Seven small planets whose surfaces could harbour liquid water have been spotted around a nearby dwarf star. If such a configuration is common in planetary systems, our Galaxy could be teeming with Earth-like planets. See Letter p.456
Most stars in the Milky Way are much smaller and dimmer than the Sun. Low-mass stars that are about 80 times1 the mass of Jupiter have core temperatures that are just high enough to convert hydrogen into helium. The brightness of these stars is less than one-thousandth that of the Sun. An example of such a star is TRAPPIST-1, which is located only 12 parsecs (39 light years) away2. Last year, Gillon et al.2 announced the detection of three Earth-sized planets around TRAPPIST-1. On page 456, the authors3 report that the star has, in fact, seven planets, all of comparable mass and size to Earth.
During the past decade, thousands of planets have been discovered beyond the Solar System using a method called transit photometry. When a planet passes in front of (transits) its host star, a small amount of stellar light is blocked, exposing the planet and providing information about its size. Low-mass stars make good targets in the search for Earth-sized planets because a large fraction of the stellar surface is blocked during a transit, making these planets easier to detect.
In 2010, Gillon and colleagues began monitoring the smallest stars in the vicinity of the Sun, using a dedicated 60-centimetre robotic telescope called TRAPPIST (the Transiting Planets and Planetesimals Small Telescope) in Chile. After their initial TRAPPIST-1 discovery2, the authors carried out intense ground-based observations of the star, in addition to 20 days of continuous monitoring using NASA's Spitzer Space Telescope. In the current paper, Gillon et al. present evidence for 34 clear transits, which they attribute to a total of seven planets (Fig. 1).
The authors' results show that the TRAPPIST-1 system is extremely compact, flat and orderly. The six inner planets have orbital periods of between 1.5 and 13 days that are all 'near-resonant' — in the same time that the innermost planet makes eight orbits, the second, third and fourth planets revolve five, three and two times around the star, respectively. Such an arrangement causes the planets to have periodic gravitational influence on one another. This effect results in small shifts in the observed transit times, which the authors used to estimate the planets' masses.
The planetary system is strikingly reminiscent of that of Jupiter and its Galilean moons, albeit scaled up in mass by a factor of about 80. Io, Europa, Ganymede and Callisto orbit Jupiter with periods of between 1.7 and 17 days, also in near-resonance. This resemblance suggests that the TRAPPIST-1 planets and the Galilean moons formed and evolved in a similar way4.
In the past few years, evidence has been mounting5,6 that Earth-sized planets are abundant in the Galaxy, but Gillon and collaborators' findings indicate that these planets are even more common than previously thought. From geometric arguments, we expect that for every transiting planet found, there should be a multitude of similar planets (20–100 times more) that, seen from Earth, never pass in front of their host star. Of course, the authors could have been lucky, but finding seven transiting Earth-sized planets in such a small sample suggests that the Solar System with its four (sub-)Earth-sized planets might be nothing out of the ordinary.
Gillon et al. will soon step up their search for planets around the smallest stars in the vicinity of the Sun with the project SPECULOOS (Search for Habitable Planets Eclipsing Ultra-cool Stars; see go.nature.com/2l8bfpv), which will use four ground-based 1-metre telescopes and increase the authors' sample of stars by a factor of ten. In addition, NASA plans to launch TESS (the Transiting Exoplanet Survey Satellite), a space telescope that will spend two years identifying planets around more than 200,000 of the brightest stars in the sky, including about 10,000 dwarf stars7. Although none of the stars monitored by TESS will be as small as TRAPPIST-1, the high-precision observations obtained in space will compensate for the weaker photometry signals and allow populations of Earth-sized planets around such stars to be accurately mapped.
Excitingly, we might soon find out what conditions are like on the seven sisters of planet Earth in orbit around TRAPPIST-1. The James Webb Space Telescope, scheduled for launch next year, will be able to detect atmospheric components and thermal emission from the planets8, constraining their composition and climate, respectively. Liquid water could exist, or have existed, on any of the TRAPPIST-1 planets9, but predictions are difficult to make. For instance, although the estimated densities of the planets are consistent with their being similar in composition to Earth, they could instead be volatile-rich — containing a large fraction of water and ices — like the Galilean moons.
We also know from Jupiter's moons that a crucial factor for predicting the climate of a planetary body is the heating of its interior owing to friction caused by tides. This effect is responsible for widespread volcanism on Io and is the reason why Europa is thought to have a subsurface ocean. Tidal heating is expected for the TRAPPIST-1 planets because they exist in near-resonant orbits.
Could any of the planets harbour life? We simply do not know. But one thing is certain: in a few billion years, when the Sun has run out of fuel and the Solar System has ceased to exist, TRAPPIST-1 will still be only an infant star. It burns hydrogen so slowly that it will live for another 10 trillion years (ref. 10) — more than 700 times longer than the Universe has existed so far, which is arguably enough time for life to evolve.Footnote 1Footnote 2
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