About 40.5 light-years away — that’s roughly 240 trillion miles — a small, dim star called TRAPPIST-1 holds seven planets in its orbit. One of them, TRAPPIST-1 e, sits in a spot where scientists think liquid water could exist. That doesn’t mean it has water, or life, or even air. But it does make this little world one of the most carefully watched places in the sky.
A Tiny, Cold Star and Its Seven Worlds
TRAPPIST-1 is what scientists call a red dwarf — a kind of star that is much smaller and much dimmer than our Sun. Its surface temperature is 2,566 K (Kelvin is a temperature scale scientists use; 2,566 K is far cooler than our Sun, which burns at around 5,778 K). Because it gives off so little heat and light, its habitable zone — the range of distances where liquid water could exist — sits very close in, almost hugging the star.
Seven planets orbit TRAPPIST-1, and all seven were discovered to be packed in tight orbits. Having seven worlds around one star is already unusual. Having several of them in or near the habitable zone is rarer still, which is why this system has drawn so much attention from scientists around the world.
How Scientists Found TRAPPIST-1 e
TRAPPIST-1 e was discovered in 2017 using the transit method. Here is how that works: when a planet passes in front of its star, it blocks a tiny bit of the star’s light. Telescopes can detect that small dip in brightness. By watching many of these dips, scientists can figure out the size of the planet and how long it takes to orbit the star.
The transit method is one of the most reliable tools astronomers have for spotting planets that are far away. It works especially well around small, dim stars like TRAPPIST-1, because a planet blocks a bigger share of the light when the star itself is small. That is partly why TRAPPIST-1’s planets were spotted at all — the star’s low brightness made the signal easier to see.
Size, Mass, and What That Tells Us
TRAPPIST-1 e is slightly smaller than Earth. Its radius — the distance from its center to its surface — is about 0.92 times Earth’s. Its mass is about 0.69 times Earth’s. Mass means how much matter is packed into the planet.
When scientists look at both radius and mass together, they can make careful estimates about what a planet might be made of. A planet with a mass and size both a little smaller than Earth’s could be rocky, a little like our own world. Scientists think TRAPPIST-1 e is probably a rocky planet, but they haven’t been able to confirm what its surface or interior truly looks like. That kind of detail is still very hard to measure from 40.5 light-years away.
If you want to compare TRAPPIST-1 e with other candidates, the Most Earth-Like Worlds list shows how different planets stack up by size and other features.
The Habitable Zone — and Where TRAPPIST-1 e Sits
The habitable zone is the band of space around a star where temperatures might allow liquid water to exist on a planet’s surface — not too hot, not too cold. Scientists sometimes call it the “Goldilocks zone.” It is not a guarantee of water or life. It simply means the conditions aren’t obviously ruled out.
Because TRAPPIST-1 is so cool and faint, its habitable zone is very close to the star. TRAPPIST-1 e orbits within that zone. That is one of the main reasons scientists find it so interesting. Being in the habitable zone is a starting point, not a finish line. A planet also needs the right kind of atmosphere — the layer of gases surrounding it — to hold onto warmth and keep water liquid. Whether TRAPPIST-1 e has an atmosphere at all is something scientists are still trying to work out.
You can explore how habitable zones work for different stars using the habitable zone explorer, which lets you see how the zone shifts depending on a star’s heat and size.
Temperature: Cold on the Surface, but the Story Isn’t Over
The estimated temperature of TRAPPIST-1 e is about 250 K, which works out to roughly -23 degrees Celsius (-9 degrees Fahrenheit). That is cold — colder than a winter day in many parts of northern Canada or Russia.
At first, that might sound like it rules out liquid water. But this temperature estimate is what scientists call an equilibrium temperature — a rough guess at how warm the planet would be if it had no atmosphere at all, based purely on the starlight it receives. Atmospheres change everything. Earth’s own atmosphere traps heat and keeps our planet about 33 degrees Celsius warmer than it would otherwise be. If TRAPPIST-1 e has a thick enough atmosphere with the right gases, its real surface temperature could be warmer than 250 K suggests. Scientists haven’t confirmed what kind of atmosphere, if any, the planet has.
A Year That Lasts Just Six Days
TRAPPIST-1 e completes one full orbit around its star every 6.1 Earth days. That means one year on TRAPPIST-1 e is shorter than one week on Earth. This happens because the planet orbits so close to its star — and orbiting close means moving fast.
That closeness brings another effect scientists think about: tidal locking. When a planet orbits very close to its star, the star’s gravity can slow the planet’s spin until one side always faces the star and the other side always faces away. If TRAPPIST-1 e is tidally locked, one half of the planet would be in permanent daylight and the other in permanent night. Scientists think this is likely, though it hasn’t been confirmed. A tidally locked world is very different from Earth, but it doesn’t automatically rule out the chance of liquid water — especially near the boundary between the day and night sides.
What We Still Don’t Know
Honest science means keeping a clear list of what hasn’t been answered yet. For TRAPPIST-1 e, that list is long. Scientists haven’t confirmed whether it has an atmosphere, what gases might be in it, what its surface looks like, or whether any water is present in any form. Current telescopes are beginning to study the TRAPPIST-1 planets in more detail, but getting clear answers about small, rocky worlds this far away is genuinely difficult work that will take time.
TRAPPIST-1 e is not a confirmed Earth twin, and it is not a world we know anything definitive about yet. What it is, so far, is a planet the right size, in the right neighborhood, around a star we can study — and that is already enough to make it worth watching carefully.