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Based on recent developments in astronomy, pertaining to a greater degree of measurement, the mass of the Oort cloud around Sol is apparently only about 1.5 times the mass of earth, in contrast to over a thousand times, roughly, as previous thought, which got me wondering, if there might be a scenario where an Oort cloud would have substantial mass, and moreover, would consist of planets—full-on planets—rather than ours which consists of a handful of planetoids and a whole lot of comets.

I know the field (of astronomy) is currently evolving, and there's a great deal of unknown aspects regarding the formation of solar systems but one idea I do have is, planetoids drifting through an outer system and being captured upon the stars initial formation, with an outer system with a rather high density of interstellar gas, and so encouraging the formation of said planets.

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  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Apr 9, 2023 at 15:24
  • $\begingroup$ The gravitational interactions that kick some planetesimals out to very large distances, populating the Oort Cloud, shift the orbits of planets by very tiny amounts. So you simply can't produce an Oort Cloud of "full on planets". $\endgroup$ Apr 9, 2023 at 19:30
  • $\begingroup$ At the distance of the Oort cloud the sun's gravity is so weak, that if you'd put a cloud of Jovians there, their mutual scattering perturbations would quickly depopulate the cloud. $\endgroup$ Apr 11, 2023 at 12:13
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    $\begingroup$ Article on oort cloud exoplanets arxiv.org/abs/2306.11109 $\endgroup$
    – sno
    Feb 11 at 0:41

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Just to clarify - the solid materials that make up the inferred Oort cloud were not formed there. They were formed much closer to the star (or another star - see below) and then thrown outwards by interactions with other bodies. Only some fraction of the material that is thrown outwards remains part of the Solar System; much of it will escape.

One of the main theories to explain the structure and dynamics of our Solar System is the "Nice model", which has a variant in which an ice-giant planet, similar to Neptune, is ejected outwards after an encounter with Jupiter. If the ejection has a low enough speed, the planet may stay in the Oort cloud and indeed, this is a possible explanation for the presence of a planet 9 out there somewhere.

I don't think there is any scenario where a cloud of planetary-sized objects forms an Oort cloud. During star formation, the circumstellar disk forming the solids and from which gas can be accreted onto planets is in the inner regions. A recent study (thanks sno) shows that even throwing out a single planet, such that it remains in the Oort cloud is an unlikely event (see also Portegies Zwart 2021). Capture of the odd planet or two is also possible, but unlikely, although some fraction of the current Oort cloud may be in the form of captured objects Portegies Zwart et al. 2021).

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