Question: Why doesn’t gravity cause Mercury or any other planets to fall into the Sun?
Answer: Although the Sun’s gravity is trying to pull Mercury towards it, it doesn’t fall into the Sun because this force is countered by Mercury’s own momentum.
Mercury is travelling incredibly fast at over 107,000 miles per hour as it orbits the Sun, effectively trying to escape the stars gravitational pull as is the case with all the planets in our solar system.
It just happens that the two massive forces are currently in balance meaning that Mercury neither gets pulled in or completely fly’s away.
It may seem amazing that all the planets are in such a fine balance with our stars gravity. However you need to remember that over the billions of years our solar system has been evolving many potential planets would have been sucked into the sun or flung out into outer space.
As time has gone by though things have settled down a bit into the way we know them today, however this doesn’t mean things will stay how they are now forever.
The universe is constantly changing with stars, planets and moons gaining, and losing mass which effects their gravitational forces.
Generally though any significant changes take place over thousands and millions of years so barring something like a huge meteor strike things here should stay as they are for a long time yet.