Mercury Venus earth and mars were formed by what?

Answer 1

The combining of asteroid-sized debris

The earth and other inner planets (terrestrial) were formed by what's know as "The Big Bang Theory" which occurred approximately 4.5 billion years ago. The inner planets (Mercury, Venus, Mars, and Earth) were formed from a huge inner stellar cloud of gas and dust, known as the solar nebula. Matter in the cloud such as oxygen, carbon, iron, and nitrogen became condensed and clumped together. A massive shock wave from a nearby supernova or an interstellar wind of a supergiant star jump started the process and compressing the matter. Once the cloud collapsed, it began to rotate. In short, rocky dust and gravel made of heavier elements collided and stuck together in much larger clumps, known as planetesimals. They grew rapidly and became larger bodies known as protoplanets. Once the nuclear fusion came from the protosun, the resulting blast of solar wind and radiation swept through solar system and began to heat the surfaces of the inner planets. That is the short version of how the inner planets were formed.

Answer 2

the combining of asteroid-sized debris