What is a protoplanet?
A whirling cloud of gas or dust that becomes a planet by condensation during formation of a solar system. As the central body, or protostar, of the system contracts and heats up
I think you mean "protoplanet hypothesis". In fact, astronomers usually call it the Nebular Hypothesis A protoplanet is a planet-like object that hasn't fully developed into a planet. Why that hypothesis? It's because it is the best we have to describe the origin of the solar system according to the Laws of Physics.
There are several, but one of the more popular is that a protoplanet about the size of Mars originally shared Earth's orbit. It crashed into Earth, and threw off a gigantic cloud of debris that coalesced into the Moon. If you'd like to read more about this, look up "Theia", the name given to this hypothetical protoplanet.
It's generally believed that a protoplanet about the same size as the present-day Mars struck Earth early in the solar system's formation.This impact ejected material from Eath that eventually became the moon. This seems so likely that this hypothetical protoplanet has even been given a name, Theia.
The current most widely subscribed-to theory is the Giant Impactor Hypothesis, which supposes that a protoplanet about the size of Mars struck the proto-Earth while the solar system was forming. This "splashed" a lot of magma from Earth, and some of it coalesced in orbit to form the moon. This hypothetical protoplanet has been given the name Theia, who was the mother of the moon goddess Luna in Greek mythology.
No one was around at the time, so we're not really sure, but the current hypothesis in favor is called the "Giant Impact Hypothesis." It posits that a protoplanet about the same size as the current planet Mars struck the proto-Earth while it was still forming, and "splashed" a lot of rock off. This eventually formed the Moon, while the rest of this protoplanet (named Theia) joined with the Earth.
Saturn is the outer neighbor to Jupiter. Jupiter's inner neighbor is a little trickier to answer; there are several classifications of planets, and not all of them are commonly honored. The nearest planet, of all classifications of planets (excluding asteroids), to Jupiter is Pallas, a protoplanet. The nearest planet in a step up by size is Ceres, a dwarf planet (both Ceres and Pallas, as well as Vesta, another protoplanet, orbit within the Asteroid Belt)…
The current thinking is that during the early Solar System, in what is called the Bombardment Period, a protoplanet collided with the young Earth and knocked off a big chunk (about 1/80th of the planet's mass). For a while, the debris of this collision circled Earth with Saturn-like rings, but eventually these consolidated into the Moon.
Ceres and Vesta. Ceres is a Dwarf planet (as it is just about massive enough to form an approximate spherical shape). It is around 975km in diameter. Vesta is an asteroid that is thought to be a remnant protoplanet with a mean diameter of about 530 km, although it is a lot more irregular in shape and so is classified as a 'small solar system object'.