An international team has discovered that the ultracold dwarf star TRAPPIST-1 is orbited by seven planets comparable to Earth in size. For all but the outermost planet, the researchers were also able to obtain density estimates and concluded that the data “broadly suggest rocky compositions.” Most intriguingly, all the planets could potentially house liquid water on at least limited regions of their surfaces. Following are the essential facts about the discovery.
The host star: TRAPPIST-1 is located about 39 light-years from Earth in the constellation Aquarius. Its radius is about 12% and its mass 8% of the Sun’s. The star’s name derives from the Transiting Planets and Planetesimals Small Telescope at the La Silla Observatory in Chile. The new work was accomplished with the help of several telescopes, including the Spitzer Space Telescope, which monitored the star in the IR for 20 days.
The discovery technique: The seven planets were discovered with the transit technique. When a planet passes in front of its host star, it briefly blocks some of the stellar light. The larger the planet, the more light is blocked. The telescopes trained on TRAPPIST-1 measured those intensity dips and thus obtained precise determinations of the planetary radii. Because TRAPPIST-1 is so small and its orbiting planets are so close, each planetary transit blocks out a relatively large chunk of starlight—about 1%. For that reason, the TRAPPIST-1 system is well suited for the transit method.
Mass determinations: The transit method does not directly give planetary masses. But sometimes a planet is retarded in its orbit by the gravitational pull of its companions, and sometimes it is advanced. By combining computer modeling with observations of the variations in orbital periods—changes in the time separating transits—the research team could estimate planetary masses, though not very precisely.
The habitable zone: The seven planets known to orbit TRAPPIST-1 lie within 0.011 and 0.063 astronomical units from the star. (The AU is the Earth–Sun separation distance.) The figure shows planet–star distances in terms of energy received from the star. Planets e, f, and g, the authors note, could feature water oceans. The other four planets might have surface water, depending on detailed effects of clouds and tidal heating.
What’s next: In addition to continuing to search for other star systems that host multiple planets, the research team hopes to study the atmospheres of the TRAPPIST-1 planets. The goal is to look for molecules, such as O2, and chemical abundance profiles that would be indicative of biological life. With the Wide Field Camera 3 aboard the Hubble Space Telescope, a complementary research team has ruled out cloud-free hydrogen atmospheres for the innermost two planets. But detailed characterization of planetary atmospheres will have to await the launch of the James Webb Space Telescope, scheduled for October 2018.
The paper: M. Gillon et al., Nature 542, 456 (2017)
