How do planets have EXACTLY the right speed to stay in orbit?
In truth, NO PLANET has EXACTLY the correct speed. The planet of the solar system closest to having the perfect speed at present moment is Venus. All planets have some eccentricity, even if only a small amount. Circular orbit is a special case of general elliptical orbit…and what? Do you demand that a planet form in a perfect circular orbit to what fraction of a nanometer? Planets that form too eccentric do not really “survive”. It is just like evolution of lifeforms. Those species who do not develop the traits favorable for life do not get to reproduce, and do not get to extend the span of their species’ lifetimes. Those species who DO develop favorable traits for survival DO get to live Similariy, planets too eccentric for a stable orbit might be slung in to interstellar space and never see their original parent star again…or they might crash in to the parent star. The small fraction of planets that “survive” end up forming the actual planetary systems that we know and love.
Because the planets formed from the remnants of the solar nebula, they were already in orbit when they formed. But actually, the natural tendency of an object moving in the vicinity of a star, unless it is moving faster than escape velocity, is to go into orbit around it. If you change the speed of an orbiting object, it will go into a new, and probably more eccentric orbit, but it takes a great deal of energy to make it either escape from orbit or fall into the sun. Billions of years ago, when the solar system was much denser, there were lots of interactions between objects, and many of them were flung out of orbit or into the sun, and others changed their orbit due to the interaction. Over time, the remaining planets settled into stable orbits in which their mutual gravitational interactions balanced out so they no longer change orbits. Perfectly circular orbits *do* require exactly the right speed and direction. Otherwise, the planet’s position oscillates around a perfectly circular
If a planet was to move just a bit too fast, then it would drift away, slow down (as gaining altitude takes away speed) and settle to a higher orbit. That is how rockets and satellite adjust their altitude: by slight speed adjustment. The elliptical orbit is the result of having a body in orbit that trades a bit altitude for speed and, half a rotation later, trades speed for altitude. Bodies oscillate between higher altitude/lower speed and lower altitude/higher speed. A circular orbit is a special case that is hard to achieve and would be unstable, as any perturbation — gravity of other planets, a meteor impact, even the light pressure form the Sun — is enough to cause a slight deviation from perfect circle.
They don’t need to. There is a wide range of speeds to stay in orbit around a star. For example, the Earth would orbit around the sun at any speed from 1.6 to 42.1 km/s. At the lower speed it would fall into the sun, and at the higher speed it would escape. The Earth’s actual orbit speed (average) is 29.78 km/s, comfortably inside this range.
