TRAPPIST-1 Exoplanets May Have Too Much Water for Life

The TRAPPIST-1 system seems at first to be a good place to look for extraterrestrial life. It has seven rocky planets roughly the same size as Earth, and some of them are in the “habitable zone” of the star where liquid water could exist on the surface. Now, researchers from School of Earth and Space Exploration at Arizona State University say there might actually be too much water for the TRAPPIST-1 planets to harbor life.

Astronomers first announced the discovery of three exoplanets around TRAPPIST-1 in 2016. Then, a year later we learned of four more exoplanets in the system. TRAPPIST-1 is only 39 light years away, making it an excellent way to study the behavior and conditions on exoplanets. While all the planets are similar in size to Earth (no gas giants have been detected), only three of the planets (TRAPPIST-1e, f, and g) are orbiting at a distance that would allow them to have liquid water. Having liquid water is a requirement for life as we know it, but it turns out you might want some land, too.

All the TRAPPIST-1 planets were discovered with the transit method. A telescope watches a star for small dips in brightness. These dips can give away an exoplanet passing in front of the star from our perspective. That can tell us how large an exoplanet is. By watching the way transit signals vary over time, astronomers can also estimate an exoplanet’s mass. Put those together, and you’ve got an approximate density.

The Arizona State University used this data to create computer models of six of the seven planets in the TRAPPIST-1 system. They didn’t analyze the outermost planet, TRAPPIST-1h, because not enough is known about its properties. The innermost planets in the system (b and c) are believed to be about 10 percent water by mass. The more distant TRAPPIST-1f and g are a whopping 50 percent water. Exoplanets TRAPPIST-1d and e are in the middle of the system and have water masses in between the others.

You’ve probably heard Earth is 70 percent water, but that’s surface area. Water makes up just 0.2 percent of Earth’s mass. So, the TRAPPIST-1 planets could be incredibly wet. The outer planets would have more than 1,000 times the volume of water we have on Earth. The researchers point out this could impede the development of life because there are certain chemical processes that occur on dry land. In addition, the pressure of all that water pressing down on the mantle could prevent most volcanic activity. Without the carbon dioxide from volcanic activity, even planets in the habitable zone could have fallen victim to a runaway snowball effect.

Since TRAPPIST-1 is a cool red dwarf, all these planets orbit very close — less than the distance of Mercury’s orbit around the sun. That means they’re exposed to more radiation and solar flares. They’re probably also tidally locked so the same face always points toward the star. Having a lot of water could help disperse the heat and absorb radiation, so maybe there’s still some hope.

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