Traditionally, planets been put into one of two categories: rocky or gas giant. However, researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) have now announced the discovery of a third type of planet: a gas dwarf. The research was described in Nature and was presented today at the 224th Meeting of the American Astronomical Association.

While our solar system is home to small rocky planets and huge gas giants, there are other types of planets elsewhere. Using data from NASA’s Kepler mission, the researchers have discovered that smaller gas planets are actually quite common around other stars. A planet’s category is determined by the composition of the host star.

"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," lead author Lars A. Buchhave said in a press release.

Exoplanets are much too distant to detect the light they reflect, as they get washed out by the glare of their host star. Instead, Kepler is able to identify exoplanets and their size by how much light the planet block outs when it transits the star. Researchers use the planet’s orbital radius and the mass of the star to determine the planet’s mass. When mass and size have been determined, astronomers can calculate the density, which provides information about the composition of the planet.

This method isn’t without its faults, as dimmer and smaller planets closer to the size of Earth are considerably harder to calculate the mass. Buchhave’s team circumvented this obstacle by measuring the metal content (metallicity) of the host star. As the planets were formed from the same material, the content of the star is likely to match the content found in the planets.

After analyzing 600 exoplanets around 400 stars, they found that there were clear dividing lines regarding size and composition. Planets more than 3.9 times the size of Earth were most likely gas giants, and anything smaller than 1.7 times the size of Earth was likely rocky. The group in the middle represents the gas dwarfs, which have rocky cores that can accumulate gas, forming atmospheres thick with hydrogen and helium.

Distance from the star plays a role in how large the rocky core can grow before developing the thick atmosphere, with planets more distant to the sun growing quite large before becoming a gas dwarf. This shows that there is no upper limit to the size of rocky planets.

Additionally, the team also discovered that small, rocky planets have metallicities similar to our Sun, while systems with more gas giants have about 50% more metal than our sun.

"It seems that there is a 'sweet spot' of metallicity to get Earth-size planets, and it's about the same as the Sun. That makes sense because at lower metallicities you have fewer of the building blocks for planets, and at higher metallicities you tend to make gas giants instead,” he added.

Of course, metallicity of a star isn’t the only factor that goes into regulating which types of planets will be present in the system; there are many others to consider as well. Additionally, as the data comes from Kepler that studies inner planets transiting the star, it does not include data on some of the more distant exoplanets in the system, which may alter the results. Future study will utilize outer planets when more information is available.