Is a black/white dwarf (remnant of a star after it stops out emitting heat or light) much like a gasball?
The poster obviously mean “where that system doesn’t also orbit a star.”
The problem with this, of course, is that the number of bodies in the categories besides gasballs is far too large for the general public to relate to. When you’ve got “too many to list” then the term becomes meaningless to anyone but scientists.
Another solution would be to just recognize that “planet,” like “continent,” is a popular term and not even try to define it in scientific terms, because there is no non-arbitrary scientific definition. If the general public wants to call Pluto a planet, why not?
Nope. I was actually unaware of that. Obviously, I am not an astronomer. It only just recently occurred to me that such a thing could be possible, but I don’t recall hearing about one.
What they are saying is that a “xball at the center of an orbital system” can describe all planets with moons. But you mean an xball that has moons that did not form around a star. And the answer is, I don’t know of any that have been actually discovered, but don’t see why not. Brown dwarf stars can have planets, so I don’t see why a really large gas giant that isn’t quite big enough to be called a brown dwarf can’t have them.
** Steve MB** is obviously referring to Jupiter and Saturn. Each is the center of a system consisting of rings and dozens of moons.
There probably are “star systems” where the central object is a brown dwarf or gas giant, not a star. I don’t believe we’ve discovered any, but that’s just a limitation of our observing equipment.
Could there be a system similar to the solar system except, instead of having a star in the center, there is a giant hunk of rock?
After reading another thread, it occurred to me that my assumption that “Luna” is the Earth’s moon might not be correct. If so, please correct me and know that this is the (main) reason why my post above didn’t make sense.
The original definition of ‘planet’ applied to the five objects in the night sky, other than the moon,that didn’t maintain a fixed positional relationship to one another. ISTM that whatever definition of ‘planet’ we have, these five - Mercury, Venus, Mars, Jupiter, Saturn - belong in it. They were THE planets for thousands of years.
Then along came Galileo & Co., and it became clear that one big property the five planets had in common was that they orbited the sun, not the earth. And in fact the earth orbited the sun as well. So six planets. Then when Uranus and Neptune were discovered, they (a) orbited the sun, and (b) were bigger than anything else in the solar system besides Jupiter, Saturn, and the sun itself. You couldn’t exactly say they weren’t planets. Eight now.
As **Colibri **says, the definition of ‘planet’ has been tinkered with to keep the number of planets from getting large. I disagree that anything was special about 10; if there had only been four objects in the asteroid belt, kids in 1900 would have grown up with a 12-planet solar system, IMHO.
But any definition of planet that would have overwhelmed the original eight was going to be right out, so when it became clear that there were a shitload of asteroids, the handful of asteroids that had been declared planets got un-declared.
And that’s basically what happened with Pluto and the Kuiper Belt Objects. Pluto-killer Mike Brown could have been the discoverer of the tenth planet - for a while. But he realized that the same thing was likely to play out with KBOs that did with the asteroids: that if Pluto and the KBO he’d discovered were both called planets, then once again we’d likely have a shitload of planets, not just 10 or 11 or 12.
So that’s why we have eight planets. There’s the original five, plus Earth, Uranus, and Neptune, because there’s no way to keep them out without tossing out some of the original five. But you can toss out the asteroids and the KBOs without tossing out any of the eight planets.
The reason this isn’t as likely is that this system has to form out of something.
Regular solar systems form out of gas and dust clouds that are mostly hydrogen, so that’s why the Sun is mostly hydrogen.
And that’s also why there are gas giants that are also mostly hydrogen. Jupiter and Saturn very likely contain more rocks than the Earth does, but since they have so much more hydrogen they’re mostly gas. If the inner rockball planets once had that much gas that was all lost very early in the formation of the solar system. So maybe proto-Earth was once a lot more like Jupiter, but it lost almost all of that gas.
Anyway, if you try to imagine a small cloud of space dust and gas collapsing into a denser state, there’s a continuum on how much gas gets collected. You can have super-massive stars that are millions of times bigger than the sun, massive stars that are just much bigger than the sun, “regular” stars that are like the sun, and smaller stars. List of nearest stars and brown dwarfs - Wikipedia
These smaller red dwarf stars are much more common than brighter stars like the sun, the only reason we don’t see them with the naked eye when we look up at the night sky is that they’re much less bright than stars like the sun. Barnard’s Star is only 6 light years away, but it was only discovered in 1916, because it is a red dwarf and you can’t see it with the naked eye.
So then there are brown dwarfs, which are even less massive than red dwarfs. The distinction is that red dwarfs are massive enough to fuse hydrogen, while brown dwarfs aren’t. And that grades all the way down to planets like Jupiter. So there’s no reason that there couldn’t be gas giant type planets out there with smaller bodies orbiting them. And there are rocky bodies out there in interstellar space as well. The reason there’s probably not many systems of rocky bodies in interstellar space is that the gravitational forces holding the systems together wouldn’t be that strong, compared to stars, red dwarfs, brown dwarfs, and gas giants. So those rocky systems are going to be torn apart very easily, most before they even form. So the system will consolidate into one body, and the rest of the bodies will get ejected. And in any case, if these bodies formed without a central star to vaporize the volatiles, they’ll just form as ices. So any random bodies out in interstellar space are probably mostly iceballs rather than rockballs. Or, if they’re massive enough, they’ll be gasballs if the body is warm enough that the volatiles are gasses.
I didn’t mean there was something inherently special about 10, just that that’s been the de facto limit that’s been employed recently. The first 4 asteroids to be discovered were at one time classified as planets, and weren’t demoted for about 40 years until a lot more asteroids were discovered. So during that time there were in fact 12 “planets.” (I wonder what the astrologers did about it?) And the major moons of Jupiter and Saturn at one time were referred to as “secondary planets.”
A slight fact-check digression – stellar masses max out at somewhere around 150-300 solar masses, because anything bigger would generate energy so fast it would quickly blow itself apart.
That said, there are probably rocky planets out there that got ejected from their original solar systems, probably during the demolition-derby stage of solar system formation.
There are at least four planetary mass objects in interstellar space listed on Wikipedia;
PSO J318.5-22 with around 6 times Jupiter’s mass
Cha 110913-773444 with around 8 times Jupiter’s mass
and 2MASS J1119–1137, a binary system with two planets around 4 times as massive as Jupiter.
https://en.wikipedia.org/wiki/2MASS_J1119–1137
The last one at least is definitely a system of planetary mass objects orbiting each other with no star involved, assuming that the masses quoted are correct.
The number of rockballs isn’t unmanageable: I listed all of them in our Solar System, six (or maybe seven) in total. There are lots of rocks, of course, but most of them aren’t balls.
There are too many iceballs to list, but that’s going to be a problem no matter how you categorize things. It doesn’t help to say “there are lots of KBOs” or “there are lots of Plutoids” instead of “there are lots of iceballs”.
And yes, Rucksinator, by “Luna”, I mean the world where Neil Armstrong came in peace for all mankind. It orbits the Sun (albeit in an orbit which is extremely perturbed by Earth).
But can a comet have dunes?
Some scientists agree and I recall reading last year that there was a movement afoot to restore Pluto’s status as a planet. Fingers crossed!
I believe they’ve been reclassified as cull obsidian.