With the current news about the possibility of a planet in the Oort cloud, I was thinking, why is the matter in the Oort cloud distributed so? It makes sense that inside the planet region there is not a huge concentration of comets and meteors, but why is the matter inside the cloud itself not agglomerating? It seems like there is enough matter to form not one or two, but many more planets.

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    $\begingroup$ What news is that, do you have a link or keyword? The shape of the Oort cloud is inferred from the comets coming from all directions, as if from a cloud surrounding us (or like interstellar vagabonds). I guess that the reason they haven't agglomerated is that it is so empty out there. The distance between comets is of the order of the distance between planets. They don't get together very often. $\endgroup$
    – LocalFluff
    Mar 22, 2015 at 10:16
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    $\begingroup$ I've probably read about the planet elsewhere, but here is one link: astronomynow.com/2015/01/16/… It has been on the news several weeks ago. $\endgroup$
    – L.R.
    Mar 22, 2015 at 10:19
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    $\begingroup$ It probobly is agglomerating, just very slowly. Compared to the orbits of the known planets, the Oort cloud is enormous, and the matter in it is enormously spread out, so it takes much longer to agglomerate. It's also worth pointing out, if I understand the article right, that those 2 planets are only theorized at this point. We know nothing about them. They might be captured, meaning, not formed in our solar system. They might not exist at all, or, there might be more out there. $\endgroup$
    – userLTK
    Mar 23, 2015 at 12:20
  • $\begingroup$ @userLTK This should probably be an answer. $\endgroup$
    – called2voyage
    Jun 24, 2015 at 17:04
  • $\begingroup$ I'd be wary of the paper that article is based off of because of the frighteningly small sample size of just 13. $\endgroup$ Jun 24, 2015 at 23:49

1 Answer 1


Simple answer: the material is just too spread out.

Forming a planetary-size object takes LOTS of collisions to build up enough matter to start gravitationally attracting the surrounding stuff. The Oort cloud is very far from the sun (starting at ~1000 AU, compared to a maximum of ~50 AU for Pluto), meaning that the icy fragments are moving very slow in their orbits. Furthermore, the material is spread out over a huge volume, especially since the Oort cloud is a spherical shell (the matter is spread in 3 dimensions), as opposed to the 2-d disk that makes up the vast majority of matter in the Solar system (Sun, Planets, Asteroid & Kuiper Belts). The timescale for these objects to accrete to any graviationally significant size is far beyond the lifetime of the Solar system (or even the universe).

  • $\begingroup$ I will also say that a planet existing that far out from a star does not necessarily mean that it formed there - in fact, it's likely that many small planets are "ejected" from the inner solar system during giant planet formation. $\endgroup$ Jul 22, 2015 at 22:10

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