
The planets around TRAPPIST-1. Image credit: NASA/R. Hurt/T. Pyle [Click for larger image]
There were previously three planets known about (well, two for sure and another maybe). The new results have confirmed six, with another tentative seventh. All seven have been identified through the “transit method”, whereby the planets pass in front of the star from our point of view. This gives us an estimate of their size (relative to the star), but not their mass – at least that’s usually the case. The seven planets are labelled “b” to “h” (by convention, “a” is reserved for the star itself).
Finding Earth-mass planets isn’t new anymore. Finding lots of planets in one system isn’t new. Finding planets very close to Earth isn’t new. So what’s so interesting about TRAPPIST-1? I think it’s fair to say that the news was overhyped in much of the media coverage, I guess partially because of the embargo period which tends to get people wound up. Getting people talking about exoplanets is good – but the community needs to be careful that people don’t think that these results are all the same, and so stop paying attention. Having said that, while some aspects of this result are “incremental”, there are some really neat bits in there when you dig down.
A packed system

Environment of the planets in TRAPPIST-1 compared with our Solar System. Original source & more info: Gillon et al. (2017) [Click for larger image]
There have been many articles and commentaries about the possible conditions on the seven planets, three of which lie in what is normally called the “habitable zone” (i.e. where the surface temperature could allow liquid water) and all of which are classed as “temperate”, meaning that the right atmospheric density and composition could allow for liquid water.
There are a lot of “coulds” in there, and we simply have no idea about their atmospheres – though it’s very possible that future telescopes, such as the planned Extremely Large Telescope and the soon-to-be-launched James Webb Space Telescope, will be able to make progress on that front.
For now, all we know is how much starlight falls on the planets, and therefore what the temperature of a bare lump of rock would be. As shown in the image above, even though they are similar in that sense to Earth and Venus, that doesn’t tell us much. That’s perfectly illustrated by the fact that Earth (nice and temperate) and Venus (inhospitable hellhole) are so different. The Earth’s average surface temperature would be -15°C without the atmosphere, but is increased to about +15°C by the atmosphere. Venus, on the other hand, receives twice as much light from the Sun, but with its thick atmosphere has a surface temperature of about 500°C. Put simply, although the right atmosphere could lead to nice temperature planets, the wrong atmosphere could equally lead to a very inhospitable planet.
Multiple transits

Triple-transit of three TRAPPIST-1 planets observed in December 2015 by the VLT. A cartoon of the planets passing in front of the star is shown at the bottom. Original source & more info: Gillon et al. (2017) [Click for larger image]
Resonances
For me the really interesting thing is quite how much has been learned from transits. I mentioned above that the transit method normally only gives us the size of the planet. To get a planet’s mass (and therefore its density), we normally need to use another method, usually the Radial Velocity method, to pick out the weak gravitational effect of the planet on the star, causing the star to wobble a little bit. However, the TRAPPIST-1 system has another feature that helps us out – the planets are so close together that we can measure their gravitational effects on each other.There’s also a sort of synchronicity between the orbits. Take the 4th and 5th planets, called “e” and “f”, for example: for every two times planet “f” orbits the star, planet “e” orbits three times. That means that every other time “f” gets to a particular point in its orbit, planet “e” is in the same place. Having that repeatable pattern is something called a “mean motion resonance”, and there are links between all the planets. The image on the right shows the orbital periods and the “resonances” relative to the inner-most planet.
We see this in our own Solar System. Three of Jupiter’s largest moons (Io, Europa and Ganymede) have orbital periods in the ratio of 1:2:4 (for Io:Europa:Ganymede) – so for every once Ganymede goes round, Europa goes round twice and Io four times. This keeps their orbits locked, means Europa’s orbit is slightly elliptical, and ultimately helps generate the heat that create Europa’s subsurface ocean. There are also resonances between the orbits of asteroids and Jupiter, as well as between Kuiper Belt objects and Neptune (with Pluto and Neptune having periods in a 3:2 ratio).

Transit Timing Variations of TRAPPIST-1, comparing the simulations (red) with the observations (black datapoints). Original source & more info: Gillon et al. (2017) [Click for larger image]

Masses and radii of the TRAPPIST-1 planets, compared with models. Original source & more info: Gillon et al. (2017) [Click for larger image]
To get the masses better we need radial velocity measurements, but that could be tricky. I had a play with the Radial Velocity Simulator from the Nebraska Astronomy Applet Project, and put in the values for the closest planet (star mass = 0.08 Msun, planet mass=0.003 Mjup, semi-major axis=0.01au, inclination=90deg, ecentricity=0) to see the size of the effect. The planet’s mass are so small that it would only cause the star to wobble by about 2m/s, which is, in principle, within reach of the best instruments on the largest telescope, but the issue may well be that the star is too faint to get a strong enough signal. I’m not an expert on the intricacies of radial velocity measurements, though, and would be delighted to be proved wrong!