The Science of the Prospective Planets in Interstellar

[CuriousJorge]

I recently watched Christopher Nolan's Interstellar and came up with a question regarding one of the planets orbiting the black hole, Gargantua. If the planet that Dr. Mann landed on was close enough to remain in Gargantua's orbit wouldn't it need to remain at a velocity faster than the speed of light? Light isn't quick enough to escape a black hole's gravity; everybody knows that, so how can a planet be close enough to Gargantua to be in orbit, yet fast enough to escape being pulled in?

[mistermack]

This is an off-the-cuff answer, so if it's wrong, I was only guessing.

An orbit is essentially a stable state. If the planet goes any closer, it speeds up, which sends it back out. If it strays any farther out, it slows, which sends it back inwards.

So if nothing else is interfering, the orbit naturally stabilises at a certain level. It would have to lose kinetic energy to move closer.

 

Rather like the Solar System, a planet doesn't need to be close, to orbit a black hole. It can be a long way out, or close in. It depends on the kinetic energy of the planet.

Our Solar System is actually orbiting a black hole that is at the centre of the Milky Way. And we are a hell of a long way away from it.

Actually about 25,000 light years away.

Of course, our orbit is influenced by all the other matter in the Galaxy so we're not simply orbiting the black hole in isolation.

Edited May 26, 2017 by mistermack

[swansont]

The planets were not inside the event horizon. None of the orbits would require moving above c

[Janus]

I recently watched Christopher Nolan's Interstellar and came up with a question regarding one of the planets orbiting the black hole, Gargantua. If the planet that Dr. Mann landed on was close enough to remain in Gargantua's orbit wouldn't it need to remain at a velocity faster than the speed of light? Light isn't quick enough to escape a black hole's gravity; everybody knows that, so how can a planet be close enough to Gargantua to be in orbit, yet fast enough to escape being pulled in?

There are more than one region around a black hole. There is the event horizon as previously mentioned, where nothing inside of it can escape. Then there is the Photon sphere. This marks the distance at which a photon will maintain a circular orbit around the BH. for a non-rotating black hole this is located at a distance of 1.5 time that of the event horizon. Outside of the photon sphere is the region where material objects can maintain an orbit.

If the BH is rotating, there is a third region called the ergosphere. It will take the shape of an oblate spheroid touching the event horizon at the poles and extending outward at the equator of the BH. This marks a region where objects can only orbit in the same direction as the BH rotates. (if the BH is rotating, photons can only orbit in the equatorial plane but in either direction)

 

This is an off-the-cuff answer, so if it's wrong, I was only guessing.

An orbit is essentially a stable state. If the planet goes any closer, it speeds up, which sends it back out. If it strays any farther out, it slows, which sends it back inwards.

So if nothing else is interfering, the orbit naturally stabilises at a certain level. It would have to lose kinetic energy to move closer.

 

Rather like the Solar System, a planet doesn't need to be close, to orbit a black hole. It can be a long way out, or close in. It depends on the kinetic energy of the planet.

Our Solar System is actually orbiting a black hole that is at the centre of the Milky Way. And we are a hell of a long way away from it.

Actually about 25,000 light years away.

Of course, our orbit is influenced by all the other matter in the Galaxy so we're not simply orbiting the black hole in isolation.

The SMBH at the center of our galaxy only makes up a small fraction of the total mass that our solar system orbits. It is in the order of 4 million solar masses while the galaxy has 100-400 billion stars, and we orbit ~2/3 of the way out from the center.