Life as we know it can't exist without water, so the search for extraterrestrial life often begins as a search for water. A new study on the density of the seven planets in the nearby TRAPPIST-1 system has found that these worlds may have even more water than Earth does, but in a cruel case of having too much of a good thing, that might be enough to drown out any hopes of finding life there.
TRAPPIST-1's seven planets were discovered just over a year ago, and since then they've been the subject of intense study. Astronomers have calculated the diameters and densities of the planets, what they might be made of, how the closely-knit group avoids crashing into each other, and of course, what the chances are of finding life.
Judging by existing data, the TRAPPIST-1 planets are surprisingly light for their mass and volume. At a glance, the most obvious explanation to astronomers is that the worlds must be quite gassy, with thick atmospheres – but that doesn't quite line up with observations.
"The TRAPPIST-1 planets are too small in mass to hold onto enough gas to make up the density deficit," says Cayman Unterborn, co-author of the study. "Even if they were able to hold onto the gas, the amount needed to make up the density deficit would make the planet much puffier than we see."
The next candidate to plug the hole is water, either in the form of liquid or ice. So the scientists on the new study set out to determine the composition of the worlds. Using a physics calculator program known as ExoPlex, the researchers plugged in as much of the available data as possible on the star and planets in the system, as well as comparisons to other stars.
The team found that water appears to account for as much as 15 percent of the mass of the innermost planets, TRAPPIST-1b and 1c, and over 50 percent for the planets 1f and 1g. That's an absolutely huge amount of water – by comparison, Earth is only 0.02 percent water by mass. That may sound like a promising start to the search for life, but the scientists warn against getting your hopes up.
"We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live," says Natalie Hinkel, co-author of the study. "However, a planet that is a water world, or one that doesn't have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life."
Interestingly, the planets also appear to be too close to the star for ice to have formed there in the first place. They lie inside what's known as the "ice line," the distance from a star where water builds up on a planet in the form of solid ice. According to the study, the planets of TRAPPIST-1 most likely formed at least twice as far out, before migrating inwards.
Of course, it's always wise to keep in mind that these calculations are incomplete, and are estimations at best. Another recent study on the composition of the TRAPPIST-1 planets found that the worlds were at most only 5 percent water by mass. The discrepancy is likely due to different datasets and methods.
The latest research was published in the journal Nature Astronomy.
Source: Arizona State University
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