The planetary discriminant is a measure of how dominant a body is within its region of the solar system. For (true) planets, it is $>10000$ and for dwarf planets it is $<1$. (See this answer to "How many planets are there in this solar system?" which explains the related details.)

Is it possible, given what has already been observed about our solar system, for there to be an object with "planetary discriminant" µ between that of Ceres and Neptune? In other words, could the clear cut line between planets and dwarf planets get fuzzier?

Obviously it would have to be a massive body pretty far out. Is there a region in the solar system which could be devoid enough of many small objects, which also could be home to a large enough body that we also haven't observed yet? How does the math work out?

  • $\begingroup$ @ThePopMachine note that there was a mistake in the data used in the answer you quote, which has since been corrected in Wikipedia. There is an object with planetary discriminant between Ceres (0.33) and Neptune (24,000) - Mars, at 5,100. This doesn't change the four-orders-of-magnitude gap in discriminant between the planets and the dwarf planets, but it does change the wording of your question. $\endgroup$ Commented Jun 22, 2016 at 19:07
  • $\begingroup$ @EmilioPisanty: Feel free to edit the question and title to make this valid. $\endgroup$ Commented Jun 23, 2016 at 0:26

2 Answers 2


I know this is an old question, but recent developments in astronomy have caused this to become relevant again. Early in 2016 there was a paper published by Konstantin Batygin and Michael E. Brown (here) which indicates the possible existence of a new planet, dubbed "Planet 9". Subsequent work by other authors have lent more evidence to its possible existence, e.g., a paper by Fienga et al. (here) which concludes that including Planet 9 in orbital dynamic models decreases residuals.

This being said, the original paper specifically states

...our calculations suggest that a perturber on a $a'\sim700\:AU$, $e'\sim0.6$ orbit would have to be somewhat more massive (e.g. a factor of a few) than $m'=10\:M_\oplus$ to produce the desired effect.

Such a planet is certainly massive enough to have cleared out its orbit under normal circumstances. For reference, Neptune has a mass of $17\:M_\oplus$ while Uranus has a mass of $14.5\:M_\oplus$. The special feature of this planet is that it is located $700\:AU$ distant from the Sun which is in a very ambiguous and unknown region of our solar system. It is not clear at all how much, if any, debris in the form of comets resides out there. There are models suggesting a disk (referred to as the Hills Cloud) exists at those distances, implying that this new planet could reside inside an extended cometary disk. Of course this is only speculation, but it is a distinct possibility.

If the above-mentioned disk exists, we have to consider a few scenarios.

  • Planet 9 was "inserted" into this disk by being expelled from the inner solar system early on in its formation. This is a possible cause for its current position and suggests one of two outcomes.
    1. It has existed here long enough that it has cleared out its orbit, thus making its planetary discriminant well within the "planetary" side of the definition.
    2. It has either not existed in the disk long enough to clear the orbit, or has achieved some sort of equilibrium such that it can't now clear the orbit (considering most of the clearing occurs during formation). Here is where you may run into an ambiguous case. For all intents and purposes we may consider this a planet, but there remains the possibility that, despite it being more massive than the Earth, it could formally be considered a dwarf planet or else exist in this fuzzy region as you suggest.
  • Planet 9 formed at its current orbit (plus or minus some migration). In this scenario I think it is clear that it would have cleared its orbit and resoundingly be considered a true planet.

Now these are all wild suppositions. I don't have any of the math to back it up, and I'm not sure the numbers to plug into the various equations are well known, or even exist. I'm merely remarking on the possibility that the situation you asked about now seems like it could exist.

  • $\begingroup$ I'm not sure that 'planet 9' could have formed at its current orbit, which is much too elliptical for stuff to accrete from a disk. It could, of course, have formed in a circular orbit of equal size, and subsequently been torqued into a more elliptical orbit, but that feels (to me) rather unlikely, as it would have been very far for stuff to really influence it. My understanding is that it's more likely to have been a closer-in formation, ejection into a very elliptical orbit, and then a slight circularization over time. $\endgroup$ Commented Jun 22, 2016 at 19:16
  • $\begingroup$ (For more details on those mechanisms, see e.g. this article.) $\endgroup$ Commented Jun 22, 2016 at 19:17
  • $\begingroup$ @EmilioPisanty I certainly agree with you. The most likely scenario is that it formed close in and got pushed farther out during early formation. But I'm trying not to discount any possibilities as we just don't have enough evidence to say one or more is definitely wrong. $\endgroup$
    – zephyr
    Commented Jun 22, 2016 at 19:23

Given the the discovery of Pluto was made based on the search for a body to explain orbits being different from that predicted by known bodies, it seems very unlikely with the even more precise and powerful calculations available today that we've missed a large body of any kind in that region, let alone a whole planet. I think we have simply amassed too much high precision data for that to be the case. Although I'd like to be wrong.

I would also suggest that the "clear cut line between planets and dwarf planets" is not at all clear cut, but a rather arbitrary line drawn by a committee. I personally consider it artificial and any decision of that sort that can demote a body the size of Pluto from planet to dwarf-planet is one I can only view with derision. The measure chosen to justify the decision seems more to do with the need to have a definition for the "rule book", rather than any scientific need. I don't think I'm the only one with that view.

However, a chunk of rock ( or ice ) in that region large enough to be a planet by any reasonable common sense view, much less the IAU definition, seems unlikely to have gone undiscovered to date. It's not as if we're not looking.

  • 1
    $\begingroup$ First of all, this appears to be partly an anti-dwarf planet rant. Second of all, the line is not arbitrary because of the four orders of magnitude difference. The question I'm asking is precisely whether the line could become more arbitrary with a new discovery. You can't answer that question by just rejecting the premise because you dislike the outcome of the deplanetization of Pluto $\endgroup$ Commented Apr 1, 2016 at 20:21
  • $\begingroup$ This answer is indeed more of a rant than anything else, but moreover it has been disproven by events since its posting. It is indeed possible that there is a ninth planet, even in the full IAU sense, much further out than Neptune, and there is some theoretical evidence for it. It's not as if we're not looking - but we simply haven't looked hard enough to rule out such bodies. For more details see e.g. Emily Lakdawalla on the subject. $\endgroup$ Commented Jun 22, 2016 at 19:12

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