The TRAPPIST-1 planets’ are all closer to their home star than Mercury is to the Sun. Three of them might be “habitable zone” where liquid water could exist.

TRAPPIST-1 has at least seven planets, all small enough to be Earth-like.

In two papers released this week, teams of scientists talked about what their atmospheres might look like. They also provided a greater sense of their composition.

Astronomers now say that water appears to be present in significant quantities on all of the planets. In some cases up to five percent of the planet’s mass.

To get a better idea of their density, and what they might be made of, the researchers tracked each planet’s orbit. So, because the planets are so close together, their gravitational fields tug at each other. Scientists measured the power of these tugs and placed that data into a sophisticated computer-modeling algorithm. Thus, they were able to get an idea of how dense each planet was.

Researchers found that the planets weren’t dense enough to be made of just rock and metal. Volatiles—elements, and compounds with low boiling temperatures—must be present, they say, and the best explanation is water.

For the paper in Nature Astronomy, the scientists used NASA’s Hubble Space Telescope to probe the atmospheres of four of the planets in or near the star’s habitable zone. They wanted to make sure these planets did not have puffy, hydrogen-rich atmospheres. That’s because hydrogen is a greenhouse gas, and such an atmosphere would make the planet far too hot for life.

“Densities, while important clues to the planets’ compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life,” said Brice-Olivier Demory a study co-author from the University of Bern in a news release.

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Thumbnail image: This artist’s concept allows us to imagine what it would be like to stand on the surface of the exoplanet TRAPPIST-1f, located in the TRAPPIST-1 system in the constellation Aquarius, as of Feb 2017. Credit: NASA