Discovery of Extraterrestrial Oceans: Alien Worlds Reveal Unimaginably Deep Water Reservoirs, 500 Times the Depth of Earth

 

Astronomers announced in December 2022 that they have discovered the first genuine water planets, which differ from any existing in our solar system. These exoplanets are slightly larger than Earth but have densities that fall between rocky planets like ours and the gas giants in our solar system. The astronomers believe that these planets possess global oceans at least 500 times deeper than Earth's oceans, which they described as a mere wet coating on a rocky sphere.

This illustration by an artist depicts Kepler-138 d in the foreground, with Kepler-138 c and Kepler-138 b visible in silhouette as they transit their star. Kepler-138 c and Kepler-138 d are water planets slightly larger than Earth, but their oceans are significantly deeper and warmer than Earth's, with a hot, steamy water vapor atmosphere enveloping them. Image courtesy of NASA/ESA/Leah Hustak (STScI)/Hubblesite.

These two worlds orbit a red dwarf star known as Kepler-138, located 218 light-years away in the Lyra constellation. Scientists have previously speculated about the existence of water planets like these but only had tantalizing clues until now. This discovery was made using data from the Hubble and Spitzer space telescopes.

Leading the research effort was Caroline Piaulet from the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, Canada. The team's findings were published in the peer-reviewed journal Nature Astronomy on December 15, 2022, and a free preprint version of the paper is also available on arXiv.

Water planets with extensive global oceans

While we typically regard Earth as a water planet due to its predominantly ocean-covered surface, these two newly discovered exoplanets, Kepler-138 c and Kepler-138 d, appear to have oceans that are at least 1,000 miles (1,600 km) deep. This depth is approximately 500 times greater than the average depth of Earth's oceans.

In our own solar system, moons like Europa and Enceladus, which harbor oceans beneath their icy crusts, serve as the closest analogies. However, these moons are much smaller than planets and primarily composed of ice. The newly discovered planets are comparable to larger and warmer versions of these ocean moons. As Piaulet described:

Imagine larger versions of Jupiter and Saturn's water-rich moons, Europa and Enceladus, but brought much closer to their star. Instead of an icy surface, they would possess extensive water-vapor envelopes.

Strange densities offer clues

How did astronomers identify these water planets? The answer lies in the data obtained from the Hubble and Spitzer space telescopes. Upon analyzing the data, the researchers noticed something peculiar. Although both planets are slightly larger than Earth, they are not as dense as Earth or other known rocky planets. Nonetheless, their densities exceed those of the gas or ice giants in the outer solar system. This raises the question: how is this possible?

Björn Benneke, a co-author from the University of Montreal, explained:

We previously believed that planets slightly larger than Earth were composed of metal and rock, resembling scaled-up versions of our planet, which is why we called them super-Earths. However, we have now shown that these two planets, Kepler-138 c and d, differ significantly in nature and that a substantial portion of their total volume likely consists of water. This evidence represents the strongest indication thus far for the existence of water planets, a planetary type that astronomers have theorized about for a considerable time.

Water planets, but unlike Earth

Although the oceans on these planets are comprised of water, they are believed to differ considerably from Earth's oceans. The evidence suggests that these oceans are considerably warmer and subjected to high pressures. There may not even be a distinct boundary between the ocean surface and the planet's atmosphere. For instance, Kepler-138 d's atmosphere is hot and likely composed of steam. Piaulet elaborated:

The temperature in Kepler-138 d's atmosphere likely surpasses the boiling point of water, and we anticipate the presence of a thick and dense atmosphere predominantly consisting of steam on this planet. It is conceivable that beneath this steam atmosphere, liquid water exists under high pressure, or even an alternative water phase that occurs under high pressures, known as a supercritical fluid.

Both planets also have close orbits around their star and lie outside the habitable zone where rocky planets could sustain liquid water. This means that the presence of liquid water, as we typically envision it in the form of lakes or Earth-like oceans, is unlikely. However, due to the planets' proximity to their star, their atmospheres are overheated and steamy, with liquid water existing at greater depths under high pressure.

Two 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, similar in size to Mars, with a mass estimated to be 0.07 times that of Earth. Kepler-138 e, on the other hand, is the farthest from the star. It lies just within the inner boundary of the habitable zone and completes an orbit around its star in 38 days. Astronomers have yet to determine its exact size, but the study suggests it is larger than Kepler-138 b, with a mass approximately 0.43 times that of Earth. Unlike the first three planets, Kepler-138 e does not transit in front of its star, making it more challenging to study. The paper states:

While the transits of Kepler-138 b, c, and d are observed in the Kepler light curve, Kepler-138 e, although expected to be larger than Kepler-138 b, does not exhibit a transit. We interpret this as indicative of Kepler-138 e likely having a non-transiting orbit configuration.

Overall, this discovery demonstrates that not all super-Earths are rocky in composition. Some, such as Kepler-138 c and d, may primarily consist of water. Astronomers anticipate finding more water planets as our instruments and techniques become more sensitive to detecting and studying planets located farther from their stars.

In conclusion, an international team of astronomers has identified two exoplanets located 218 light-years away that qualify as water planets, with global oceans 500 times deeper than Earth's oceans.