# What precisely leads to planets like COCONUTS-2b to orbit so far away from their host stars, 6000 AU in its case?

Taking our Solar System as an example, most gas giants formed relatively close by (a few AU) and drifted away to reasons I don't know, from an explanation I recall reading. Simply orbiting a few AU away leaves Jupiter very cold. From what I've read, simply observing the bodies orbiting the Sun a few 40 or so AU away reveals they are very sparse in number, and small dwarf planets at best.

This too, is for a fairly large sun like ours, which I assume would have had a much larger protoplanetary disc compared to your average red dwarf star, which COCONUTS-2a is.

How would a planet, that too a gas giant of all things, find itself orbiting a very small star which would have a weaker gravitational pull compared to a Sun-like star like ours at a ludicrous distance of 6000 AU?

Would the gravitational pull not be very weak there for a star like COCONUTS-2a? Not to mention, how would a planet like 2b make it there? Drifting a few AU away is understandable and probable, but assuming it formed an AU at best from 2a, how would it end up at 6000 AU?

• It's 2b, not 2B. The difference is crucial and actually maybe it should be 2B indicating a binary companion. The accuracy with which the mass is known is questionable. This could be a binary system of an M dwarf and a low-mass brown dwarf. Sep 29, 2021 at 17:29
• @ProfRob Even for a brown dwraf wouldn't orbiting each other at a such a huge distance from each other be very peculiar ?
– Hash
Sep 29, 2021 at 17:50
• Not especially. Most M dwarf binaries have separations of 10-1000 au, but wider examples exist. I'm just saying that the mass and it's identification as an "exoplanet" are rather uncertain. As such it can form just like wide binaries do, by fragmentation of a collapsing cloud with angular momentum. In fact this is a more obvious explanation than the alternative IMHO. Sep 29, 2021 at 18:45

2B or not 2b? That is the question.

The published paper - Zhang et al. (2021) - defines COCONUTS 2b as an exoplanet based upon the mass-ratio of 2b/2A, which is of order 0.02. I think this is a bit arbitrary and it just looks like a wide, low-mass binary system, with a secondary that is a low-mass brown dwarf ($$\sim 10 M_{\rm Jupiter}$$).

As the authors say, it is very unlikely that this object formed close to its primary star and was then ejected, because its current orbital binding energy is so small that it would imply very fine tuning of the ejection mechanism to have it now orbiting at 6500 au rather than being ejected entirely.

As for the origins, the authors say

However, given its wide orbital separation, COCONUTS-2b probably formed in situ, like components in stellar binaries via the gravitational collapse of molecular cloud [sic].

i.e. It forms from the fragmentation of a collapsing molecular cloud, just like other binary systems, not like the planets in our own Solar System and therefore should perhaps be COCONUTS 2B.

There are two major theories for the formation of wide-orbit exoplanets. This is discussed here: https://www.exoplanets.ed.ac.uk/news/formation-of-planets-on-wide-orbits

The first major theory is called GI (Gravitational Instability). The theory is that a protoplanetary disk could fragment. Then the fragment could coalesce separately through gravitational collapse into a large gas giant planet or even a smaller star. These are often also called "cold-start" planets.

The second major theory is CA (Core Accretion) in which the planets form through standard accretion methods and then get a gravitational kick into a wider orbit. Some models of our Solar System indicate there once might have been another gas giant planet that was fully ejected through this mechanism: Could an ejected "extra ice-giant" still be lurking in distant solar orbit?

For the planet COCONUTS-2b, it seems more likely that it formed through Gravitational Instability since it is so far away from its host star and so large and also cold. However, we may never know what the mechanism was. A much more unlikely scenario is an orbital disruption from another passing star.

• I don't think either of these are runners for a separation of 6000 au. Sep 29, 2021 at 18:52
• @ProfRob If gas giants can be ejected from a young solar system, couldn't large separations also be possible? Sep 29, 2021 at 19:43
• AFAIK it is a "Neptune" that may have been ejected from the Solar System, not something that is 6 Jupiter-masses. What would eject such an object? A close encounter with another star perhaps. Is it really a "planet" or just a low-mass binary companion? The key thing is it can't have formed in situ via either of the methods described. Sep 29, 2021 at 21:29

Almost commented, but got too long... this is an anecdotal/layman type answer to attempt to compliment the other specific, informational(good), answers.

So just on a really basic note; if there aren't any other large objects nearby/between them, what else would you expect to happen?

Not to oversimplify but isn't this just a 'matter' of relativity.
We expect what we have because we know it best/it makes sense relative to our local conditions. I think this orbit only seems odd because we're a narcissistic species. Even if what we've currently observed in our local cluster makes this odd, in the expanse of the universe I doubt we could say which is more common, something like our solar system or a system like the described. (would love to see links to prove me wrong!)

Total speculation, but to answer your question specifically, because the density of the matter in that portion of space was precisely, at an atomic level, what was needed to create that orbit in that system. I know technically you can say that about well... everything. But you get what I mean. I don't think there has to be some special mechanism that left these bodies in perfect balance - except the results of the creation of the universe (big bang, or whatever your religion).

P.S. on re-reading my own answer and ProfRobs, they're more similar than I intended - but going to leave it as a conversational style answer.