Module 8 · Travelling by gravity, and beyond
Other worlds and our galaxy
Interactive 9 min
A planet and its star orbit their common centre of mass, so the star wobbles. That wobble is how thousands of worlds were found: the NASA Exoplanet Archive reached 6,000 confirmed planets on 17 September 2025 [1]. The Sun orbits the Milky Way at about 230 km/s, one lap in about 230 million years [2].
Try this. Add the motions of the last step: the Earth's spin, its orbit, the Sun's lap of the galaxy. Which is fastest?
Sources: [1] 2025 Exoplanet Archive News (NASA Exoplanet Archive); [2] Solar System: Facts (NASA Science)
Step 1 of 3
How we find other worlds
A planet doesn’t simply orbit its star — both swing around their shared center of mass, the barycenter. The star traces a tiny circle in response to the planet’s pull. We can’t see most exoplanets directly, but we can detect that wobble: the star’s light shifts blue then red as it approaches and recedes. Jupiter makes our own Sun loop by about one solar radius; that telltale dance is how thousands of distant worlds were discovered.
Text version of this interactive
How we find other worlds
A planet doesn’t simply orbit its star — both swing around their shared center of mass, the barycenter. The star traces a tiny circle in response to the planet’s pull. We can’t see most exoplanets directly, but we can detect that wobble: the star’s light shifts blue then red as it approaches and recedes. Jupiter makes our own Sun loop by about one solar radius; that telltale dance is how thousands of distant worlds were discovered.
The Milky Way and the galactic year
Pull back further than any orbit so far. Our Sun is one of a few hundred billion stars in the Milky Way, riding a spiral arm some 26,000 to 28,000 light-years from the center. It orbits the galaxy at roughly 230 kilometres per second — yet the galaxy is so vast that one lap, a “galactic year”, takes about 230 million years. The last time the Sun was here, dinosaurs were just beginning. The same gravity that holds a moon holds a galaxy together.
You are never standing still
Sitting perfectly still, you are being carried by four motions at once, each riding on the one above it. Earth’s spin sweeps you eastward at 0.46 km/s at the equator. Earth carries you around the Sun at 29.8 km/s. The Sun carries the whole system around the galaxy at 230 km/s. And the galaxy itself is falling toward the Great Attractor, so that against the oldest light in the universe — the cosmic microwave background — you are moving at about 370 km/s. Every one of those motions is a fall: a body going as straight as it can through curved space. The counter above shows how far you have come since this slide opened.
About this interactive
- Licence
- MIT
- Credits
- © 2026 Mateusz Wojczal. Earth texture: Solar System Scope, CC BY 4.0. Three.js (MIT). Inter and Roboto Mono (SIL OFL 1.1).
- Source
- https://github.com/ulams-dev/ulams/tree/main/demo-content/gravity