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star · ultra-cool red dwarf with seven planets

TRAPPIST-1

An ultra-cool red dwarf barely bigger than Jupiter, with seven Earth-sized planets packed tight around it.

Also called: the seven earth sized planets star.

🚀 Travel there

What it is

Star: A ball of gas heavy enough to fuse hydrogen in its core. Every kind of thing there is →

Can you see it?

No. No telescope on Earth will show you this one; it is here to be understood, not seen. It is here because it is worth knowing about — and because leaving it out would make the sky look emptier than it is.

Worth knowing about TRAPPIST-1

TRAPPIST-1 is barely wider than Jupiter, even though it is a real star. In weight, only about a twelfth of the Sun's mass -- just barely enough to light itself. On the 100-meter-track scale model of the Milky Way, TRAPPIST-1 would be about 4 centimeters from the Sun -- roughly the size of a walnut. Light takes 40 Earth years to get here from TRAPPIST-1, so what you are looking at is TRAPPIST-1 as it was 40 Earth years ago.

How bright it is
Puts out roughly two thousand times less light than the Sun, and most of that is infrared -- heat rather than the light your eye can use.
What it is made of
A red dwarf, and about as small as a star can be: any lighter and it would never have started fusing hydrogen at all, and would have ended up a brown dwarf instead.
What makes it worth knowing
The most Earth-sized planets known around any one star -- and the star itself is smaller than some planets elsewhere.
Seven worlds
Seven planets, all roughly Earth-sized, and every one of them orbits closer than Mercury does to the Sun. A year on the innermost lasts a day and a half.
How we know: Found by the transit method: telescopes watched the star for weeks and caught a tiny repeating dip in its light each time a planet crossed in front.
How long it will last
A red dwarf burns its fuel so slowly that this star will still be shining trillions of years from now, long after the Sun has died. No red dwarf anywhere has yet had time to end.
The numbers
Distance from us40 light-years

How anyone knows this

Nobody flew a tape measure out there. Every number on this page was worked out from the ground:

Next to it

The SunstarNaked eyeProxima CentauristarWith effortSiriusstarNaked eyeVegastarNaked eyeArcturusstarNaked eyeBeta PictorisstarNaked eye

Then go outside

Not this one. But the sky over your head is full of ones you can see, and the Planetarium knows which.

🌙 Plan an evening outside🌌 Everything else in the sky