Among the abundance of recently discovered planets around other stars, one identifiable group has puzzled astronomers. Ranging between two and four times the Earth's radius, the objects have spurred debate on whether they are small gas giants – Neptune's smaller cousins – or if they are primarily made of water. The possibility these are rocky worlds like our own has already been largely dismissed.
Whether life could thrive in oceans thousands of kilometers deep is a great unknown, but it certainly seems more likely than on a world of gas. New research favors the water world theory, meaning that if life can exist under such conditions, the universe may be filled with it.
Before we started finding planets beyond our Solar System, our expectations were shaped by what we could see. Many astronomers expected a mix of rocky worlds the size of Earth or smaller as well as much larger gas giants. Others predicted a smooth curve, with larger sizes increasingly rare. Instead, we have found an abundance of planets similar in size to Earth and a second category that have a radius between two and four times larger. There’s an intriguing gap at 1.8-2.0 Earth radii. Harvard University's Dr Li Zeng has identified planets above this gap as likely made mostly of water, whether in liquid or solid form.
Most of the planets in question were measured through the decrease in light collected by the Kepler space telescope as they passed across their star’s face. This tells us their size but not mass. However, in some cases we have been able to conduct follow-up measurements with the Gaia space telescope to observe the wobble the parental star undergoes in response to the planet's orbit, thus revealing their mass.
The most common size for the worlds Kepler found is 2.5 Earth radii, Zeng reports in Proceedings of the National Academy of Sciences. That makes for a volume 16 times that of the Earth. Those whose mass could be measured cluster between 6 and 15 Earth masses, only a little less dense than Earth (Neptune, being mostly gas, is less than a third as dense as Earth). The finding is consistent with at least a quarter of the planet being water or ice around a metallic and rocky core. Any atmospheres are probably thin veneers around the planet, similar to our own.
Many of these planets exist in their star's habitable zone, where water can exist in liquid form, so they are candidates for life. In modeling how these planets formed, Zeng reached conclusions relevant closer to home. He explains Uranus and Neptune as mergers of smaller icy cores that prevented the build-up of hydrogen and helium Jupiter experienced.