Solar System Simulator · What if? · the Sun
What if the Sun disappeared?
Not a video. Delete the Sun in a real gravity simulation and watch every planet fly off in a straight line at exactly the speed it had. Then read the numbers that explain why.
Straight lines, not spirals
Ask people what the planets would do and most say they'd spiral inward, or keep circling out of habit. Neither. The only thing bending Earth's path into a circle is the Sun's pull. Take that away and there is nothing left to turn it, so Earth keeps exactly the speed and heading it had at that instant — about 30 km/s, along the tangent of its old orbit — and travels in a straight line forever. That is Newton's first law with the training wheels off: the circle was the Sun's doing all along.
In the simulator the lesson removes the Sun's mass for real and lets the integrator run. The overlay draws Earth's straight line, and the debrief reports its speed before and after (they match) and how far it has gone. After the four months the lesson runs, Earth is roughly 329 million km from where the Sun used to be — about 2.6 million km a day.
Nothing happens for 8 minutes 20 seconds
The one thing the simulator cannot show: gravity is not instantaneous. Changes in gravity travel at the speed of light, and light takes 8 min 20 s to cross the 149.6 million km from the Sun to Earth. So the real Earth would keep circling a Sun that no longer existed for another eight minutes, still lit by sunlight already on its way — and then, in the same instant, go dark and fly straight. Neptune, 30.1 times farther out, wouldn't notice for about 4 h 10 min.
What if it disappeared for just one second?
Almost nothing. Earth's orbit is a circle only because the Sun bends it by 5.9 mm/s² — the acceleration you'd measure if you fell toward the Sun from Earth's distance. Switch that off for one second and Earth departs from its circle by half of 5.9 mm/s² times one second squared: about 3 mm. Then the Sun is back and the pull resumes. Eight minutes later, Earth would experience a one-second night. Nobody could measure the wobble.
How fast would each planet leave?
Each planet keeps its own orbital speed, and the inner planets move fastest:
| Planet | Speed it keeps | Gravity's delay |
|---|---|---|
| Mercury | 48 km/s | 3 min |
| Venus | 35 km/s | 6 min |
| Earth | 30 km/s | 8 min |
| Mars | 24 km/s | 13 min |
| Jupiter | 13 km/s | 43 min |
| Saturn | 9.6 km/s | 1 h 19 min |
| Uranus | 6.8 km/s | 2 h 40 min |
| Neptune | 5.4 km/s | 4 h 10 min |
Speeds are for circular orbits at each planet's mean distance, from the same orbital elements the simulator uses; the delay is the light-time from the Sun.
Would the Moon stay with Earth?
Yes. The Moon is held by Earth's gravity, and Earth is still there. The pair leaves together, the Moon still circling Earth once a month, while the two of them drift off in a straight line. The simulator keeps integrating the Earth–Moon pull after the Sun is gone, so you can zoom in and check.
What the simulator does
- The Sun is deleted for real — mass, gravity, everything — and the view holds on the spot where it was, switched to true-scale distances so straight lines look straight.
- The overlay draws Earth's path; the trails show every planet leaving along its own tangent.
- The debrief reports Earth's speed before and after, how far it has gone, and the 8-minute caveat the simulator can't show.
Questions people ask
Would the planets fly off immediately?
No. Gravity propagates at the speed of light, so Earth would keep orbiting for 8 minutes 20 seconds after the Sun vanished and Neptune for about 4 h 10 min. The simulator removes the Sun instantly, which is the one place it departs from reality — the lesson says so.
Which direction would Earth go?
Straight along the tangent of its orbit, at about 30 km/s — roughly 2.6 million km every day, forever, with nothing to slow it down.
Would the Moon still orbit Earth?
Yes. Earth's gravity is untouched, so the Moon keeps circling it once a month while the pair travel away together.
Is this a real simulation or an animation?
A real one. The simulator integrates the gravitational pull between every pair of bodies each frame (velocity Verlet, energy-conserving), and the lesson genuinely removes the Sun's mass. What you see afterwards is whatever the physics does.
Keep going
Every number on this page comes from the simulator's own physics or the real mission record it is checked against. Where the simulation and reality differ, the page says so.