The idea that planets grew from a spinning cloud began with a philosopher
Long before telescopes could see a single planet around another star, the philosopher Immanuel Kant argued that the Solar System condensed from a slowly turning cloud of gas. His guess, refined over two and a half centuries, is now the standard account of how planets form, though one crucial step remains unexplained.
The Swedish thinker Emanuel Swedenborg sketched parts of the idea in 1734. Kant, who knew his work, went further in 1755, proposing that gaseous clouds rotate, collapse under gravity and flatten until they produce stars and planets. In 1796 Pierre-Simon Laplace reached a similar picture on his own: a hot, contracting cloud around the young Sun that spun faster as it shrank and threw off rings of gas that became planets.
Laplace's version ran into trouble, and the 20th century produced a parade of rivals, including planetesimal, tidal, accretion and capture theories. A famous objection supposedly raised by James Clerk Maxwell, that rings spinning at different speeds could not clump, turns out to have been misattributed, an error traced to popular writing by George Gamow. The modern form, the solar nebular disk model, is usually credited to the Soviet astronomer Victor Safronov, whose 1969 book set out most of the key problems and many of their answers.
In today's telling, stars are born inside giant clouds of cold hydrogen, some around 300,000 times the Sun's mass, which fragment into dense collapsing cores. Every newborn star ends up surrounded by a disk of gas and dust; around a million years in, perhaps all young stars have one. As the disk cools, grains of rock and ice form, and once bodies reach about a kilometre across, gravity takes over and they snowball quickly into objects the size of the Moon or Mars. Giant planets form farther out, beyond the frost line, where ice makes the building blocks heavier, and must grab their gas before the disk vanishes.
The biggest puzzle sits in the middle: nobody knows how centimetre-sized pebbles stick together to make kilometre-sized bodies. Solving it may explain why some stars have planets while others have nothing at all, not even a belt of dust.
Source: Nebular hypothesis
