How the Heck is a Gas Planet Formed?

Originally worded "do we have any idea how Jupiter was formed?" Carlos expanded on this question in a private discussion over at G+, and I thought that his later form of the question made for a better post title!

Jupiter was formed in much the same way that all the other planets were, so here's a whistlestop tour of the beginnings of the solar system:

We started out as an enormous cloud of gas and dust which started to collapse, rotating as it did so (for much the same reasons as the water in your sink rotates around a plughole rather than going straight down it). Most of the gas and dust in this disk headed for the centre, collecting together and forming the protosun*.

The rest - a tiny percentage of the original cloud, but still a fair amount by any human standards - went into forming the rest of the objects in the solar system, the largest of which are the planets. These all began as specks of dust and ice colliding with, and sticking to, other specks of ice and dust. Collision after collision raged in the spinning disk of stuff, and specks became lumps, which collided further and became... even bigger lumps.

Eight of these lumps began to dominate their respective orbits, hoovering up smaller lumps and building themselves up to the kind of lumps you just don't argue with because they have their own noticeable gravity wells. These gravity wells attracted even more lumps of dust and ice, and even gained the attention of the gases mixed up in the protoplanetary** disk. These gases started to accrete around the lumps, now large enough to be pulled into a spherical shape by their own gravity, with the gases forming an atmosphere.

The fifth lump from the Sun managed to gather together more of the ice and dust than the third one did - about fifteen times as much stuff, by current estimates - and therefore got a more solid gravitational grip on the gases in its region of the disk.

Once the protosun stopped being a protosun and became an actual, burning fiery sun*** the temperature in the inner solar system rose dramatically. There's a kind of fuzzy, intangible temperature-barrier about as far out as the asteroid belt. Inside this barrier volatile compounds such as water, carbon dioxide, ammonia and methane tend to evaporate, and the inner planets struggled to hold on to a lot of it (although they managed to keep some of it, given that we have all of these things in our own atmosphere****!) Outside this region things are cooler, so there's less evaporation going on.

Over time, the solar nebula (the gassy remnants of the protoplanetary disk) dissipated, partly being driven away by the solar wind. Jupiter, with its greater mass and distance from the source of the wind, would have lost less of its atmosphere in this way, and has kept much of it until today.

Jupiter's atmosphere has a very similar composition to that of the Sun, and if it had a bit more mass***** it'd be more like the Sun in a big, bright shiny way, too: we'd live in a binary star system.

* 'Proto' is from ancient Greek, meaning 'first'. 'Protosun' just labels the ball of gas and dust that was to become the Sun, before it did so.
** There's that 'proto' again.
*** I tried to find a post in which I'd described this process, but I don't appear to have done one. Plenty of people have asked about what happens when stars die, but evidently no-one's asked how they get started in the first place!
**** Well, not too much ammonia. Thankfully.
***** About a hundred times more. I didn't want to put that in the main body as I think it makes things feel a bit less exciting.


  1. Just for the sake of argument: can we hypothesise that if Jupiter, Saturn, Uranus and Neptune were closer, under the influence of the Sun's "intangible temperature-barrier", as you put it, they could well be "naked" planets, like Mercury, Venus, Earth and Mars?

  2. If they formed closer then I'd be fairly confident that they'd have much smaller atmospheres.


    Early extraosolar planet detection found only planets known as 'hot Jupiters' - gas giants orbiting close to their parent star. There was some initial worry that our solar system was a bit odd, and that we'd have to rethink our theories behind star system formation. Even now, a large fraction of the planets we've found outside our solar system are hot Jupiters, but we're not so worried that we've got stuff wrong. I'll leave it to the reader to hypothesise why...


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