In contrast to any in our solar system, astronomers announced in December 2022 that they had discovered the first actual water worlds. Both of these exoplanets are marginally bigger than Earth. However, their density is in the middle of that of our rocky planet and the gas-giant outer planets of our solar system. The global seas on these planets, according to these astronomers, must be at least 500 times deeper than the average depth of the oceans on Earth, which they have referred to as nothing more than a wet veneer on a stony ball.

The two worlds orbit a red dwarf star called Kepler-138, which is 218 light-years away in the direction of our constellation Lyra the Harp. Scientists have theorized that global water worlds like these exist. They’ve had some tantalizing hints before now. But these are the first specific worlds for which there is now good evidence. The astronomers made the discovery using data from the Hubble and Spitzer space telescopes.

This artist’s illustration shows Kepler-138 d in the foreground, Kepler-138 c and Kepler 138 b, seen in silhouette transiting its star. Kepler-138 c and Kepler-138 d are water worlds a little larger than Earth. But their global oceans are much deeper than Earth’s and warmer, with a hot, steamy water vapor atmosphere above them. Image via NASA/ ESA/ Leah Hustak (STScI)/ Hubblesite.

Caroline Piaulet of the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, Canada, led the team effort. The researchers published their peer-reviewed findings in Nature Astronomy on December 15, 2022. A free preprint version of the paper is also available on arXiv.

Water worlds with deep, global oceans

We think of Earth, of course, as a water world, with its surface mostly covered by oceans. But these two new exoplanets – Kepler-138 c and Kepler-138 d – appear to have oceans at least 1,000 miles (1,600 km) deep. That’s about 500 times deeper than the average depth of Earth’s oceans.

The ocean moons in our own solar system, such as Europa, Enceladus and others, would be the closest analogy. But both of those are much smaller moons, not planets, with a crust of ice over their oceans. These new planets are kind of like much larger and warmer versions of the ocean moons. As Piaulet said:

Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star. Instead of an icy surface, they would harbor large water-vapor envelopes.

Weird densities provide clues

So, how did the astronomers discover these water worlds? The Hubble and now-retired Spitzer space telescopes provided the data. When the researchers studied the data, they found something unusual. Both planets are just a little larger than Earth. But they aren’t nearly as dense as Earth or other known rocky planets. They are, however, still denser than the gas or ice giants in the outer solar system. How could that be?

Co-author Björn Benneke at the University of Montreal said:

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. However, we have now shown that these two planets, Kepler-138 c 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.

Water worlds, but not like Earth

While the oceans are composed of water, as on Earth, the researchers believe them to be quite unlike our own oceans. The evidence suggests that they are significantly warmer and under very high pressure. There may not even be a sharp boundary between the top of the oceans and the atmospheres of the planets. For example, Kepler-138 d’s atmosphere is hot and probably composed of steam. Piaulet said:

The temperature in Kepler-138 d’s atmosphere is likely above the boiling point of water, and we expect a thick dense atmosphere made of steam on this planet. 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.

Both planets also orbit close to their star, and are not in the habitable zone where temperatures on rocky planets could allow liquid water to exist. This means liquid water in the ways we are used to seeing it, in lakes or more Earth-like oceans. But because these two planets are much closer to their star, their atmospheres are overheated and steamy, with liquid water deeper down at high pressure.

Comparison of the interior structures of Earth and Kepler-138 d. Image via Benoit Gougeon. (University of Montreal)/ Hubblesite.

2 other planets in the Kepler-138 system

The Kepler-138 system also contains two additional planets.

Kepler-138 b is also rocky and even smaller, close to the size of Mars, with a mass calculated to be 0.07 times that of Earth. Kepler-138 e, on the other hand, is the farthest from the star. It is just inside the inner edge of the habitable zone, and orbits its star in 38 days. Astronomers aren’t sure yet exactly how large it is, but the study suggests it is larger than Kepler-138 b, with a mass 0.43 times that of Earth. Unlike the first three planets, however, its orbit doesn’t transit in front of its star, making it harder to study. The paper says:

The transits of Kepler-138 b, c and d are detected in the Kepler light curve, but while Kepler-138 e should be larger than Kepler-138 b, its transit is not detected. We interpret this as originating from a likely non-transiting configuration of Kepler-138 e’s orbit.

Overall, the findings show that not all super-Earths are rocky. Some, like Kepler-138 c and d, can be largely water. And astronomers expect to find more of them, as Benneke noted:

As our instruments and techniques become sensitive enough to find and study planets that are farther from their stars, we might start finding a lot more of these water worlds.

Bottom line: An international team of astronomers says that two exoplanets 218 light-years away are true water worlds, with global oceans 500 times deeper than Earth’s oceans.

Source: amazingastronomy.thespaceacademy.org