Inspired by this BBC article and the corresponding journal article about the M dwarf star GJ 3512.

The Jupiter-like world is unusually large compared with its host star, contradicting a widely held idea about the way planets form.

On the other hand, binary stars are very common, comprising (according to Wikipedia) about one third of star systems.

Naively, the same processes that create binary star systems could be expected to also create star systems comprised of a single star and a Jupiter-like "failed star" - the only difference being that in the latter case the would-be second star wasn't large enough to ignite.

What is the difference between these scenarios? What makes the small-star-and-large-planet, according to the standard models of planet formation, unlikely, when binary stars are common?

(In non-technical terms if possible, please.)

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    $\begingroup$ There are several planets discovered by gravitational microlensing that have properties similar to GJ 3512 b, so it may turn out not to be particularly unusual. $\endgroup$
    – user24157
    Commented Oct 12, 2019 at 11:01

1 Answer 1


It's important to realize that binary stars form much differently than planets do. Assuming that both stars form in situ (i.e. excluding scenarios where one is captured from outside the system), there are several main ways for a binary star system to form from a molecular cloud. The most widely-accepted model at the moment is the fragmentation hypothesis, where the protostellar cloud splits during its collapse, forming two clouds which will each collapse into separate protostars.

Planets, on the other hand, form not from collapsing clouds but in protoplanetary disks around stars. The dynamics of these systems are substantially different, as there are interactions between the gas, dust, and protoplanets - perhaps a more complicated system than a collapsing gas cloud. We therefore see planets arise through different processes, such as gravitational instabilities within the disk or pebble accretion.

The paper being referenced found that the GJ 3512 system appears incompatible with the pebble accretion models the team tested - that is, rocky planetary cores being built up through collisions. In a low-mass star system, the cores should move inwards too quickly to accrete enough mass, and tweaking other initial conditions didn't help. That's what the BBC article focuses on.

What the BBC ignores is that the authors were able to reproduce the system by assuming that the planet(s) arise through gravitational instabilities within the protoplanetary disk, and this model is borne out by the simulations. It may not have been widely considered, but it still works. That's one of the key results to pay attention to.

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    $\begingroup$ Can you expand on what prevents fission or fragmentation from resulting in one (small) star and one (large) planet rather than two stars? Is it just that it isn't possible for the two new protostars to have such a high mass ratio (1:270 in this case according to the BBC) or is it more complicated than that? $\endgroup$ Commented Oct 2, 2019 at 1:48
  • $\begingroup$ @HarryJohnston As I understand it, the physics behind mass ratios is not well-understood, (though it's not a field I have any expertise in). I wish I could give you a better answer on that. $\endgroup$
    – HDE 226868
    Commented Oct 2, 2019 at 3:35

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