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There is somewhat of an abstract way of generalising the notion of planets.

Standard definition of planets is, obviously: "planets are the objects formed from the residual material surrounding a newly formed main sequence star through a complex formation process".

Let me introduce a bit more general, by no means commonly known, definition of "generalised planets as bound macroscopic objects formed from some former accretion process taking place near some gravitating body over many orbital periods".

Clearly, the latter definition is more general, as it doesn't specify the type of accretion event and the type of the central body. Now, for example, the accretion mechanism can be very different from that in protoplanetary discs: consider, for example, accretion on a compact object (a neutron star or a black hole), accrection on supermassive black hole, accretion from remnants of binary neutron stars merger, or any other possible accretion process.

As noted in some answers, some stars also form in accretion-involving processes in the vicinity of other stars. To avoid including stars into 'generalised planet' category, the definition specifies that the objects should be formed over many orbital periods, implying that they should be sufficiently bound with respect to the primary.

The question: Are there generalised planets other than planets/exoplanets?

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Question expanded and explained in a bit better detail. –  Alexey Bobrick Nov 22 '13 at 0:20
I think the logical next step is to think about the various accretion-type processes which are known to exist. Here's a starting point: accretion disks around newly forming stellar systems, accretion onto super-massive black holes at the centers of galaxies, accretion from red giants onto white dwarfs (SN Type Ia), planetary accretion (e.g. - rings of Saturn). I'm sure I left some out, but I would then ask physically speaking, which ones allow for the creation of what you're calling "generalized planets". Maybe shocked gas (from colliding galaxies) can produce generalized planets? –  astromax Nov 22 '13 at 16:22
Also, would you happen to have any sources for generalized planets? I've searched around and have only found generalized planetary formation theory (adsabs.harvard.edu/full/1967SvA....11..156B), which seems to be about the general process of planetary formation, not about generalized planets. –  astromax Nov 22 '13 at 16:25
@astromax, reviewing possible accreting systems is definitely a good place to start. One other necessary thing to do would be to formulate at least some sufficient criteria for anything to form. Shocks seem like a possible way to go, gravitational instability - perhaps another one, some coagulation+accretion - yet another one. I guess making an estimate for the conditions for each of these or some other mechanisms would definitely give some insights. I will possibly do some myself and add in here, and would also warmly welcome any input you might like to bring in. –  Alexey Bobrick Nov 22 '13 at 17:29
@astromax, as for the term "generalised planets", I took the liberty to make it up myself. I haven't happened to see its usage in any sources, but the problem of various types of instabilities operating in accretion discs and leading to formation of clumps has definitely being studied. –  Alexey Bobrick Nov 22 '13 at 17:31

4 Answers 4

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  1. If a star passes through a molecular cloud then a new disc could form providing another chance for planet formation.

    • Old pre-main-sequence Stars: Disc reformation by Bondi-Hoyle accretion. Reference 1
  2. If two stars merge then they throw off an excretion disc which could form new planets.

    • A binary merger origin for inflated hot Jupiter planets. Reference 2
  3. Do you count brown-dwarfs and sub-brown-dwarfs as stars, and their satellites as planets? Reference 3

  4. If a star has a high mass or high metal content it would burn through the main-sequence very quickly to become a white dwarf while still being within the star cluster it formed in. It then could pass through a cloud, as per item 1 on this list, and form a new disc.

  5. In a supernova some of the ejecta may fail to escape and so create a fallback disc. This could form planets around neutron stars. (and black holes)

    • A debris disk around an isolated young neutron star. Reference 5
  6. As you mention in an answer to your own question, the common envelope phase in a binary star can form a new disc. This can be extended to a multiple-star system: whenever one of the stars in the system turns into a red-giant, AGB-star, or planetary-nebula this creates a common envelope if the stars are close enough together, allowing new discs to form creating new generations of planets.

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Remember you can include hyperlinks to improve your answer using markdown, here you can find a guide explaining how to do it. –  harogaston Aug 3 at 16:15
Greatly many thanks for a very good and interesting reply! –  Alexey Bobrick Aug 4 at 12:14

As per your definition, a star formed in a binary system (hence near some gravitating body) is a generalised planet.

