The Many Planets of TRAPPIST-1

Wherever you see an alien planet in sci-fi films or television, there’s always something weird going on in the sky. How else would you know you’re not looking at Earth? So in everything from Avatar to Star Wars we get double stars, panoplies of moons, other planets in the same system—so many disks visible even without a telescope. Sometimes it seems you could throw a rock from one planet and have it land on another.

See what I’m saying?

Reality is, of course, much more prosaic. Space is huge and often boring to look at. Large planets like Jupiter appear to us as points of light, and even if you look from, say, Titan, Saturn’s other satellites will appear quite small. But that’s not to say the improbable can’t happen. It’s a big universe, and we are already beginning to discover some true wondrous vistas out there—such as the utterly perplexing star system of TRAPPIST-1.

TRAPPIST-1 generated a media buzz back in 2017, when researchers pieced it together from grainy telescope data. It is a red dwarf, forty light-years away and only eighty-four times the mass of Jupiter, around which circle a whopping seven rocky, possibly habitable planets. The outermost is just 0.06 astronomical units from its star. Mercury, by comparison, comes to about 0.3 at perihelion. These worlds are so close together that if you stood on one, the others would appear about as large as the Moon does in our sky, or even larger.

An artist’s conception, from NASA. Note that as close as the planets are to each other, they aren’t quite this close.

The star itself was first discovered in 1999, as part of an all-sky survey by astronomer John Gizis. It appeared completely ordinary and received a dull string of numbers as a name. Things picked up in 2015, when a team of Belgian researchers used the direct transit method1 to detect the first three planets (b, c, and d), and also gave the star its name, TRAPPIST-1, after a Catholic religious order, the Trappists. It was an important find, but not too remarkable…

Then the Spitzer Space Telescope discovered four more planets in February 2017. The data came in—really a huge amount of data, gleaned from a distant point of light—and we built a picture of this star system, so totally unlike our own. Seven terrestrial planets, up to six of them capable of hosting liquid water. A complicated ballet of worlds, all within a fifth of Mercury’s orbit. The possibility of diverse ecosystems and even civilizations there. Suffice it to say, few planetary systems have gotten as much exposure as TRAPPIST-1.

Artist’s representation of two planets transiting TRAPPIST-1. Sadly, our telescopes aren’t quite this good yet. ESA/Hubble, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons

A few things to note: due to the compact size of this system, all of these planets are likely to be tidally locked, and they probably won’t have moons, since the gravitational perturbations of other worlds would dislodge them. You won’t find anything quite like Earth around TRAPPIST-1. Nevertheless, early observations promise an exciting and dynamic selection:

  • TRAPPIST-1b: 1.374 Earth masses2, very roughly. This world is large, with a thick atmosphere comprising either water vapor or carbon dioxide. Surface temperatures may be over 1,400 K—there won’t be any life here, unless it’s floating in the clouds.
  • TRAPPIST-1c: 1.308 Earth masses. We suspect this one to be similar to b, though its hot, thick atmosphere isn’t quite so hot and thick.
  • TRAPPIST-1d: A steep drop in size, at 0.388 Earth masses; it receives a little more sunlight than we do. Despite being in the habitable zone, a University of Washington model indicates that it, too, may be hot and Venus-like.
  • TRAPPIST-1e: This is the exciting one. 0.692 times Earth’s mass, with a relatively thin atmosphere and temperatures ideal for liquid water, it holds the greatest chance for habitability. The James Webb Space Telescope is slated to observe it after launching in October of this year.
  • TRAPPIST-1f: Moving further out we find a world almost the same mass as Earth, just four percent larger. Unfortunately it’s not very dense, indicating a water content of at least twenty percent, and with such a thick envelope it would start to look more like Neptune than our own planet. Expect a thick and mostly gaseous soup.
  • TRAPPIST-1g: The second-largest planet in the TRAPPIST-1 system, at 1.321 Earth masses, g is expected to be fairly chilly. It may nevertheless have liquid water; some simulations suggest a thick, abiotic oxygen atmosphere (hundreds of times the ambient pressure on Earth) over an ocean many kilometers deep.
  • TRAPPIST-1h: This is the smallest of the seven, at 0.326 Earth masses. It is (relatively) far from its star, and cold, so it is probably an ice ball—though tidal heating might produce a Europa-style ocean.
One of JPL’s unforgettable travel posters, showing a fanciful view of TRAPPIST-1e.

There’s the list! Seven planets, each an enigma waiting to be unraveled. I look forward to seeing what the James Webb Space Telescope discovers. Further telescopes—truly massive things—may yield actual pictures of TRAPPIST-1’s worlds, maybe continents and seas. And of course, there is the distant possibility that we may one day send a probe there…

My next post should go up on Monday—the sixtieth anniversary of a very important event. Until then, take care!

Further reading:


  1. Probably the most convenient way to detect exoplanets—you watch a star and see if anything big passes periodically in front of it. This is why so many known exoplanets are gas giants with extremely close orbits. The downside is, the transit method only works if the planet’s orbital plane is pointed towards Earth.
  2. Mass estimates vary, even on Wikipedia, but for this post I will take them from table citing a recent study, published in 2020.

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