Hope was ignited in the science community when researchers discovered that three of the seven Earth-size planets orbiting TRAPPIST-1, a cool red dwarf about 40 light-years from Earth, are within the star’s habitable zone and could have flowing water on their surfaces. But while the presence of water undoubtedly increases the likelihood of habitability for these planets, it doesn’t automatically make them safe havens for life. In fact, an overabundance of water suggests just the opposite, and new research conducted by scientists at Arizona State and Vanderbilt Universities indicates that the TRAPPIST-1 system actually has too much water to support life.

Each of TRAPPIST-1’s planets are roughly the size of Earth and are tightly packed together, with all of their orbits keeping them closer to their host star than Mercury is to the Sun. While the exoplanet’s are similar in size to Earth, measurements of their masses and volumes show that they’re much less dense. They’re too light to be rocky and, unlike other low-density planets of similar size, too compact to be primarily composed of atmospheric gas.

“The TRAPPIST-1 planets are too small in mass to hold onto enough gas to make up the density deficit," said Arizona State University geoscientist, Cayman Unterborn, in a press release. "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.”

With rock and atmospheric gas ruled out, the research team determined that the system’s abundant component is likely water. However, just how much water is needed to make up the exoplanets’ masses remained unknown.

To find out, Unterborn and Alejandro Lorenzo, another member of the research team, developed software called ExoPlex, which merged all of the available data for the TRAPPIST-1 system into one platform. By analyzing the host star’s chemical composition, along with the mass and radius of each planet, the software estimated that the two innermost planets (marked “b” and “c” on the image below) have less than 15 percent water by mass, while two of the outer planets (marked “f” and “g”) have over 50 percent water by mass. Keeping in mind that Earth is just 0.02 percent water by mass, the difference is pretty substantial.

To learn more about how astronomers discover and identify habitable exoplanets, check out our FREE exoplanet guide!

