Eli5 Who do all the planet not eventually get sucked into the sun?
In: Physics
Earth will be engulfed by the sun as the sun dies. The sun will expand before earth can fall into it.
A long time ago, lots of stuff did get sucked into the sun. The remaining planets are just the ones who were too fast to get sucked in, but not fast enough to completely escape. If they were to slow down, then yes they’d get sucked in, but there’s no friction in space so how would they ever slow down?
The planet orbiting the Sun is trying to move in a straight line, and thus away from the Sun. That’s called momentum. The Sun is pulling the planet with gravity towards the Sun, causing the planet to “turn” slightly in its path. Enough small turns over and over, you get a circle. An orbit is when the planets speed and the pull of gravity are balanced making a perfect circle. Thanks to physics, momentum and gravity never wear out or decrease, so the orbit goes on forever. So momentum is pushing out just as much as gravity is pulling in. There is balance and it doesn’t fall into the Sun.
If the sun lasted much longer then they probably would. The orbits are decaying with time, but they’re decaying so slowly that the sun would actually end up dying before the planets were sucked in.
To get sucked in, they would have to lose their momentum and energy. But what’s going to do that in space? Nothing really.
A well lubricated, low friction clock pendulum can swing for a very long time, a massive planet flying through basically empty space experiences essentially zero friction. They can keep flying in a circle around the sun because they have the speed to do that and aren’t losing it.
But how did they all end up with enough energy to keep flying around the sun? Wouldn’t some of them have less speed and a path that would eventually hit the sun? Absolutely, and that happened a few billion years ago. The current planets, moons, asteroids, and meteors in their present configurations are not all that ever existed in our solar system. Lots of things fell into each other when the solar system was young.
Will they ever fall into the sun? It there a very small amount of friction slowing them?
Yes. The inner planets will end up in the sun. Definitely mercury and venus. Probably earth. Probably not mars. Not because they fall it, but because the sun will grow to a red giant before it dies and engulf them.
But ignoring the sun ever dying, foreseeably the planets would eventually decay. There is gravitational waves, and gravitational orbits do trigger tiny amounts of these waves that carry away energy and momentum. In a few trillion or more years, longer than the existence of the entire universe so far, the planets might fall into the sun. Sun of course by then will be a small, cold black ball rather than a star. Life as we know it will be long dead and impossible to exist.
Before we toilet bowl into the Sun, something else will happen.
Eventually the Sun will turn into a Red Giant, engulfing Mercury, Venus, and probably Earth too. After that it will have burned up all of its fuel and shrink into a White Dwarf. The end
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