Some TRAPPIST-1 planets may be water worlds

There’s so much water on some of TRAPPIST-1’s seven Earth-sized planets that any life lurking there might be difficult to detect.

New estimates of the makeup of these potentially habitable worlds suggests that two of them are more than half water, by mass, researchers report March 19 in Nature Astronomy. Earth, by comparison, is less than 0.1 percent water.

TRAPPIST-1’s planets are so wet that most of the water probably isn’t even liquid, but ice formed under high pressure, says Cayman Unterborn, an exogeologist at Arizona State University in Tempe. That would change the chemistry happening on the planet in a way that could make any signs of life tricky to distinguish from geochemical processes.

TRAPPIST-1 is a cool, dim star about 39 light-years from Earth. Since the star system’s discovery in 2017, it’s been a prime focus for scientists seeking life outside of our solar system because some of the seven planets might have the right conditions to host life (SN: 12/23/17, p. 25). They’re rocky rather than gaseous, and at least three are at a distance from the star that could let them host liquid water.

Unterborn and his colleagues used previous estimates of the mass and diameter of TRAPPIST-1’s planets to calculate the worlds’ densities. Then, the team used a computer program to test different compositions of basic planetary building blocks to determine which makeups would yield planets with those densities.

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Frozen or liquid water is less dense than rock, but more dense than a gas. So a less dense planet might have a higher proportion of water or gases compared with a denser, rockier world. But Unterborn doesn’t think the TRAPPIST-1 planets are massive enough to hold onto much of an atmosphere — it would probably escape into space. So the team concluded the lower densities in this system probably come from the presence of water.

The researchers focused on four of the seven planets for which they had the best data. The first and second planets from the dwarf star are probably less than 15 percent water by mass, still far wetter than Earth, the researchers found.

The fifth and sixth planets, both in the habitable zone, are more than half water — a volume so large that the water pressure alone could force much of it into a form of ice, Unterborn says. He estimates that on the fifth planet, TRAPPIST-1f, liquid water extends down about 200 kilometers — about 20 times deeper than the Mariana Trench on Earth. Below that, a nearly 2,300-kilometer layer of ice stretches almost halfway to the center of the planet.

These water estimates might throw a wet blanket on the chances of finding life on any of TRAPPIST-1’s planets, if it exists at all. The thick covering of ice and water might mess up some of the geological processes that, at least on Earth, help regulate the planet’s temperature over long periods of time. If so, that might be an impediment to life getting a foothold. Having so much water might also slow or halt the movement of building blocks of life, such as carbon and phosphorus (the backbone of DNA), into oceans. That could make it harder for us to detect whether certain molecules in the water are hints of the presence of living organisms, or just the by-products of geological processes.

It doesn’t rule out life, Unterborn says, but it does make it harder to find. When it comes to understanding the way a planet’s geologic composition affects chemical processes, “the vast majority of data that’s out there is for one planet, and it’s ours,” he says. The TRAPPIST-1 system is “such an extreme of rocky planet chemistry.”

Updated estimates of the TRAPPIST-1 planets’ masses were published in February (SN Online: 2/5/18), and this study doesn’t use those numbers, says Billy Quarles, a physicist at the University of Oklahoma in Norman who wasn’t part of the study. Based on the newer estimates, TRAPPIST-1’s planets aren’t quite as wet as this study predicts. But the big-picture conclusion — that some of the planets contain far more water than Earth — still holds up, he says.

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