In the sci-fi film Waterworld, Earth’s polar ice caps melt entirely, raising sea levels to extreme heights. While Earth’s oceans may never reach those levels, astronomers have discovered planets far beyond our solar system that fit the description of true “water worlds.” These exoplanets, primarily composed of water, offer exciting insights into the diversity of planetary formation. However, unlike Earth’s oceans, these worlds exist under extreme conditions, making them vastly different from our blue planet.
Discovery of Kepler-138’s Water Worlds
Scientists at the University of Montreal have been studying a distant planetary system called Kepler-138, located about 218 light-years away in the Lyra constellation. Their research, conducted through the Trottier Institute for Research on Exoplanets, revealed two exoplanets—Kepler-138 c and Kepler-138 d—that appear to be made mostly of water. The Hubble and retired Spitzer space telescopes provided data for this groundbreaking discovery.
NASA’s Kepler Space Observatory first identified the Kepler-138 planets in 2014. Follow-up observations with the Hubble and Spitzer telescopes allowed researchers to determine that these planets lack the rocky solidity seen on Earth. Instead, their low density and unique composition suggest they’re mostly water, covered by a thick layer of steam.
What Makes Kepler-138 c and Kepler-138 d Unique?
Kepler-138 c and d are larger than Earth, with volumes more than three times its size and masses about twice as great. This size difference is intriguing, as most planets larger than Earth that have been studied so far resemble rocky “super-Earths.” However, these two planets stand apart as “mini-Neptunes” with a large proportion of their volume likely consisting of water. Researchers estimate that water occupies up to half of their volume.
This discovery challenges previous assumptions that planets slightly larger than Earth would automatically have rocky compositions. “We previously thought that planets a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth,” explained Björn Benneke, study co-author and professor of astrophysics at the University of Montreal. “However, we have now shown that these two planets, Kepler-138 c and d, are quite different in nature.”
How Scientists Studied the Water Worlds
Direct observation of water on Kepler-138 c and d isn’t possible due to their distance and the limitations of our technology. Instead, researchers used indirect methods to measure the planets’ density by comparing their size and mass. This approach allowed astronomers to deduce that materials lighter than rock but heavier than hydrogen or helium must make up a significant part of these planets. Water is the most plausible candidate.
Images released by NASA depict an artist’s rendering of Kepler-138 d as a large water world with a steamy atmosphere. According to Benneke and his team, this dense, steam-filled layer likely results from surface water reaching high temperatures under intense atmospheric pressure. The planet’s surface environment is unlike any ocean on Earth, as it would exist under conditions unsuitable for typical marine life.
What Lies Beneath the Steam?
While Kepler-138 c and d may not have fish swimming in their oceans, scientists believe they contain supercritical water—a high-pressure, high-temperature state in which water behaves as both a liquid and a gas. This supercritical fluid could cover a rocky core, but researchers anticipate that these planets lack a clear ocean-atmosphere boundary as seen on Earth.
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Instead, Kepler-138 d likely has a dense atmosphere of steam surrounding an ocean with high-pressure water. “Only in that steam environment could there possibly be high-pressure liquid water or even water in another phase that happens at high pressures,” said Caroline Piaulet, lead researcher at the Trottier Institute.
Additional Planets in the Kepler-138 System
In 2014, astronomers confirmed the existence of three planets in the Kepler-138 system. The discovery relied on the “transit method,” which identifies planets as they pass in front of their star, causing a temporary dip in brightness. Kepler-138 c and d, which are likely water-rich, are not located in the habitable zone—the distance from a star where conditions would allow liquid water on a rocky surface.
However, scientists recently identified evidence of a fourth planet, Kepler-138 e, in the Hubble and Spitzer data. Unlike its siblings, Kepler-138 e appears to lie within the habitable zone of the Kepler-138 star. This newly found planet is smaller and orbits farther from its star than the other three. Since it doesn’t seem to transit its star, details about its composition remain a mystery. Further observations may reveal whether Kepler-138 e also harbors water or has the potential for life.
Could Life Exist on These WaterWorld?
Despite the abundance of water on Kepler-138 c and d, these planets aren’t likely to support life as we know it. Their high-pressure, high-temperature conditions make them inhospitable for typical marine organisms. In contrast, the newly discovered Kepler-138 e, within the habitable zone, may offer more favorable conditions for life, though its composition remains unknown.
The Kepler-138 system reminds us that our galaxy holds a vast array of planets, many with properties vastly different from Earth’s. Each discovery of Waterworld brings new questions about the possibilities of life beyond our solar system and how planets form under various conditions.
A New Frontier in Planetary Exploration
The discovery of Kepler-138’s Waterworld has opened an exciting new chapter in exoplanet research. Kepler-138 c and d demonstrate that water-rich planets can exist outside our solar system, challenging our understanding of what planets can look like. While the Kepler-138 planets are extreme and likely uninhabitable, they add to the growing list of planetary types and inspire further study of the universe’s diversity.
As our technology advances, we may uncover more about these distant water worlds and perhaps even identify planets with conditions more like Earth’s. The Kepler-138 system, with its fascinating mix of steam-shrouded oceans and potentially habitable zones, will continue to be a focus of astronomers seeking answers to the mysteries of the cosmos.
For now, Kepler-138 remains a distant reminder of the extraordinary variety that exists within our galaxy, showing us that even worlds completely covered in water are possible, though they may not look anything like the oceans we know.
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