Planets are resilient things. They can survive a lot of punishment from their host stars, with some planets having survived being broiled and others even having survived being engulfed as their parent star swells into a red giant. Amazingly, the means by which planets form is no less hardy, for instance being formed in the debris left after a supernova as pulsar planets. So if the method for planet formation is so rugged, what would happen if you started out with an extremely massive star? Could the largest stars in the Universe form smaller stars as if they were planets?
It may at first seem strange to think of stars forming the way planets do. Generally, it’s believed that the star forms first and planets start to slowly assemble from the stellar leftovers afterwards. This disparity has caused problems in the past for theorists, many of whom are still a little uncertain as to whether brown dwarfs prefer to form like stars or like planets. Seemingly, the answer could be both.
Planets can actually form in one two two ways. The more traditional picture is of planets accreting from a dusty disk around a young star, slowly building up from a jumble of ice. This simply takes too long. The stellar wind would blow away the dust before any planets could start to condense. In the other picture, the star has a much thicker disk of dust around it. Tiny instabilities in the disk start do destabilise it and clumps start to form*, inexorably collapsing in on themselves, condensing into planets. Most planets formed this way would be gas giants, but some would be large enough to ignite as stars. In fact, this paper uses some rather elegant maths to find that if you consider a star 100 times as massive as the Sun, objects could form in orbit with as much as 0.3 solar masses. That’s heavy enough to be a red dwarf star. In fact, it’s three times as massive as Proxima. Even for those not attaining such a high mass, it seems quite likely that a few brown dwarfs would form.
All that said, this isn’t likely to happen too often. Stars with 100 solar masses are the rarest of the rare. Exceedingly hot O-stars which burn through all of their fuel rapidly in just a few million years. They only form in the densest interstellar clouds where they can find enough mass to collapse from. This does lend strength to the idea that these massive stars may have planets and orbiting stars forming though. Forming a star around a star would require an extremely dense, massive disk of material. Massive enough to fragment into collapsing starlets and dense enough to shield those starlets from the intense radiation from the central star. Any stars or even planets which form around such a star would form with huge orbital radii. But then, we’ve already found planets orbiting very far from their parent star, such as Fomalhaut b. Such monstrous disks of dust have also been found around some extremely massive stars.
One final thing to note is that low mass stars live for much much longer than the most massive ones. A few thousand times longer, in fact. Which means that while it’s a lovely image to imagine a hulking hypergiant with a system of stars in tow, just like Jupiter and its moons, a system like this wouldn’t last long. Quite rapidly, the hypergiant would evolve into a luminous blue variable and then a Wolf-Rayet star and start to puff off its outer layers, losing mass. Any orbiting stars would find their orbits steadily less circular and more eccentric until– BOOM! Finally, their giant parent would explode as a supernova and they’d be scattered long before they’d even finished forming. They might even start forming planets of their own.
It’s interesting to wonder how many red dwarfs and brown dwarfs might have formed this way, before being kicked out into the dark. Or even planets. Interstellar planets, too small for us to ever even know they exist, rich in heavy elements from supernova condensates.
* This always happens in my saucepan when I make bechamel sauce…
Image: The Carina Nebula. ESO/IDA/Danish 1.5 m – R.Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron.
Amit Kashi, & Noam Soker (2010). Forming Low Mass Stars and Brown Dwarfs in Protoplanetary Disks of Very Massive Stars preprint arXiv: 1002.4693v1