By Lucianne Walkowicz
From across the galaxy, just shy of 40 light years away, seven long shadows sweep across our planet. These shadows belong to the planets of the TRAPPIST-1 system, seven Earth-sized worlds whose discoveries were announced with great fanfare this past week.
Led by Michael Gillon of the University of Liege, an international team of astronomers deployed both space and Earth-based telescopes to make these incredible discoveries: three of the TRAPPIST-1 planets were revealed last year, with the recent announcement unveiling an additional four.
Each world passes in front of its parent star from our point of view, creating those shadows - called “transits” - that dim the light of the star during the planet’s passage.
It might seem that these days, planet discoveries are a dime a dozen: more than three thousand planets are known around other stars, and their existence implies that there are hundreds of billions of planets in our galaxy alone.
Seven years ago, no one – neither a random person on the street, nor the most learned astronomer – could tell you whether any planets that might be like Earth existed.
Today is a different story: we know from the discoveries of planet-hunting missions like NASA’s Kepler Telescope, TRAPPIST, and a myriad of other projects, that rocky planets must be out there in abundance.
This revolution in planet hunting is certainly incredible, but also begs the question: in a galaxy awash with more planets than stars, what’s the big deal in finding seven more?
As it turns out, the TRAPPIST-1 planet system is remarkable for a number of reasons. For one thing, the star at the system’s centre is exceptionally cool - it emits only around five hundredths of one percent of the energy a second that our sun does, and most of that light is in infrared energy.
Because the star’s light is so feeble, its planets must be cuddled in very close orbits to be warmed enough to have any hope of liquid water, and this is exactly the case: all seven of the TRAPPIST-1 planets orbit closer to their star than the planet Mercury does to our own sun.
The TRAPPIST-1 planets are so close, in fact, that it takes only a day and a half for the innermost planet to complete its full trip around its star (or “year”), and even the outermost known planet orbits in a mere 20-ish days.
Incredibly, three of these planets orbit at just the right distance from their star where they could be the right temperature to have liquid water on their surface.
Not only are the TRAPPIST-1 planets close to their star, they’re also close to each other - so much so that if you were standing on the surface of one, you would be able to see others in the sky, in some cases looming as large as our own moon.
Hints from the timing of these orbits also tells us that the close proximity of these planets was likely not always the case – in all probability, they formed further from their star and moved inwards over time, eventually coming to their current compact arrangement.
Today, the little gravitational tugs the planets exert on each other can actually be seen in tiny differences in how long each planet takes to orbit – these “transit timing variations” appear as the planets’ shadows sweep across our telescopes, either slightly ahead of or behind schedule.