“An international team of astronomers, including space scientists from the University of Leicester, has found a system of seven Earth-sized planets just 40 light-years away.”
It’s a pretty exciting discovery: using ground and space telescopes, including NASA’s Spitzer Space Telescope and ESO’s Very Large Telescope, the planets were all detected as they passed in front of their parent star, a small red dwarf star known as TRAPPIST-1, blocking some of the light coming from it. It’s an effect known as a planetary transit.
Three of the planets lie in the “habitable zone” of TRAPPIST-1 and could harbour oceans of water on their surfaces, increasing the possibility that the star system could play host to life. Note, that’s “could”. More later.
This system has both the largest number of Earth-sized planets yet found and the largest number of probably rocky worlds in the habitable zone.
I’m a co-author on the Nature paper detailing this discovery. My small part? Working with one of our PhD students, Alex Chaushev, we observed TRAPPIST-1 in early July of last year using a telescope at the South African Astronomical Observatory in Sutherland, South Africa.
At the time there were thought to be perhaps four planets orbiting the star, and our aim was to observe one of the planets transiting the star.
By measuring the dip in the star’s brightness during a transit, we can infer the size of the planet. By observing successive transits, we can also constrain the orbit of the planet, and that was the primary aim of our observations.
Puzzlingly, we didn’t see a transit at the expected time. This was actually a vital clue, since it indicated there was more going on in the TRAPPIST-1 solar system than we were previously aware.
Thanks to a whole programme of observations with different telescopes and the NASA Spitzer satellite over the last few months, we now know that there are at least seven planets in the system, all roughly the size of Earth. The reason we didn’t see our expected transit from South Africa, was because we were confusing different planets with each other, and getting their orbits all wrong.
You can learn more, and see some cool graphics, by watching this short video produced by the media people at ESO:
You can read the paper itself here.
ESO have produced loads of graphics, videos and information sheets here.
And TRAPPIST-1 has its own website too.
Back to the science:
This amused me on Monday evening, after those teasers at NASA announced they were to host a news conference on a discovery beyond the solar system, and whole hyping machine went into over-drive:
NASA have called an exoplanet announcement for Thursday. What do we think?
— Active Astronomy (@ActiveAstro) February 20, 2017
Of course, it’s not Earth 2.0. And neither is this Something Incredible like, say, the discovery of signs of life would be.
Professional astronomers have grown somewhat weary over the years of yet another announcement of the discovery of “Earth-like planets”, aka Earth 2.0. In many cases, it was pretty obvious immediately that the planet either wasn’t really like Earth at all, or there just wasn’t sufficient evidence to fully support the claim. I’ve criticised previous claims myself, and I’ve seen some pretty cynical tweets and posts in the last couple of days.
Planets which are likely made of rock have also been found before of course, including in their stars’ habitable zones, like the planet around our nearest star, Proxima Centauri b. The excitement with TRAPPIST-1 is the sheer number of rocky planets, with three in the habitable zone.
For TRAPPIST-1, we’ve been careful in the press releases to avoid the term “Earth-like”, because we don’t have enough evidence to claim that.
The main information a transit gives you is the radius (i.e. size) of the planet, and the length of its orbit.
We have been able to glean some information about the masses of the planets from an effect called Transit Timing Variations (TTVs). Each successive transit of a planet should happen at a specific, predictable moment. But because of the presence of the other planets in the system and their gravitational influence, the transits of these planets can happen slightly earlier or later than expected. The difference between the expected time of transit, and when it actually occurs, is related to the masses of the planets.
The graphic above doesn’t give the uncertainties on the masses of the TRAPPIST-1 planets, but if you read the paper you can see from Table 1 that they are quite large. In fact, modelling TTVs is a complex business. More than one set of solutions can fit the data, and we don’t yet have a unique solution for the masses of TRAPPIST-1’s planets. So, there are still more observations to be made and work to be done. But we can be fairly confident that their masses and densities are in the right ballpark for them to be rocky worlds.
What we cannot say is whether these planets definitely have water on their surfaces, or show evidence for life, or that they harbour alien civilisations. Three of the planets exist in the habitable zone of TRAPPIST-1, where the surface temperature is right for water to exist as a liquid. So they might. But for all sorts of reasons they may be more like Venus, or Mars.
And planets in such close orbits to a red dwarf star like TRAPPIST-1 could suffer all sorts of nasty effects, like deadly flares of charged particles and x-rays from the parent star, that preclude any life developing. We just don’t know, and shouldn’t make outlandish claims without more evidence.
In the near future, it should become possible to try to see if some of these planets have atmospheres, using forthcoming telescopes like NASA’s James Webb Space Telescope (the successor to the Hubble Space Telescope), due for launch in 2018, and the European Extremely Large Telescope, which has begun construction in Chile.
And so, in summary: