Falling into Black Holes
Earth's Orbit if the Sun Collapsed to a Black Hole
What would happen to Earth's orbit around the Sun, if the Sun were to suddenly collapse into a black hole?
Our home planet has been in a stable orbit around our local star for nearly five billion years, and is likely to remain so for at least another five billion years. The Sun's gravitational force tugs Earth into a nearly circular path about 150 million kilometers (93 million miles) from our solar system's central fire. The stable delicate balance between Earth's inertia and the Sun's gravity keeps us from either flying out of the solar system or diving into the Sun.
What if the Sun were to magically collapse into a black hole? What would happen to this delicate balance? Never mind that there is no way concievable to modern astrophysics that the Sun would collapse like this. Also never mind that we would miss the Sun's warming light and energy. Here we are only going to concern ourselves with the impact of such a hypothetical event on the stability of our annual trip around the Sun.
This is basically a multiple choice question. What would happen to Earth's orbit if the Sun were to suddenly collapse into a black hole? A) We would be sucked into the black hole B) We would fly off into interstellar space C) Nothing.
Most people would probably pick choice A, but that is wrong. The correct answer is C, nothing, nada, zip, zilch. There is a popular image that black holes have an irresistable gravitational force from which nothing can escape. It is true that once an object has fallen into a black hole there is no escape. But a planet or another star can remain in a stable orbit around a black hole. The gravitational force on a planet orbiting a star depends on the masses of the star and planet and on the distance between them. But as long as the planet is outside the surface of the star, the radius, or size, of the star has no effect whatsoever. Changing the mass of the Sun would affect Earth's orbit as would changing the Earth Sun distance. However changing the Sun's radius, without changing its mass, would not affect our orbit. That is true even if the change is as drastic as collapsing from a little less than a million miles to the few miles size of a black hole. Even though an object that falls into a black hole can never escape, an object at a distance from a black hole is no more likely to fall into a black hole than into a star of the same mass. Contrary to the popular image of black holes, they are not celestial vacuum cleaners sucking up everything around them. They are not the hoovers of the heavens.
