NASA Kepler: 'Neapolitan' exoplanets come in three flavours

This artist's conception shows a planet forming from a disk of gas and dust surrounding a young star. 

Credit: David A. Aguilar (CfA)

The planets of our solar system come in two basic flavors, like vanilla and chocolate ice cream.

We have small, rocky terrestrials like Earth and Mars, and large gas giants like Neptune and Jupiter.

We're missing the astronomical equivalent of strawberry ice cream - planets between about one and four times the size of Earth. NASA's Kepler mission has discovered that these types of planets are very common around other stars.

New research following up on the Kepler discoveries shows that alien worlds, or exoplanets, can be divided into three groups, terrestrials, gas giants, and mid-sized "gas dwarfs", based on how their host stars tend to fall into three distinct groups defined by their compositions.

"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."

"That's where you'll find rocky worlds, which are the only kind that we would consider potentially habitable," says lead author Lars A. Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA).

Buchhave presented his research today in a press conference at a meeting of the American Astronomical Society.

Kepler finds exoplanets using the transit method, looking for a star that dims as a planet passes in front of it from our point of view.

We can learn the planet's size from how much starlight it blocks. However, to determine the planet's composition we need to measure its mass, so its density can be calculated.

A rocky planet will be much denser than a gas giant. Unfortunately, the smaller a planet, the harder it is to measure its mass, especially for the dim and distant stars examined by Kepler.

New research finds that exoplanets can be divided into three groups, terrestrials, gas giants, and mid-sized "gas dwarfs," based on how their host stars tend to fall into three distinct groups defined by their compositions. 

All three are portrayed in this artist's conception. 

Credit: J. Jauch

Buchhave and his colleagues took a different approach. They measured the amount of elements heavier than hydrogen and helium, which astronomers collectively call metals, in 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.

The team took follow-up spectra of more than 400 stars hosting over 600 exoplanets.

Then, they conducted a statistical test to see if the sizes of the planets fell into natural groups, along with the stellar metallicities.

They found two clear dividing lines - one at a size 1.7 times as large as Earth and the other at a size 3.9 times larger than Earth.

They infer that these boundaries also mark changes in composition. Planets smaller than 1.7 Earths are likely to be completely rocky, while those larger than 3.9 Earths are probably gas giants.

More information: Research Paper: v509/n7502/full/nature13254.html