Aliens in TRAPPIST-1? Nearby Worlds Could Host Life—but There Is Too Much Water to See It

Although we normally think of water as key to life on Earth and elsewhere, it could actually be more difficult to find evidence of life on planets with immense stores of the stuff, a new study on the TRAPPIST-1 planetary system suggests.

Two TRAPPIST-1 planets, researchers report in the journal Nature Astronomy, are very, very wet. With both having more than half their entire mass made of water, it may be very difficult indeed to distinguish biological signatures from plain old geochemical ones.

TRAPPIST-1 is an ultracool M-type red dwarf star sitting just 40 light years from the sun. Seven planets have been discovered orbiting the star, of which at least three are sitting in the star's habitable zone.

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An illustration of the Trappist-1 exoplanet system. Although we normally think of water as key to life on Earth and elsewhere, it could actually be more difficult to find evidence of life on planets with immense stores of the stuff, a new study on the TRAPPIST-1 planetary system suggests. Dan Tell/California Academy of Sciences/JPL-Caltech/NASA

Astronomers are particularly excited about M dwarfs like TRAPPIST-1 because they may be the most common hosts of habitable planets in the Milky Way.

Researchers used previous estimates of the planets' masses and diameters to figure out their densities. They then used a computer program to work out what planetary components could add together to fit these results. Lower planet densities in the TRAPPIST-1 system seemed to suggest water.

Related: Could Trappist-1 host alien life? Two planets in the solar system could hold on to their atmospheres

Narrowing their efforts to four planets, they estimated that two were less than 15 percent water—still far more than Earth's measly 0.1 percent—and two were more than half water. These two incredibly wet planets are both in the habitable zone of the star, which means at least some of that water could be liquid.

However, intense pressure has probably squashed much of that liquid into ice, ScienceNews reports.

Cayman Unterborn, one of the study's authors and an exogeologist at Arizona State University in Tempe, told ScienceNews that liquid water on one of these planets, TRAPPIST-1f, stretches some 125 miles deep, and 1,400 miles of ice stretches below that—nearly halfway to planet's center.

These massive stores of water are bad news for alien-hunters. For one, a thick coating of ice and water could get in the way of processes that regulate a planet's temperature.

Related: Nearby Trappist-1 solar system has two planets that appear warm and friendly to life

Also, vast amounts of water could actually make it harder for chemicals like carbon to move into oceans. On Earth, chemical building blocks of life like carbon are drawn down into oceans via continental weathering and erosion. Without exposed land, it will be tough for scientists to tell if certain molecules in water are a sign of life or just a remnant from other geological processes, the study authors wrote.

When it comes to geological activity and chemistry, "the vast majority of data that's out there is for one planet, and it's ours," Unterborn told ScienceNews. The TRAPPIST-1 system, in comparison, is "an extreme of rocky planet chemistry."

"Although M dwarfs may be the most common habitable planetary host in our Galaxy," the study authors wrote in the Nature Astronomy paper, "they may be the toughest on which to detect life."

Aliens in TRAPPIST-1? Nearby Worlds Could Host Life—but There Is Too Much Water to See It | Tech & Science
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