Astronomers believed there was an undiscovered planet between Mars and Jupiter until quite recently. The existence of the hypothetical planet seemed truer when Giuseppe Piazzi discovered what was considered to be planet Ceres in 1801. A year later, Heinrich Olbers discovered what was thought to be planet Pallas.
Olbers soon realized that Ceres and Pallas used to be part of the same planet. This belief was reinforced when planets Juno and Vesta were discovered. Ceres, Pallas, Juno, and Vesta were later reclassified as asteroids and considered remnants of a hypothetical planet called Phaeton.
Astronomers of the day thought that Phaeton had broken up and created the four large asteroids and every other one in the asteroid belt between Mars and Jupiter today.
Some astronomers thought that Phaeton had broken up after it exploded, was destroyed by Jupiter, or had smashed into another celestial body. Some think that this celestial body is Nemesis, a hypothetical star that was believed to be in our solar system.
However, today’s astronomers have disproved the existence of Phaeton. They say that the asteroids in the asteroid belt have always been asteroids. They were stuck between Mars and Jupiter after the big bang and would have formed into a planet if it weren’t for Jupiter’s massive gravitational pull that kept them apart.[