Imagine a planet about 2.5 times the radius of Earth orbiting a distant star. Do you picture something like our own rocky, ocean-strewn world? Or is it something more akin to a small version of a gassy planet like Neptune? Now, two new papers suggest that there may be a third option were not taking seriously enough.

As more and more confirmed exoplanets roll in, one thing is clear: The Sun is not orbited by every type of planet that exists. Our solar system basically has two flavors of planet rocky and gassy. The inner planets make up the former group, and the outer planets make up the latter. But in recent decades, astronomers are finding a lot of worlds that break that familiar mold.

Astronomers general consider super-Earths to be exoplanets that have up to about four times the mass of Earth and a radius up to about 1.6 times that of Earth. But because theres been such a wide variety of planets found around other stars, the exact definition of a super-Earth is still largely debated.

To muddy things further, mini-Neptunes are what astronomers usually call planets that have masses several to 10 or more times that of Earth (Neptune is about 17 times the mass of Earth), plus a radius more than 1.6 times that of Earth. The assumption was that even the smallest of these mini-Neptunes are too girthy to be rocky worlds, so they were thought to be made of mostly hydrogen and helium, like a normal gas giant.

In the community, there was this stance that we were really looking at two populations [super-Earths and mini-Neptunes], Jrmy Leconte, an astrophysicists at the French National Centre for Scientific Research (CNRS) and coauthor of a new paper on the topic, tells Astronomy. But without an easy way to directly capture images of either type of planet, the exact dividing line remained fuzzy.

Now, based on Lecontes and others recent work, its starting to look like super-Earths and small mini-Neptunes might be two versions of nearly the same thing.

On June 15, scientists at the Laboratoire d'Astrophysique de Marseille published a paper in The Astrophysics Journal Letters that suggests mini-Neptunes dont have to rely on hydrogen and helium for their fluffy compositions. Instead, the study claims, mini-Neptunes could be irradiated ocean worlds with dense, rocky cores and ultrathick atmospheres of water.

Such an atmosphere, the researchers says, could be the result of an intense greenhouse effect caused by the rocky planets fiery host star boiling water from its surface. Such a scenario, the study suggests, could keep the planets mass in check, while also greatly increasing its radius.

Furthermore, Lecontes recent work, published June 9 in Astronomy & Astrophysics, focuses on how host stars can irradiate the surfaces of a rocky, water-rich exoplanets. This process, the research suggests, can indeed create an expansive atmosphere that balloons a solid rocky world into what looks like a shrunken gas giant.

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Mini-Neptunes could be super-Earths with bloated atmospheres of water - Astronomy Magazine