A Strong Case for TRAPPIST-1 Planets

TRAPPIST continues to be my favorite astrophysical acronym. Standing for Transiting Planets and Planetesimals Small Telescope, the acronym flags a robotic instrument at the La Silla Observatory in Chile that is operated by the the Institut d’Astrophysique et Géophysique (University of Liège, Belgium) in cooperation with the Geneva Observatory. The name is a nod to the branch of the Cistercian order of monks called Trappists, whose beer is world-renowned and closely associated with Belgium itself (although also brewed in the Netherlands and a few other countries). A jolly telescope indeed.

You’ll recall TRAPPIST-1 as the far more approachable term for the red dwarf star 2MASS J23062928-0502285, a bit over 39 light years away in the direction of the constellation Aquarius. A 2016 paper in Nature announced three rocky planets orbiting the star, one of which could conceivably be in its habitable zone, where liquid water can exist on the surface. Now we have a helpful follow-up from the 8-meter Gemini South telescope in Chile. A team led by Steve Howell (NASA Ames) has been able to rule out any close stellar companion.

That’s good news for planet hunters because what we have been looking at are fluctuations in the light of this small star (TRAPPIST-1 is only about 8 percent of the Sun’s mass), making the assumption that these were caused by the three planets mentioned above. Howell used the Differential Speckle Survey Instrument (DSSI) at Gemini South to demonstrate that there was no hitherto undiscovered small star complicating an already complicated planetary detection.

“By finding no additional stellar companions in the star’s vicinity we confirm that a family of smallish planets orbit this star,” says Howell. “Using Gemini we can see closer to this star than the orbit of Mercury to our Sun. Gemini with DSSI is unique in being able to do this, bar none.”

Speckle imaging, which is what the DSSI instrument Howell used in this work does, works by taking numerous extremely short exposures, allowing astronomers to combine the images and eliminate distortion caused by the Earth’s atmosphere. We wind up with high-resolution images that duplicate what the same telescope would produce if it were in space.

Image: Artist’s concept of what the view might be like from inside the TRAPPIST-1 exoplanetary system showing three Earth-sized planets in orbit around the low-mass star. This planetary system is located only 40 light years away. Gemini South telescope imaging, the highest resolution images ever taken of the star, revealed no additional stellar companions, providing strong evidence that three small, probably rocky planets orbit this star. Credit: Robert Hurt/JPL/Caltech.

Ongoing work may detect further planets in this system, but for now, what we have are the original three. The inner worlds are in orbits of 1.5 and 2.4 days respectively, both far too hot for liquid water on the surface; in fact, these two would receive four and two times the radiation the Earth does from the Sun respectively. The third planet’s orbital period has proven difficult to constrain, so that all we can say is that it is between 4 and 73 days, though this Gemini Observatory news release pegs the most likely period at 18 days, which would evidently be in the habitable zone. Confirming that would add to the sizzle of the recent Proxima Centauri b discovery.

Thus we continue to learn about TRAPPIST-1, a promising candidate for still more detailed work in coming years. M-dwarfs are small enough that planets in their habitable zone have short orbital periods. That means frequent transits, giving astronomers the chance to analyze their planets’ atmospheres by studying starlight as it is filtered through them. The odds on the Proxima Centauri planet transiting are slight (we’ll soon know more), but in TRAPPIST-1 we have the potential for follow-up with space-borne instruments and the coming generation of extremely large telescopes (ELTs) on the ground, to learn what kind of atmospheres such planets have, and whether we may eventually find biosignatures within them.

The Howell paper is “Speckle Imaging Excludes Low-Mass Companions Orbiting the Exoplanet Host Star TRAPPIST-1,” Astrophysical Journal Letters Vol. 829, No. 1 (abstract). The original work on TRAPPIST-1 is Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star,” published online in Nature 2 May 2016 (abstract).