If Earth were replaced with another planet in our solar system, the planet closest to the Sun would be Mercury.
The reason for this is based on the concept of the order of planets from the Sun, known as the "heliocentric model." In our solar system, the planets orbit the Sun in nearly circular paths, with the Sun at the center. These orbits are arranged in a specific order, with the innermost planet being Mercury, followed by Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Mercury's proximity to the Sun is primarily due to the process of planetary formation and the gravitational forces at work. When the solar system was forming about 4.6 billion years ago, a protoplanetary disk of gas and dust surrounded the young Sun. Within this disk, solid particles began to accumulate and collide, gradually forming larger bodies known as planetesimals. Through further collisions and gravitational interactions, these planetesimals eventually coalesced into the planets we know today.
Mercury happened to form closer to the Sun than the other planets because the protoplanetary disk was denser and hotter near the Sun. This higher density and temperature allowed more solid material to come together, resulting in the formation of a smaller and rocky planet like Mercury. As the protoplanetary disk dissipated over time, the remaining material available for planet formation became less abundant farther from the Sun, leading to the formation of larger planets like Jupiter and Saturn in the outer regions of the solar system.
Thus, if Earth were replaced with another planet, the new planet would follow the heliocentric model and be closer to the Sun, with Mercury remaining as the closest planet to our star.