Astrobiology Top 10: Welcome to TRAPPIST-1

As 2017 comes to a close, Astrobiology Magazine is counting down the best stories from the past year. Nothing has boosted astrobiology more than the discovery of exoplanets and, thanks to the latest generation of telescopes, we are finding more and more worlds in the habitable zones of stars. In particular, astronomers are starting to take note of red dwarf stars, which are the most common type of star and which make up the majority of nearby stars. At number 7 in our countdown, Sheyna Gifford presents one of the greatest exoplanet discoveries yet, namely a bounty of seven terrestrial worlds orbiting a red dwarf just 39.3 light years away, with at least three of the worlds within the star’s habitable zone. This story was originally published on 23 February 2017.

The TRAPPIST-1 star, an ultra-cool dwarf, has seven Earth-size planets orbiting it. This artist’s concept appeared on the cover of the journal Nature on Feb. 23, 2017. Credits: NASA/JPL-Caltech

If you were standing on TRAPPIST-1D looking out at the other six planets in your system, they would look large – something like our moon does to us – but be moving much faster. For all seven of the planets circling around TRAPPIST-1, revealed by NASA as the largest catch of Earth-like planets ever discovered, are packed tightly around their ultra-cool dwarf star, giving us a tantalizing opportunity to peer directly into the atmospheres of possibly habitable worlds.

The TRAPPIST-1 discovery has many important consequences. For one, before today, no more than three Earth-like planets had been found anywhere in a single star system. TRAPPIST-1, named after the Transiting Planets and Planetesimals Small Telescope in Chile that first discovered it, has three Earth-like planets – all within the red dwarf’s the habitable zone. Then there’s the convenience and proximity. Positioned only 40 light-years away and powered by a star only a fraction as bright as our own, this system is cool and relatively close to us. As a result, we are able to use ground-based and satellite-based telescopes to study the fundamental properties of these fast-moving worlds (the swiftest travelling around its star in only 1.5 days).

Already, the Spitzer space telescope has spent more than 500 hours watching planets 1b through 1h fly across the face of their star. In the process, the 13-year-old satellite was able to discern many facts about these seven planets including their speeds, distances, and – this is how we know they are Earth-like – their masses. By knowing the masses of these planets plus their distances, and watching their shadows crossing between us and their sun, our scientists determined their sizes and compositions. So we now know that all seven of these worlds are probably rocky, like ours.

Findings like the TRAPPIST-1 system gives us something to look forward to as we prepare to launch our next generation of planet-hunting satellites. When the eye opens on the James Webb telescope next year, we’ll have a direct line-of-sight into the atmospheres of the seven planets, where we can look for signs of water, oxygen, ozone, and other life-associated chemicals.

Credit: NASA/JPL