Of course in practical and exact terms this question is un-answerable. Nevertheless, the effort to approximate a reasonable answer should be both interesting and informative. For instance, consider definitions. For the question to be meaningful what is meant by “planet” and “the universe” must be clearly defined.
Let’s define the “jar” first. Some think that the universe is infinite, truly unbounded. If the universe really is infinite in the ultimate mathematical sense of the word then the number of galaxies, stars, and planets inside it would also be infinite, and the problem ends there, with a an admittedly big, although somewhat ambiguous answer. So let’s add the qualifier “observable” to limit the size of the jar to more manageably stupendous proportions.
But since when we look across vast intergalactic distances we are also looking backward across billions of years of universal history we need to address the question of timing. Adding “now” to the end of the question, as in “How many planets are in the observable universe now?” still seems ambiguous. I say this because galaxies at the extreme limit of the currently observable universe have “now” receded far beyond our ability to see them forevermore. I welcome suggestions as to how to set a reasonable temporal boundary for this question.
Then there is the endlessly debated but (to me at least) ever entertaining question of how to define the word “planet”. Do we dare include the diminutive dwarf planets? I like big numbers and this is my question, so on the grounds that a dwarf planet is still a planet I’m inclined to say yes, let’s let ‘em in. This adroitly avoids the problem of where to draw the line between “real” planets and the dwarfs. Unfortunately, we still have an even bigger problem of where or how to make the cut between dwarf planets and lesser bodies. Any suggestions?
On similar grounds I would also include starless “rouge” planets too. Does a planet stop being a planet just because it goes rouge? It wasn’t its fault. Why demote it from planet status simply because other bigger bodies bullied it about? So I’m inclined to let the rouges in too, but contrary opinions are fun to consider and so are welcome as well.
Timing comes up again, as in WHEN is a planet a planet? I’m not fond of the part of the IAU’s definition that requires planets to have cleared their orbits of other bodies. A consequence of this rule would be that “planets” could technically never collide. But early in our solar system’s history a Mars sized body named Theia collided with a Venus sized proto-Earth, producing the Earth/Moon system. Where Theia and the proto-Earth planets before the collision? I would argue that they were. Therefore in my opinion once an object is big enough to be a planet, it’s a planet, unless …
… it’s a moon, which is an object orbiting a planet. But if two objects are big enough to both be considered planets if they weren’t in a mutual orbit, are there two planets, or one? Since such a dual object can also be known as a double planet, and the “planet” part of the term is sigular, I would say one planet. (This is spliting hairs, but not planets.) You might think that such objects would be rare, and maybe they are, but in our solar system the Earth/Moon system is sometimes called a double planet, and Pluto and Claron have been as well (before the Deplanitization Decree). If the Moon was orbiting the Sun instead of the Earth it would be known as a dwarf planet. (Adding up the number of planets in the Sun’s domain will be important in this discussion, so this paragraph is not as unimportant as it may have at first seemed.)
So how do Brown Dwarfs enter in to this question? Planets have been found orbiting BDs, and these orbiters are always called planets, so BD orbiting objects in the right mass domain are in, IMO. BDs themselves should be out, because these are “failed stars”, not planets. But if a planet sized body orbits a BD that in turn orbits a star, is it a planet, or is it a moon? Any opinions?
And then there’s the question of the how or where to draw the line between a BD and a massive planet. Very recently MargaritaMc informed us of a breakthrough in distingushing bewteen the smallest stars and BDs, in this topic: http://asterisk.apod.com/viewtopic.php?f=31&t=32575 In that discussion Ann provided this comment about the division between BDs and planets which prompted me to post this question in the first place:
So you can see that this is a multi-facited question. The answer is beyond our reach, but the path toward the answer should be interesting and informative.Ann wrote:This is beginning to look like old news now, but I have been thinking about it, and it is really so interesting. In particular, the relation between mass and radius in planets and brown dwarfs is very interesting.Isn't it remarkable? Jupiter is a sort of sub-stellar/brown dwarf/gas giant standard when it comes to radius and volume. However, Jupiter appears to be larger than the smallest stars. I googled the mean radius of Jupiter (69,911±6 km), and the radius of the Sun (695,500 km), and even though the uncertainty seems to much greater when it comes to the radius of the Sun than when it comes to the radius of Jupiter, I nevertheless used these two figures to conclude that the raidus of Jupiter appears to be pretty much 10% the radius of the Sun. However,http://en.wikipedia.org/wiki/Brown_dwarf#Distinguishing_low-mass_brown_dwarfs_from_high-mass_planets wrote: A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter.
At the high end of their mass range (60–90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron-degeneracy pressure, as it is in white dwarfs; at the low end of the range (10 Jupiter masses), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. This can make distinguishing them from planets difficult.
Gas giants have some of the characteristics of brown dwarfs. For example, Jupiter and Saturn are both made primarily of hydrogen and helium, like the Sun.
Saturn is nearly as large as Jupiter, despite having only 30% the mass.So the smallest hydrogen-fusing star may have a radius 8.7% that of the Sun, but Jupiter may have a radius 10% that of the Sun. Then again, this is without a very reasonable margin of error.http://www.noao.edu/news/2013/pr1311.php wrote: Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”
It might be more correct to say that Jupiter is, for all intents and purposes, the same size as the very smallest M-type stars.
This makes me wonder how large a gas giant planet can be. Several massive gas giants have been found among exoplanets. I remember having read about hot Jupiters whose atmospheres have been puffed up from the intense radiation of their suns, so that the planets were very large. But when it comes to gas giants at greater distances from their suns, how large can they be? Is Jupiter once again a sort of gold standard of the size of small stars, brown dwarfs and gas giant planets?