The nebular hypothesis states that the Solar System had condensed out of an interstellar nebula that had collapsed, with the planets condensing in orbit around the Sun. Originally proposed by Kant and Laplace in the 18th century, it became generally accepted in the 19th century, though a stellar-collision model became popular in the early 20th century. It was revived in the mid-20th century, and it has become generally accepted. Naturally this condensation happened over a very long period of time and is inconsistent with Young Earth Creationism.
There are several observations that are most consistent with the nebular hypothesis.
- Observations of protoplanetary nebulae around Stars and protostars.
- Chemical-composition gradients in the Solar System and in Jupiter's Galilean satellites.
- Inner ones: refractory materials like iron-nickel and metal silicates, what most rocks and minerals are made of.
- Outer ones: volatile materials like water, ammonia, and methane ices, and for the outer planets, hydrogen and helium.
- On objects with low geological activity like the Moon and Mars, cratering greatly in excess of extrapolation from present-day cratering rates. This is a consequence of leftover protoplanetary material being swept up.
However, there are complications like the numerous "hot Jupiters" that have been observed around other stars, some of which have highly-tilted and highly-eccentric orbits. This suggests that giant planets can spiral inward from their formation orbits and have near-collisions with each other as they do so.