The huge "mega" comet C/2014 UN271 (Bernardinelli–Bernstein) 80 miles wide is coming towards Earth from the edge of our Solar System.
It is supposed to only get to the middle of our Solar System . What is preventing it from coming farther? Would it be the gravity of Saturn? Why can other comets come farther than this one?
If it did approach Earth, would the size of this comet make it a potential "planet killer"?
Objects in space follow the laws of gravity. Since the sun is by far the biggest object in the solar system, nearly all objects (except moons) are in elliptical orbits around the sun.
There are believed to be many ice-rock bodies in orbit around the sun at great distance. This is called the Oort cloud. Most of these objects will never come near the sun, their orbits are a long way from the sun.
But sometimes these objects can be perturbed. Oort cloud objects are so far from the sun that they move at about 100m/s (compared to the Earth which moves at 30000 m/s around the sun) So a small change in velocity can make a big change in orbit. If the speed is increased by 50 m/s the comet will leave the solar system. If the speed is decreased by 100 m/s the comet will start to fall directly into the sun.
Each time a star passes by the sun, the comets will get a random push of about ±1m/s. But these comets are moving so slowly that many stars will pass by the sun before the comet has moved significantly. This is like a random walk, eventually some comets will end up on orbits that will bring them down to the orbit of Neptune, some will end up on orbits that come down to the orbit of Saturn, and some will end up on orbits that come right down into the inner solar system.
However we just don't see many comets that are on orbits that only reach to Neptune. Comets are mostly small and dark. We only see those comets that come right into the inner solar system, warm up, and develop bright coma and tails.
This comet is an exception. It is so big that we can spot it even when it is far far away. It is on an orbit that will take it about as close as Saturn. Its orbit won't be significantly changed by the gravity of Saturn or any other planet. There are lots of other comets that have this kind of orbit, but most are never spotted because they are too faint.
There is no standard definition of "Planet Killer", but if a comet of this size were to hit Earth, it would be devastating. It is far larger than the impact that finished off the dinosaurs.
So the comet won't get closer than the middle of the solar system just because that is the orbit it happens to be on. But there's nothing preventing a similar sized body from coming closer.
This image from the Wikipedia entry will probably explain it best. It's closest approach to the Sun is simply due to the properties of its orbit, just like all of the planets.
