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Astronomers identify twin water worlds 218 light years from Earth

"I've sailed farther than most men have dreamed," Kevin Costner as The Mariner in Waterworld.

Waterworld (1995)

In the not so distant future, after decades of runaway climate change, all of the world’s ice caps melt and the planet is plunged into a vast, global ocean. Our only hope? The Mariner (Kevin Costner), with his slapdash ship, urine distillation machine, and a handy mutation which lets him breathe underwater. It’s tough to make a go of it in Waterworld, no matter who you are.

The truth about our planet isn’t quite so bleak. In reality, even if every glacier and iceberg, every scrap of frozen water on Earth, were liquified, it would only raise global sea levels by about 70 meters. It’s worth noting that the use of the word “only” in this context is pretty loaded. That much sea level rise would be catastrophic in just about every way that matters to us, it just wouldn’t submerge the world. Any survivors would have plenty of land on which they could struggle to survive and rebuild in the aftermath.

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If you want a true water world, you’ll have to look further afield. But we can point you in the right direction. Researchers from the University of Montreal used data from the Hubble Space Telescope and the retired Spitzer telescope to investigate a planetary system around the star Kepler-138. Their findings, which were published in the journal Nature Astronomy, reveal a twin pair of planets carrying more water weight than they have any business having.

Kepler-138 lies about 218 light years from us and it has a smattering of worlds. The first, Kepler-138b, is a small rocky world only about half the radius of Earth. It orbits close in, only 7 million miles from its parent star. That’s compared to Earth’s comfortable 93 million mile distance from the Sun. It’s not the sort of place you’d even get close to let alone sail into the horizon on your catamaran. Its planetary siblings, however, are another story.

Kepler-138c and 138d are each about 1.5 times the size of Earth and astronomers recently determined that roughly half of their volume is made up of water. To be clear, that doesn’t mean they’re half covered in water. It means that half of their entire volume, from core to cover, is water. You might be wondering how astronomers can possibly figure out how much water exists on a pair of worlds 218 light years away. It’s a good question.

Kepler-138c and Kepler-138d

Astronomy is, in large part, a practice of taking one piece of information and bumping it up against another to see if you can get a third. You can then take that new information and use it to get a fourth piece of information, and so on. We can’t experiment on these objects in a lab, so we have to work out complex mental puzzles with incomplete information. It’s basically the scientific equivalent of a cosmic Sudoku puzzle.

In this case, astronomers can measure the distance between a planet and its star. Then they measure how much the star wobbles as the planet circles around it. Those two figures can tell you a planet’s approximate mass, but mass doesn’t give you composition. A small dense planet and a large less dense one could have the same mass. We need more information.

Fortunately, both Kepler-138c and 138d transit in front of their star, from our perspective. When they pass in front of the star, we can measure the amount of light reaching our telescopes and compare that to how much light we received when the planets weren’t in the way. The lost light tells us what fraction of the star was blocked by a planet and we can use that to figure out how big they are. Using mass and size, we can work out density, and that gives you clues as to what a planet is made of.

According to their measurements, Kepler-138c and 138d are nearly identical with diameters about 1.51 times that of the Earth and masses 2.3 and 2.1 times that of Earth, respectively. Split evenly, they each have a planetary density of 3.6 grams per cubic centimeter.

Astronomers then compared those densities to known objects in our own solar system. Earth, the densest object in the neighborhood, comes in at 5.5 grams per cubic centimeter. Saturn, meanwhile, is floating carefree with a miniscule 0.69 grams per cubic centimeter. Kepler-138’s twin planets come in right in the middle of that scale, leading scientists to conclude a significant portion is made up of something less dense than rock but denser than hydrogen or helium. Water fits the bill nicely.

Continuing the comparison to local worlds, we’ll inevitably come to Europa, which has a density of 3.0, dangerously close to our distant super-Earths. Europa is also covered with a global ocean, lending support to the idea that Kepler-138c and 138d are water worlds. But it’s more than that.

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If our hypotheses are correct, calling them a water world undersells what’s really going on. The numbers suggest they are covered over with a single endless sea stretching from horizon to horizon in every direction, forever. That ocean descends to depths of 2,000 kilometers, more than 500 times the depth of Earth’s deepest waters. Unlike Europa, scientists don’t expect 138c or 138d to have an ice shell. It’s unlikely they have water ice at all. They orbit 8.4 and 11.8 million miles from their star, respectively, about a tenth the distance as we are from the Sun, give or take a couple million miles. It’s hot and it’s humid, with surface temperatures above the boiling point of water at the surface. Researchers believe that both planets would have thick steamy atmospheres comprised of water vapor. Only deeper down, where the pressure gets intense, would liquid water exist in a super dense state. The Mariner wouldn’t last a second.

Fortunately, while investigating Kepler-138’s water worlds, they discovered a previously unknown fourth planet dubbed — you guessed it! — Kepler-138e. It appears to be in the planet’s habitable zone, where any water it has could remain in a liquid state at ordinary pressures. Unfortunately, 138e doesn’t transit in front of its star from our POV, leaving us to speculate about many of its other qualities. We like to imagine it’s got calm, serene seas and beautiful red dwarf sunsets.

Can’t get enough Kevin Costner? We don’t blame you! Check out all four seasons of Yellowstone, streaming now on Peacock!

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