≡ Menu

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 CO2-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.


Comments on this entry are closed.

  • Michael July 20, 2016, 16:18

    Fantastic animation Paul, it really brings a feeling out of being there in person.

    Here is some more animations and info, I just love the last picture in the slides, just like you where there. Check out the video too.


  • Andrew LePage July 20, 2016, 17:37

    A very nice result! Unfortunately, it doesn’t change my assessment from a couple of months ago that TRAPPIST-1b and c with their Venus-like sizes, greater than Venus-like stellar fluxes and probable Venus-like rotation states are not Earth-like in the least but are instead Venus-like. In fact, as mentioned in the abstract, the new data are still consistent with a Venus-like atmosphere.


    I still have my fingers crossed that sometime soon new data will come out which will clear up the current ambiguity about the orbit of TRAPPIST-1d. MAYBE it orbits inside of the HZ but we still don’t know for sure one way or the other.

    • Harry R Ray July 21, 2016, 9:48

      ME TOO! Keep in mind that there were TWO Spitzer observational runs. The now “infamous” confirming the binary nature of the transit of TRAPPIST-1d run in late February-early March totaling just over 5 hours, and setting off RAMPANT SPECULATION on the internet about either an exomoon or a binary planet. The later run(late March -early APRIL?) of 32 hours was the one that allowed the authors to predict the double transit. Seeing that it is now late July and no preprints are out for EITHER of these observational runs, I suspect that either the paper has not been completely written yet, because they may have found hints of ADDITIONAL PLANETS in the system that were TOO SMALL for TRAPPIST-1 to detect at the 3 sigma level, and the authors are trying to TEASE a 1.5 or 2 sigma level signal out of the TRAPPIST-1 data CONFIRMING the Spitzer signals; OR, what I THINK is the MORE LIKELY CASE, the paper WAS written but is not available as a PREPRINT on Arxiv or Vox Charta because it is under EMBARGO, as the PALE RED DOT Proxima Centauri paper seems to be.

  • Mike Fidler July 21, 2016, 4:10

    There has been a lot of discussion about the radiation problems from M Dwarf flares on these type of planets, yet they would be tidally locked. Radiation reaching the terminator of these planets would be traveling through 1000 Km of atmosphere if it was similar to Earth’s. So just how much of the hard radiation would be blocked by the atmosphere near the terminator or the horizon to an observer looking from the ground towards the M Dwarf?

    • Andrew LePage July 21, 2016, 9:27

      Mike, while sunlight has to traverse much more atmosphere when the sun is near the horizon compared to at zenith, the effect isn’t as great as you might think. For an exact copy of Earth’s atmosphere (and taking into account refraction, etc.), sunlight has to travel through about 40-times more atmosphere than when it is at zenith. While that would reduce the instantaneous effects of a flare by orders of magnitude near the terminator, it really won’t help maintain habitability at the terminator or elsewhere in the long run. The extra heat added to the planet’s atmosphere from flare activity would still be transferred globally through atmospheric and oceanic circulation affecting the terminator as well. In addition, it won’t affect atmospheric loss processes fed by flare activity – the atmosphere is lost globally not just on the sun-facing side of the planet.

      • Mike Fidler July 22, 2016, 0:08

        Ok, I see your point, but after the M Dwarf ages the flaring is not as intense and eventually stops. The planet may still be outgassing because of tidal heating so the atmosphere is renewed, this could mean an earlier time of the planet’s habitability and possible a closer-in habitability zone. The atmospheric circulation in the polar areas should not be mixing as much as in the equatorial jet stream. What I could see is larger earth’s having an insulating effect and be able to be closer to the red dwarf! It would seem to me that more research needs to be done to understand these areas near the terminator, especially when you start to get into super-earth’s and sub-neptunes near M and K Dwarfs.


        • Mike Fidler July 30, 2016, 10:34

          About Exobiology: The Case for Dwarf K Stars
          See Chart on page 26.
          I’m wondering how much the atmosphere at the terminator would block the UV and X-rays for M Dwarfs from M2V to M5V?
          How about looking at these planets like a large tropical island with hot deserts on the sun facing side and large deep oceans on the night side. Coral reefs near the terminator and then jungles as you progress into the day side. On the super earths with plate tectonics it would be interesting how mountain chains would develop from the tidal influence and librations.

    • Michael C. Fidler August 2, 2016, 9:07

      If these planet formed around the star in situ could they be elongated like an egg, like the oceans on earth from the pull of the sun? They may be tidally locked but can still rotate about their axis along as it pole is faceing toward the star, like Uranus.
      They would also have precession because of this rotation so there could be quite large changes in surface area being exposed to sunlight.
      At the speed they revolve around these K and M dwarfs it could make for some interesting precession of the equinoxes. A good project for Nvidia’s deep learning Titan X!

  • jack woods July 27, 2016, 22:59

    does anybody know if HARPS is looking at this star for radial velocity confirmation (ie. masses)?