3I/ATLAS

The third visitor from another star, and the biggest and fastest yet — it may be older than the Sun itself! Only three of these have ever been spotted, and all three since 2017.

🚀 Travel there

What it is

Third and largest/fastest known interstellar visitor, possibly one of the oldest comets ever observed.

Interstellar visitor: Passing through from another star, never to return. 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 3I/ATLAS

3I/ATLAS is much bigger than the other two interstellar visitors found so far.

How it moves: Moving so fast it can't be captured by the Sun's gravity — it will fly through and leave forever.
What makes it worth knowing: Possibly one of the oldest comets ever observed — some estimates suggest it may be older than our own solar system (up to ~11 billion years). Telescopes have detected methane and unusual water ice in its coma. It stays far from Earth the whole time it passes through.; How we know: Scientists worked backward from its speed and trajectory to estimate where in the galaxy it likely formed, combined with its unusual chemistry — together suggesting it formed around a very old star, long before our own Sun existed. Rung 3 of the cosmic ladder — how the solar system is arranged

How did anyone measure the Earth in the first place?

The numbers

Across	18 km
Found in	2025

What it belongs to

☀️ Everything the Sun holds, in one place 📖 How the solar system fits together

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