TRAPPIST-1 (2017): Seven Earth-Size Planets Around One Star

Michaël Gillon and colleagues discovered in 2017 that TRAPPIST-1, a tiny star about 40 light-years away, has at least seven Earth-size planets. All seven pass in front of their star, and several could have liquid surface water. It is the richest known system of Earth-size worlds.

The question

In the exoplanet hunt, small red dwarfs, far dimmer than the Sun, matter most: they are the most common stars in the galaxy. If they commonly host rocky planets in tight orbits, they could be the best places to look for habitable worlds. In early 2016 the same team had reported three Earth-size planets transiting TRAPPIST-1, an ultracool dwarf with about 8% of the Sun’s mass, only slightly larger than Jupiter. The question was what else was there.

What the paper found

• Seven planets, not three. Intensive monitoring revealed at least seven planets with sizes and masses similar to Earth’s, labeled b to h outward from the star.
• A miniature system. All seven orbit much closer to their star than Mercury does to the Sun. The whole system is not much larger than the orbits of Jupiter’s big moons.
• A chain of rhythms. The six inner planets have periods of 1.51, 2.42, 4.05, 6.10, 9.21 and 12.35 days, close to ratios of small whole numbers. That near-resonant chain suggests they formed farther out and migrated inward.
• Temperatures that allow water. Because the star is so faint, several planets have equilibrium temperatures low enough for liquid surface water. ESO’s announcement named planets e, f and g as orbiting in the star’s habitable zone.

How they did it

The planets were found by the transit method, the technique that powered NASA’s Kepler mission. Each time a planet passes in front of its star, it blocks a little light. The depth of each dip gives a planet’s size; the timing gives its orbit.

The search started with TRAPPIST-South, a 0.6-meter robotic telescope at ESO’s La Silla Observatory in Chile, operated from the University of Liège in Belgium. Its name, the TRAnsiting Planets and PlanetesImals Small Telescope, gave the star its name. The team then added NASA’s Spitzer Space Telescope, which watched the star in infrared light, plus ESO’s Very Large Telescope and other telescopes around the world. A key clue came from the VLT: a rare triple transit, three planets crossing the star at once, which showed that at least one more unknown planet had to be there.

Why it mattered

TRAPPIST-1 showed that a small, cool star can host a whole family of Earth-size planets, several of them in the temperate zone described in Kasting’s 1993 habitable-zone study. It became the best laboratory for testing whether rocky planets around red dwarfs can keep atmospheres, and a fixture of any list of the most Earth-like exoplanets.

Its layout matters too. The star is small and the planets transit often, so their atmospheres can be probed with transmission spectroscopy, the method behind Webb’s detection of carbon dioxide on WASP-39b.

What we know now

The James Webb Space Telescope has been observing the TRAPPIST-1 planets since 2022. The central open question remains whether any of them still has an atmosphere, and what it is made of.

Who wrote it

The study was led by Michaël Gillon of the STAR Institute at the University of Liège, who had also led the 2016 discovery of the first three planets. Co-authors include Amaury Triaud and Brice-Olivier Demory. The paper was published in Nature on 23 February 2017.

Read the original

Gillon, M., Triaud, A. H. M. J., Demory, B.-O., et al. (2017). “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1.” Nature 542, 456-460. doi:10.1038/nature21360

Free to read: arXiv preprint.