What would the planets do without the sun?

17 May 2016

Question

If the sun disappeared, what would happen to the planets?

We asked Astronomer Gerry Gilmore what he thought of this one...

Gerry - The planets would keep moving exactly as they are, instantaneously. People imagine that planets naturally move in circles, but they don't. Everything naturally moves in a straight line. A planet moves a little bit in a straight line and it falls a little bit towards the sun, and it moves a little bit more and falls towards.

Chris - And that's gravity tugging it inwards?

Gerry - Yes, that's right. It's falling the whole time but it's also moving in a straight line the whole time. So, if the gravity were simply to stop, the planets would just continue in the straight line they are currently moving in, that's the tangent as we call it, to their present orbit, and so they would just carry on moving away. The underlying presumption here is when all this happens is kind of interesting because people imagine that there is some sort of and absolute underlying time, a newtonian time, and that therefore all the planets will head off instantaneously at the same time. But, actually, time is different on the different planets so it takes time for gravity, and light, and everything else to come from the sun to us.

Chris - Ah right, so gravity isn't there instantly. What you're saying is if the sun did evaporate all of a sudden in an instant, then we wouldn't know for a certain amount of time.

Gerry - Exactly. If the sun suddenly went dark, then the sky would stay bright for as long as it took the light to get to us.

Chris - So you're saying that gravity propagates at the speed of light?

Gerry - Yes and so does time, actually. Everything goes at the speed of light. Both time and gravity and light all travel at the speed of light.

Chris - Why do we think that gravity propagates at the speed of light then?

Gerry - We know it does actually, it was measured early this year.

Chris - With the gravitational wave stuff?

Gerry - With the gravitational wave burst, yes.

Chris - And how did they measure that then?

Gerry - There were two detectors separated by a finite distance and you could measure the time.

Chris - The gravity wave arrives at one and they know how far away it is to the other one?

Gerry - Yes, and a few milliseconds later the other one which was a few thousand kilometers away and that's fundamental to Einstein's general theory of relativity, was a prediction from almost immediately after the theory was developed, and it was completely different than the way it works in newtonian gravity where there is this absolute time sitting there in the background that things just live in, whereas we now know that that's not correct.

Chris - So to summarise for the benefit of Munyuradzee who sent this question in. Were the sun or a star with some planets going round it suddenly to disappear it's gravitational influence would be removed, and the time it took before those planets ceased to feel the gravity from that star would be however long it takes light from that star to reach that planet.

Gerry - Absolutely. So if you're outside watching it you would see the planets popping off in their straight line orbits but, because everything happening in the time it takes the signal to get to you, you'd see it all just vanishing off very nicely. It's only if you were one one of those planets you'd think it was different.