Did you know that NASA has confirmed more than 5,000 exoplanets to date? This isn’t the result of hundreds of years of scientific discovery, either. The first exoplanets were just documented in the 1990s!
“Exoplanet” makes these discoveries sound a little like something out of a sci-fi movie. But it’s really just a word for a planet that orbits a star outside of our own solar system. In other words, a planet that doesn’t orbit our Sun.
In total, NASA has confirmed close to 4,000 planetary systems, with 847 of them having more than one planet. Each of the planets within these systems falls into one of four categories: Gas giant, Neptunian, super-Earth, or terrestrial. They can be broken down further from those broad categories, of course, but the labels give a general idea of what to expect from their appearance and composition.
Kepler-138 b, for example, is a terrestrial exoplanet, meaning it’s smaller than Earth and made up of rock and metal. Meanwhile, the newest discoveries, Kepler-138 c and Kepler-138 d, are super-Earths. They’re rocky as well, but larger than Earth, and may or may not have atmospheres.
But that’s not what makes these new discoveries so exciting. There are plenty of known super-Earth planets out there. What’s really fascinating about Kepler-138 c and its twin, Kepler-138 d, is that they might not fall into that category at all. Scientists now believe they’re covered in oceans 500 times deeper than Earth’s.
NASA Discovers Twin Exoplanets Covered in Water
Technically, Kepler-138 c and d aren’t new discoveries. We’ve known about them since 2014 when we realized that a star called Kepler-138 had at least three planets in its orbit. This planetary system is about 218 light-years from Earth in the Lyra constellation.
At the time of its discovery, NASA determined that the planets in Kepler’s orbit were largely made of rock. Now, however, using space telescopes and the W.M. Keck Observatory in Hawaii, scientists are learning there’s more than meets the eye to the faraway planets.
Each planet is made of something lighter than rock but heavier than the elements found in gaseous worlds. The most likely explanation? Water.
“We previously thought that planets that were a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, and that’s why we called them super-Earths,” Björn Benneke, the Nature Astronomy study’s co-author, said in a release. “However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature and that a big fraction of their entire volume is likely composed of water.”
“It is the best evidence yet for water worlds, a type of planet that was theorized by astronomers to exist for a long time,” Benneke continued.
The twin “water worlds” are outside Kepler-138’s habitable zone, meaning they wouldn’t support life. Not to mention, scientists believe their atmospheres are made of boiling-hot steam.
“The temperature in Kepler-138d’s atmosphere is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on this planet,” added Caroline Piaulet, research team leader. “Only, under that steam atmosphere there could potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid.”