Seven at one brew

Astronomers have found a star that has seven planets orbiting it, three of which are in the star’s “habitable zone”

This handout artist’s impression released by the European Southern Observatory shows the view just above the surface of one of the planets in the TRAPPIST-1 system. Image: AFP PHOTO / ESO/M. Kornmesser/
This handout artist’s impression released by the European Southern Observatory shows the view just above the surface of one of the planets in the TRAPPIST-1 system. Image: AFP PHOTO / ESO/M. Kornmesser/

Last month, the word “TRAPPIST” appeared in news reports around the world. Was this something to do with hunters who trap animals for their skins? Or with certain cloistered Benedectine monks?

Neither, as it turns out. Instead, this was about a remarkable discovery astronomers made over a period of several months. They used an instrument in Chile that’s known as the “Transiting Planets and Planetesimals Small Telescope”—or, in acronym form, TRAPPIST. Their discovery was christened ‘TRAPPIST-1’.

And what is this discovery? Nothing more, nothing less, than another solar system. The astronomers found a star that has seven planets orbiting it. Three are in the star’s “habitable zone”—not so close to the star that they would be scorched, not so far that they’d be frozen, but just right (you can figure out why it’s also called the “Goldilocks zone”). This means that they might have stores of liquid water and thus could support life, at least as we know it on our own planet.

Big news indeed, and one that’s caused great excitement among scientists across our own planet. We don’t know if there’s life in the TRAPPIST-1 system, but even the thought that there might be life somewhere else inspires awe, humility and a certain perspective.

But speaking of perspective, let’s give this discovery some.

First, this star is 39 light years away. That is, light takes 39 years to get from there to us here on Earth. And since light travels at, well, the speed of light, the distance from there to us is about 390 trillion kilometres. How long would it take you and me, travelling in the swiftest spacecraft we’ve produced, to get to TRAPPIST-1? About 700,000 years.

So the only way we can even conceive of traversing that nearly unimaginable distance is if we ever figure out how to travel at something close to the speed of light. That’s so far beyond humankind’s present scientific capabilities that it might as well be science fiction. So let’s not imagine we’re going to be visiting those planets anytime soon. Even communicating with TRAPPIST-1 residents, if there are any, will mean 78 years between sending that WhatsApp “Navroze Mubarak!” message and getting a reply. Not for the faint-hearted, certainly.

(Worth noting in passing here is that while 39 light years is impossibly far away for us humans, it is next to nothing on the scale of the universe. Most celestial objects out there are millions and billions of light years distant.)

Second, if TRAPPIST-1 is so far away, you might wonder how we figured out that this particular star has these planets. It’s not as if we can simply look through even the most powerful telescope on Earth and see it and its seven planets neatly arranged in a line. It’s simply too far away to allow for that. Yet we do have a time-lapse video of TRAPPIST-1, taken over 24 hours (take a look here:—really just a sample of data collected over nearly two-and-a-half months). You still don’t see neatly arranged planets, though. Instead, there’s a shifting pattern of grey-scale squares, like horribly low-resolution images of blinking lights. Which is what it really is—a series of lo-res images of TRAPPIST-1.

What’s fascinating about that time-lapse sequence, though, is what astronomers were able to deduce. The flickering of the brighter squares, which represents the star itself, tells us that something is crossing in front of it and blocking part of its light. If the brightness varies regularly, as it does seem to, that suggests an object that is not just crossing, but actually orbiting the star (see my column The Variation Tells The Tale,, for an explanation). And what do we call such an orbiting object? That’s right: a planet. In the case of TRAPPIST-1, the way its brightness varies suggests not just one, but seven planets.

Here’s a brief catalogue of what astronomers have so far managed to divine about TRAPPIST-1: The star itself is much cooler and smaller than our Sun—only about the size of Jupiter; that each of its seven planets is about the size of our Earth; that they are probably rocky and might all have stocks of liquid water; that three are in that Goldilocks zone where life is at least possible. Wait, there’s more. About each planet—which, remember, we “see” only as shadows that dim the light of the star—we know its mass, its density, the inclination of the plane of its orbit, and how long it takes to complete an orbit to the nearest second.

As ever, astronomers make some pretty profound deductions from some apparently flimsy data. In fact, it isn’t that flimsy. Blinking squares aside, there’s a treasure-trove of TRAPPIST-1 numerical data available ( that astronomers are poring over even as you read this.

Dilettantes like me get our kicks from reading their remarks as they analyse: “The bolometric luminosity is estimated to be 2.7*10^29 erg/s,” says Rishi Paudel. Meanwhile, Chelsea Huang tells all that “the flares are preserved in the light curve in case people want to look at them.” And when Tony Jebson says happily that “today seems to be planet hunters day”, that underlines the collaborative nature of this effort.

Which is why I’ve been imagining these hard-working astronomers taking a break to down some of the stuff this solar system’s name consciously refers to, the stuff brewed even today by many of those cloistered monks: Trappist beer.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter Of Numbers explores the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at Read Dilip’s Mint columns at

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