Modern views on the origin of the Solar System
The nebular hypothesis was proposed by Kant and Laplace in the 18th century; in the 20th century O. Schmidt (the idea that planets gradually gathered from cold solid particles) and V. Fesenkov put forward hypotheses of their own. In the modern view, ≈ 4.57 billion years ago a fragment of a cold cloud of dust and gas contracted under its own gravity; by conservation of angular momentum it spun faster and flattened into a disc: a proto-Sun at the centre and a protoplanetary disc around it. Dust in the disc clumped into planetesimals (km-sized bodies), which merged into planets. Near the Sun, where it was hot, only rock and metal were solid, so small terrestrial planets formed; beyond the frost line (≈ 3–5 AU) ice was solid too, so cores grew large and pulled in gas, forming the giants. Leftovers are asteroids and comets. Evidence: the planets’ orbits lie in one plane and go one way; meteorites and the Earth are ≈ 4.5 billion years old (radioactive dating); protoplanetary discs around other stars (for example ALMA images) have been imaged. Exoplanets (planets of other stars) are found by the transit method: when a planet crosses in front of its star the light dims by a fraction (Rp : Rs)². The first, 51 Pegasi b, was found in 1995; after the Kepler (2009) and TESS (2018) telescopes the number of confirmed exoplanets has passed 6000.
Sitting on a swivel chair (slowly, with an adult watching), spread and pull in your arms and notice how the spin changes – the conservation of angular momentum.