IIRC Saturn has very low density, was it lower than water? (IOW it would float) so this would be even lighter.
Every time I read that something should not exist based on current understanding or theories, especially in the field of astronomy, I cry a bit.
I am skeptical that “planet formation” is a type of subject where we could entertain any theories at all, on the human timescales we can handle.
mynegation•8mo ago
sailingparrot•8mo ago
To get a gas giant, you first need the formation of a "regular" planet through accretion of material in orbit. Once that regular planet is big enough, by capturing enough material, its' strong gravity allows it to start pulling in more and more gas, creating a gas giant.
It was believed that small stars can't possibly host those kind of gas giant, because small stars don't have enough material orbiting around them to create a planet big enough start the runaway process of gas accumulation needed to form a gas giant, because if there was enough material, the star would not have been small in the first place.
pavel_lishin•8mo ago
That's just one theory, right? There are competing hypotheses where they form much like stars do, simply by enough gas coalescing to do the same job that the "regular" planet "seed" would.
scotty79•8mo ago
sailingparrot•8mo ago
There could be a scenario where the second fragment is just not massive enough to achieve fusion, which is what you are alluding to, in that case the second fragment could give birth to a sub-brown dwarf [1].
A gas giant has a different composition from a sub-brown dwarf though, because they form differently. A gas giant is formed after the star, not at the same time. First the star forms, and the leftover material around the star starts clumping together in orbit around it, this allows the formation of a rocky core and at some point if the core gets big enough it starts the runaway process of pulling more and more gases, creating a gas giant. So a gas giant would have a higher density than a star of the same radius, because the massive rocky core is there. This was thought not to be a possible scenario around a small star, as we didn't expect a small star to have enough material left in it's orbit to allow the creation of a core big enough to start the runaway gas accumulation necessary for a gas giant.
Here, based on the mass/radius/temperature of the object, they were able to infer that it must have a core of roughly 12±2 Earth masses of dense material. Hence it's a gas giant (formed around a core, and thus after the star formation) and not a sub-brown dwarf in a binary system (formed at the same time as the star).
Obligatory disclaimer that I'm not an astronomer, just a hobbyist in the field.
[1]: https://en.wikipedia.org/wiki/Sub-brown_dwarf
scotty79•8mo ago