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One-Third of Exoplanets Could Be Water Worlds With Oceans Hundreds of Miles Deep

A new statistical analysis suggests seas hundreds of miles deep cover up to 35 percent of distant worlds

(NASA)
smithsonian.com

Scientists often search for water in space because on Earth, anywhere there is water, there is life.

Rovers on Mars are looking for present-day water or ice as well as signs of ancient rivers and oceans. They’ve scoured the moon looking for signs of ice deep in its craters and even sent a probe to look for ice on a comet. But new research suggests finding cosmic H2O may not be all that difficult outside our own solar system. Simulations based on exoplanet data suggest water worlds covered with deep oceans may actually be rather common throughout our galaxy, according to a new study published this week in PNAS.

Since 1992, astronomers have catalogued about 4,000 exoplanets orbiting around distant stars. It turns out that most of those planets fall into two size categories: smaller planets with a radius about 1.5 times that of Earth and a mass about five times our planet and larger planets with a radius 2.5 times that of our planet and ten times the mass. Jamie Carter at Forbes reports that researchers believe the planets with smaller radii are rocky worlds. They interpreted the size and mass of the larger planets as a class of planets called gas dwarfs, which have a rocky core surrounded by a halo of gas.

Using new data about the radii and mass of exoplanets collected by the Gaia space satellite, Harvard planetary scientist Li Zeng and his colleagues gather more details about the exoplanets’ internal structures.

They found that those big gas dwarfs are better explained as water worlds. But these are not water worlds like Earth, where despite covering 71 percent of the surface, water only accounts for 0.02 percent of Earth’s mass. Instead, these worlds are made of 25 percent and up to 50 percent water, with strange, vast oceans covering them. It’s possible that up to 35 percent of all known exoplanets are these vast ocean-covered orbs, Li noted at a conference last summer.

Anyone wanting to sail the extraterrestrial seas, however, can forget about it.

“This is water, but not as commonly found here on Earth,” Li says in a press release. “Their surface temperature is expected to be in the 200 to 500 degree Celsius range. Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before… reaching the solid rocky core. The beauty of the model is that it explains just how composition relates to the known facts about these planets.”

Li explains George Dvorsky at Gizmodo in an email that these planets may or may not have a defined surface. The oceans could be hundreds of miles deep, calling them: “Unfathomable. Bottomless. Very Deep.” By comparison, the deepest known spot in the Earth’s oceans, Challenger Deep in the Mariana Trench, is less than seven miles deep.

The weight of all that water would create pressures over a million times that found on the surface of Earth, leading to some very strange phenomenon at the bottom, including the formation of “hot, hard” rock-like phases of ice, like Ice VII.

So if these water worlds are so common, why don’t we have one like them in our solar system? Zeng tells Carter that it’s possible our planetary system may be an oddball because we have massive gas giants like Jupiter and Saturn floating around.

“The formation of gas giants and the formation of those close-in super-Earths and sub-Neptunes are somewhat mutually exclusive,” he says. “Our solar system had formed the gas giant Jupiter early on, which probably had prevented or interfered with the formation and growth of super-Earths and sub-Neptunes.”

In other star systems without a Jupiter-sized planet, the formation of rocky “super-Earths” and water worlds is probably pretty common.

Sean Raymond, an astronomer at the University of Bordeaux who was not involved in the study, tells Dvorsky the study seems spot on, but cautions that we don’t have direct confirmation of all these water worlds. Our current methods of detecting exoplanets are indirect, and we have to infer what we know from their radius, mass, orbiting time and other data.

“[The study’s] conclusions are statistical, meaning that the authors are not pointing to specific planets and claiming them to be water worlds but rather focusing on the population as a whole,” he says. “Still, it’s a cool paper and a provocative result.”

As to whether some form of cosmic-aquatic life may be out there, it’s hard to say. But we may get more information soon when the beleaguered James Webb Space Telescope launches in 2021. That next-gen space scope should be capable of directly detecting water on distant exoplanets.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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