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Rather fair, though I would expect that binary stars form through a gravitational collapse of the common cloud of ISM, not through the accretion ensuing the the formation of one of the stars. Or would you argue that the presence of one of the stars aids the formation of the other one? –  Alexey Bobrick Nov 22 '13 at 17:36
Depending on the density of the preplanetary disc, yes. Think on Jupiter... it is almost a star (for a wide sense of 'almost'). –  Envite Nov 22 '13 at 17:41
It is really interesting, though could be a bit of a side question! Do you know if stars can form from protoplanetary discs? –  Alexey Bobrick Nov 22 '13 at 18:04
Never studied that, and never heard of in my classes about star formation, but I do not reject it, since I have never studied that they can not! If there is enough mass on an orbit of the disc, you can get a Jupiter, a superjupiter like HD 29587 B or even a brown dwarf or a true small star. –  Envite Nov 23 '13 at 0:30

The main flaw with your definition of a "generalised planet" would be the boundary between stars and planets. And, in particular, the status of brown dwarfs, that are kind of "failed stars", that could not start to burn their hydrodgen.

With this regard, the definition of a planet as it is now is probably the best one (or the less worst one), because it is then possible to draw a line between planets and stars (and in particular brown dwarfs), because a planet is then an object formed by accretion in the debris material of a forming/newly formed star, whereas a star is, in general, an independent object formed by collapse and accretion (there are now observations of forming brown dwarfs; see for example the review by Luhman (2102) or the recent paper by André et al. (2012)). So you have two types of celestrial objects, well defined by two types of formation mechanisms (core accretion for the planets vs. gravo-turbulent collapse for the stars (brown dwarfs included). Then you can also clearly distinguish between "real" free-floating planets (that are then planets expelled from their orbit and floating around in the galaxy) and brown dwarfs (that form independently, where they lay).

Bottom line: if your object, in any environment (including any type of accretion disk) is form by core accretion: it is a planet. If your object is form by gravitational collapse (with a later, unavoidable phase of accretion onto the central object): it is a star.

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Dear @MBR, I don't mind calling stars or brown dwarfs as 'generalised planets', provided that they formed due to accretion+collapse, but not to grav. collapse alone, so I don't really see what you mean by a flaw. Yet still, I am interested in this question in general accretion disc, not only in those which surround newly formed stars. For example, can bound objects form in ADs in centers of galaxies, in the disc around neutron stars, etc. So I don't yet find your answer as it is now particularly helpful. –  Alexey Bobrick Dec 12 '13 at 14:27
@AlexeyBobrick: star also forms by gravitational collapse + accretion; for low-mass stars, 90% of the mass is actually accreted... So the problem I see with your "generalised planet" is that everything would be a planet with your definition. A definition is useful if it tells me something specific about the described object; it seems to me that your "generalised planet" definition is either too broad or too vague to tell me something useful. –  MBR Dec 12 '13 at 14:37
Do you make a statement, that no low-mass main-sequence stars would form without a binary companions? Yet, as I said, I am interested in processes happening in, I stress it, general accretion discs on timescales of many orbits. Perhaps that would make a clear dichotomy from the stars. –  Alexey Bobrick Dec 12 '13 at 14:55
How did you understand that (your first sentence) with my previous comment? I never said such a thing (but we can discuss this particular point in another question if you want). I edited my answer in order to clarify things, in particular with your peculiar focus on accretion disks. –  MBR Dec 12 '13 at 15:36
"90% of the mass is actually accreted". In any case, again, I am asking about bound objects formed in any possible way (not necessarily core accretion) in any accretion discs over many orbits (to exclude stars). I am not asking, what should be called planets, whether the definition is great or poor, and I am not asking about the conditions for forming stars here. Though that is an interesting separate question. –  Alexey Bobrick Dec 12 '13 at 16:20

Will be adding some relevant information here as I find it.

This article speculates that there might be planets forming from the remnants of common envelope phase in binaries: http://arxiv.org/abs/1312.3479 . These are not normal planets, as the material is not coming from star formation.

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