An alien planetary system that features seven roughly Earth-size planets is beginning to reveal its secrets. This week, scientists announced new insights into the nature of the worlds orbiting TRAPPIST-1, a star 39 light-years away. Astronomers know the planets’ sizes and orbits by observing the star dimming as the planets pass in front. Now, researchers have collected data from telescopes on the ground and in space, noting the exact timing of these transits. Because the planets are bunched together in closely packed orbits, they interact gravitationally with each other, causing slight delays or advances in the timing of transits. A complex computer analysis of those timing variations allowed the researchers to estimate the planets’ masses and so improve estimates of their densities by up to eightfold, they will report in a forthcoming issue of Astronomy & Astrophysics. From those densities, they estimate that the fourth planet out from the star, known as TRAPPIST-1e, is the rockiest of the seven and the most Earth-like, with the possibility of liquid water on its surface. Those closer in likely have dense, steamy atmospheres, whereas the outer ones may be encased in a thick layer of ice. Some may be 5% water, 50 times that on Earth. Other observations, made with the Hubble Space Telescope and published yesterday in Nature Astronomy, found no signs of hydrogen in the atmospheres of planets d, e, and f, but were inconclusive for TRAPPIST-1g. A hydrogen-rich atmosphere could cause such close-in planets to overheat because of the greenhouse effect, so its absence supports the idea that some of these planets may have habitable conditions. A more thorough examination of the system, including more constituents of the planets’ atmospheres, will have to wait for the keen-eyed gaze of the James Webb Space Telescope, which is due to launch early next year.