. "That's where you'll find rocky worlds, which are the only kind that we would consider potentially habitable."

In order to find these rocky worlds, the researchers measured the amount of elements heavier than hydrogen and helium within stars with exoplanet candidates. Since a star and its planets form from the same disk of material, the metallicity of a star reflects the composition of the protoplanetary disk-and its planets.

In all, the scientists took follow-up spectra of more than 400 stars hosting over 600 exoplanets. Then, they conducted a statistical test to see if the planets fell into natural group sizes. In the end, they found two clear groups: one at a size 1.7 times as large as Earth and the other at a size at 3.9 times the size of Earth. These boundaries also probably mark changes in composition; planets smaller than 1.7 Earths are probably rocky while the others are most likely gas giants.

But what about planets that fall in between 1.7 and 3.9 times the size of Earth? These are gas dwarfs, since they have thick atmospheres of hydrogen and helium. The rocky cores of gas dwarfs formed early enough to accrete some gas, though did not grow as large as gas giants like Jupiter.

That's not all the researchers found. They also discovered that the size of the largest rocky world isn't fixed, so some planets can become huge before accreting thick atmosphere and turning into a gas dwarf.

The findings reveal a bit more about the different planets that occur outside our solar system. By learning about these different categories, scientists can find out a bit more about which planets might be suitable for life