A Deeper Look at TRAPPIST-1

Small red stars are drawing increased attention as we continue to discover interesting planets around them. The past two days we’ve looked at the four worlds around K2-72, a red dwarf about 225 light years out in the constellation Aquarius. That two of these worlds have at least the potential for liquid water on the surface makes the system a prime target for further study. Now we return to another recently discussed system of note, TRAPPIST-1.

Designated 2MASS J23062928-0502285, this ultracool dwarf is also in Aquarius, though at forty light years, much the closer target. As with K2-72, we have multiple planets here (three), and also like the K2 discovery, TRAPPIST-1 orbits a star small and dim enough to make planet detection easier — a transiting world presents a clear signature and the study of planetary atmospheres is possible through what is known as transmission spectroscopy, wherein light from the star that has passed through the planet’s atmosphere is analyzed.

Today we have a paper in Nature from an international team including Michaël Gillon (University of Liège) and Julien de Wit (MIT), who have been tightly focused on TRAPPIST-1 for some time. TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) is a 60 cm robotic instrument operated out of Liège, Belgium but sited at the European Southern Observatory’s La Silla Observatory in Chile. The instrument has been studying 70 nearby dwarfs at infrared wavelengths, uncovering the TRAPPIST-1 planets with orbital periods of 1.5 and 2.4 days and an outer world with period not yet well determined.

It was Gillon and de Wit who announced the discovery of the planetary system around TRAPPIST-1 on May 2. The work received a bit of buzz because although the two inner planets are too close to the star to be in the habitable zone, a tidally locked world in these orbits could have regions near the terminator where liquid water could exist. To probe further, the researchers studied data from the Spitzer Space Telescope, allowing them to refine the planetary orbits. At this point, they realized a double transit was in the offing.

Moreover, the event was in a scant two weeks, making for frenzied work, as de Wit explains:

“We thought, maybe we could see if people at Hubble would give us time to do this observation, so we wrote the proposal in less than 24 hours, sent it out, and it was reviewed immediately. Now for the first time we have spectroscopic observations of a double transit, which allows us to get insight on the atmosphere of both planets at the same time.”

The result: A combined transmission spectrum of TRAPPIST-1b and c, meaning the team could analyze the atmospheres of both worlds as the transit occurred. The transmission spectrum was featureless, the data sufficient to show that both transiting planets have relatively compact atmospheres rather than large, gaseous envelopes like Jupiter and Saturn. That would imply rocky planets like the terrestrial worlds — Mars, Earth, Venus — in our own Solar System.

Image: Comparison between the Sun and the ultracool dwarf star TRAPPIST-1. Credit: ESO.

That’s a useful insight because we have no other information about the nature of these planets. Their masses have not been measured, and we have no other data about the kind of planets that can exist around ultracool dwarf stars (TRAPPIST-1 is an M8 dwarf) because the TRAPPIST-1 worlds are our first transiting example.

The excerpt below shows the team’s reasoning, building on the fact that the lack of features in the combined spectrum rules out certain kinds of atmospheres:

…the first observations of TRAPPIST-1’s planets with HST allow us to rule out a cloud-free hydrogen-dominated atmosphere for either planet. If the planets’ atmospheres are hydrogen-dominated, then they must contain clouds or hazes that are grey absorbers between 1.1 μm and 1.7 μm at pressures less than around 10 mbar. However, theoretical investigations for hydrogen-dominated atmospheres predict that the efficiencies of haze and cloud formation at the irradiation levels of TRAPPIST-1b and TRAPPIST-1c should be dramatically reduced compared with, for example, the efficiencies for GJ 1214b… In short, hydrogen-dominated atmospheres can be considered as unlikely for TRAPPIST-1b and TRAPPIST-1c.

Image: The binary transit visualized. Credit: NASA/ESA/STScl.

With an extended gas envelope ruled out, we wind up with a range of possible atmospheres, ranging from the CO 2 -dominated Venus to an Earth-like atmosphere with heavy clouds or a depleted atmosphere like what we see on Mars. To push further into the possibilities, the team has formed a consortium called SPECULOOS (Search for habitable Planets Eclipsing ULtra-cOOl Stars), the good news being that they are building larger versions of the TRAPPIST instrument in Chile that will focus on the brightest ultracool dwarf stars in the southern hemisphere. Consider the effort an attempt to build the kind of pre-screening tools that our future space telescopes like the James Webb instrument will need for their target list.

The paper is de Wit et al., “A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c,” Nature 20 July 2016 (preprint). The discovery paper is Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star,” published online in Nature 2 May 2016 (abstract). An MIT news release is available.