Kepler Mission Helps Discover That A Majority Of Small Planets Are Mini-Neptunes Or Super-Earths

A research team from the California Institute of Technology has now announced the results of a study based on its observations on all the planets discovered by Kepler. The findings show that small planets seem to come in two specific types -- and the type depends on the planet’s size.

According to The Economist, Kepler’s early discoveries showed that there is an abundance of worlds whose size is intermediate between Earth and Neptune -- the fifth and the fourth largest planets, respectively, in the solar system. However, since the gap between both planets’ size is big, the intermediate exoplanets have been categorized in two sizes.

On one side of the range are mini-Neptunes that presumably have rock and ice cores and on the other side of the spectrum are rocky worlds with little or nil atmospheres. The latter, termed as super-Earths, has been found to be the largest of terrestrial worlds that have a composition similar to the inner planets of the solar system. However, it is still unclear if there is a prototype that represents an overlap of both kinds, particularly due to the absence of accurate measurements of exoplanets’ diameters.

The Kepler discoveries, backed by the findings of the Keck Observatory in Hawaii, also show a clear difference in mini-Neptunes and super-Earths. For instance, the smallest mini-Neptunes have a diameter twice that of Earth and the maximum diameters of super-Earths are 1.75 times that of Earth. Furthermore, the gap between the two, which is a 50 percent difference in volume, indicates that bodies of intermediate size are unstable.

The research team has suggested that the gap is due to the way planetary atmospheres originate. According to The Wire, scientists also think that lack of intermediate-sized objects between the two types of worlds is a result of the bodies, which would otherwise fill the gap, having insufficient gravity to hold onto their atmospheres.

Another interesting occurrence noticed by the team is that though a large number of mini-Neptunes has been discovered -- with almost every planetary system explored by Kepler containing one -- it is surprising that there is none in the solar system. This has baffled the experts and they are banking upon future missions to help get an answer.