TRAPPIST-1: the Link between Astronomy and Beer

Today we will travel to 2MASS J23062928-0502285, better known as TRAPPIST-1, a star at about 40.5 light years from Earth. That would normally be quite a long journey; even light takes more than four decades to get there. But in our imagination we’re there in the blink of an eye.

Scarcely illuminated pub with a bottle and a glass dark trappist beer and on the wall a painting of an abbey on a remote planet
Astronomy and beer


TRAPPIST-1 is a so-called ultra-cool red dwarf star, which made the news in 2017 because Belgian astronomers discovered seven Earth-like planets in orbit around it. The terms ultra-cool and Earth-like should not be taken too literally. Anything below 2700 degrees Kelvin (more than 2400 degrees Celsius) is considered ultra-cool by astronomers. And Earth-like implies that the planets have a solid surface and roughtly the same size as our home world. In that sense, Mars, Venus and Mercury are also Earth-like planets.

The astronomers, who work at the Institut d’Astrophysique et Géophysique in Liège, used a telescope they call the TRAPPIST. The abbreviation stands for Transiting Planets and Planetesimals Small Telescope, but is of course also a reference to the rich culture of the Belgian Trappist beers. So at this star, two of my passions convene: astronomy and beer.

Schematic diagram, to scale, of the planetary system of the ultra-cool dwarf star Trappist-1, with seven planets roughly the same size as Earth
TRAPPIST-1 and its seven planets


The seven planets have been named with a letter from b to h, officially even 2MASS J23062928-0502285b to h. Remarkably, planet TRAPPIST-1a is missing. I suspect that the a is kept in reserve in case a planet is found that’s even closer to the star.

Now that we are there, we can of course come up with a few more imaginative names for the seven. And what could be more obvious than to use the names of Belgian Trappist beers in alphabetical order? There are five of those: Chimay, Orval, Rochefort, Westmalle and Westvleteren.

I can hear the reader thinking: only five Belgian Trappist beers? Indeed: Leffe, La Chouffe, Brugse Zot, Affligem, Maredsous, these are all very tasty beers. But in order to bear the honorary title of Trappist, a beer must meet three conditions. Brewing must take place within the walls of the abbey, by or under the supervision of monks of the Cistercian Order, and the proceeds must go to the monastic community or to good works.

Two planets remain unnamed, g and h. I propose to name them after the two Dutch Trappist beers, La Trappe and Zundert. Not only because those are delicious beers as well. But also because the respective monasteries, the Koningshoeven and Maria Toevlucht abbeys, are located almost in Belgium, only eight and five kilometers north of the border.

Artist impression of the planets around ultra cool dwarf star TRAPPIST-1, with apocryphal names of Belgian and Dutch trappist beers, with the Earth for scale comparison
The seven worlds around TRAPPIST-1, with the Earth for scale reference


TRAPPIST-1 is not only ultra-cool, but it’s also a real dwarf. Its diameter is only 0.11 times that of our sun. Fortunately, the seven planets are much closer to their star than Earth. As a result, some of them are into the so-called Goldilocks zone, where the temperatures are such that liquid water can exist and where conditions are favorable for the emergence of life. It is probably too hot on Chimay, Orval and Rochefort, Zundert is too far away and therefore too cold. Westmalle, Westvleteren and La Trappe are the most interesting pieces of real estate in the TRAPPIST-1 system.


Zundert is the smallest of the bunch with a radius of 4937 kilometers, La Trappe the largest with 7192 kilometers. For comparison: the radius of the Earth is 6371 kilometers, Westvleteren comes closest to that with 6657 kilometers. The mass of the planets is also in the same range as Earth’s. Gravity is therefore bearable on all seven worlds. Although I would advise against Chimay (1,374 times the mass of the Earth) for people with health issues, also because of the high temperatures.


The years are short in the TRAPPIST-1 system. Chimay orbits the star in a day and a half, Zundert takes about 18 days. That outermost planet orbits “only” nine million kilometers from the ultra-cool dwarf. It all fits well within Mercury’s orbit. A small, cozy planetary system.

The planets are so close together that from the surface of each of them the others can be clearly seen, sometimes as large as the Moon seen from Earth. This makes me think of science fiction illustrations where there are always few interesting spheres hovering above exotic landscapes.

Visualisation of the fifth planet around the ultra cool dwarf star TRAPPIST-1, renamed Westvleteren for this special occasion
At Westvleteren there are indications of the presence of liquid water

Tidally locked

Before we get the impression that these are heavenly places, I have to mention also a few disadvantages of this planetary system. All planets are tidally locked; in other words, they have a synchronic rotation. Difficult words, I know, but they mean that always the same side of the planet is facing the star. Such is the case, for example, with the Moon in relation to the Earth.

This makes it very warm on the bright side, and very cold on the dark side. Temperatures are probably most bearable at the terminator, the transition between day and night. For me as a photographer, those regions, with the star always just above or just below the horizon, also seem the most interesting locations. Imagine perpetual golden hours / blue hours / sunrises / sunsets! But be aware that on planets with an atmosphere, there can be strong winds in those transition zones between warm and cold.


Furthermore, ultra-cool dwarf stars are not as harmless as the name suggests. They tend to emit strong flames from time to time that you don’t want to be too close to. As the best site for founding a new abbey, I would therefore pick a spot on La Trappe, just on the dark side, at some distance from the terminator. An additional advantage is that it will take little effort to keep the beer cold there. Moreover, Trappist beers taste best by the fireplace when the cold winds are raging around the monastery walls.

Impressionist sketch of a fictional abbey on the dark side of one of the planets around the ultra cool dwarf star TRAPPIST-1
Fictional trappist monastery on the dark side of La Trappe

In the meantime I think Ultra Cool Dwarfs would be a great name for a post-punk folk band, but that remark is, of course, completely off topic.

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