Why do astronomers think that you have an asteroid belt and not a planet between mars and juipter?

Answer 1

There are a number of reasons. Just like astronomers can see both Mars and Jupiter in a telescope, they would be able to see a planet between them. Also, using both visual and radio telescopes, they can and do track hundreds of asteroids in the asteroid belt. They can also tell by the way Mars and Jupiter behave in their own orbits. An entire planet's gravity would effect the orbits of both planets to a much greater extent than thousands of rocks.

Answer 2

Astronomers believe that a planet did not form in the region between Mars and Jupiter because of the gravitational influence of Jupiter. Jupiter's strong gravity disrupted the formation of a planet by scattering the material in that region, preventing it from coalescing into a single planetary body. Instead, the leftovers of planet formation in this region resulted in the formation of the asteroid belt.