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The Fate of Exomoons when Planets Scatter
Planet interactions are thought to be common as solar systems are first forming and settling down. A new study suggests that these close encounters could have a significant impact on the moons of giant exoplanets — and they may generate a large population of free-floating exomoons.
[...] Led by Yu-Cian Hong (Cornell University), a team of scientists has now explored the fate of exomoons in planet–planet scattering situations using a suite of N-body numerical simulations. Hong and collaborators find that the vast majority — roughly 80 to 90% — of exomoons around giant planets are destabilized during scattering and don't survive in their original place in the solar system. Fates of these destabilized exomoons include:
- moon collision with the star or a planet,
- moon capture by the perturbing planet,
- moon ejection from the solar system,
- ejection of the entire planet–moon system from the solar system, and
- moon perturbation onto a new heliocentric orbit as a "planet".
[...] An intriguing consequence of Hong and collaborators' results is the prediction of a population of free-floating exomoons that were ejected from solar systems during planet–planet scattering and now wander through the universe alone. According to the authors' models, there may be as many of these free-floating exomoons as there are stars in the universe!
There are no confirmed exomoons yet. Rogue planets may have their own satellites as well.
Innocent Bystanders: Orbital Dynamics of Exomoons During Planet–Planet Scattering (DOI: 10.3847/1538-4357/aaa0db) (DX)
(Score: 3, Interesting) by Grishnakh on Wednesday March 14 2018, @01:11AM (6 children)
Even though they've revised the definition of "planet" in recent years (for good reason, we'd have way too many to memorize if they didn't), I'm pretty sure the definition of a "moon" is a body that orbits a planet. Well, if a moon gets ejected from a star system and is now just a ball of rock floating, alone, through the void of space, then it isn't a moon any more, it's a rogue planet or an asteroid, depending on its size I would guess. After all, if you're flying your starship through interstellar space and you come across one of these, you're not going to know its history going back billions of years, so how are you supposed to know it's an "exomoon" instead of a "rogue planet"?
Similarly, if scenario 5 happens, and the moon gets perturbed into a new heliocentric orbit, it isn't a "planet" (using the scare quotes from the summary), it's a planet, or if it's a little too small, it's a dwarf planet.
This isn't like getting divorced, where you're forever stuck with having to check the "divorced" box instead of the "single" box.
(Score: 2) by HiThere on Wednesday March 14 2018, @05:52AM (5 children)
How can it clear it's orbit if it doesn't have an orbit? So it can't be a planet, no matter how big it is, by the current definition. You've got to have an orbit before you can clear your orbit.
So what do you call a wandering rock the size of the Earth? It can't be a planet, and it's sure not a star, so I guess it has to be an asteroid.
Sorry, I really thing the current definition needs a lot of adjustments. I call those things wandering planets, moons, or asteroids basically depending on how big (massive) they are. If it's larger than Luna, I call it a planet. Otherwise if it's larger than Ceres I call it a moon. Otherwise I call it an asteroid. This terminology can't really be justified, as that would mean if, say, Europa had been ejected I'd be calling it a planet, and lots of moons are smaller than Ceres. But it's better than using the same name for all of them...though if we were to do so, the correct name to use would be planet, which is from the Greek for wanderer.
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(Score: 2) by Grishnakh on Wednesday March 14 2018, @02:36PM (2 children)
How can it clear it's orbit if it doesn't have an orbit? So it can't be a planet, no matter how big it is, by the current definition. You've got to have an orbit before you can clear your orbit.
Wrong. A "dwarf planet" doesn't clear its orbit, by definition. It's not a full-fledged planet, so that's why they created the "dwarf planet" designation to distinguish it. Similarly, a "rogue planet" can fail to satisfy the criteria for full-fledged planets (namely, orbiting a star). We already have a precedent for making a class of celestial bodies that aren't really true planets, but still have that name, by adding a modifier word in front, and "rogue planet" just follows that precedent. It's much like how we have the word "car", and then the term "toy car", or the word "diamond" and the term "faux diamond".
Sorry, I really thing the current definition needs a lot of adjustments. I call those things wandering planets,
So you don't like the term "rogue planet", but you're ok with "wandering planet"??? Now you're not even being consistent. rogue == wandering.
If it's larger than Luna, I call it a planet. Otherwise if it's larger than Ceres I call it a moon. Otherwise I call it an asteroid. This terminology can't really be justified
This is, more or less, what they're already doing. Something the size of Apophis is just an asteroid, whereas something the size of Saturn is a rogue planet. There probably isn't any black-and-white delineation between these extremes though, since we've never actually seen a rogue planet, we're only theorizing their existence at this point. My whole point earlier is that the term "moon" loses any meaning outside a star system: a moon can be the size of Earth, if it's orbiting a gas giant, or it can be the size of an asteroid, as we see right now with the moons of Mars, and a (large enough) moon can become a planet just by being moved so it orbits a star instead of a planet (which would take a ridiculous amount of energy, but still). Now of course, this is just my opinion, just as the whole flap over planets vs. dwarf planets was all based on opinion, but I just don't really see the point in using the term "rogue moon"; if it's big enough to have hydrostatic equilibrium (it's round), it's a rogue planet, if it's small, it's an asteroid. That seems simpler and easier to me.
though if we were to do so, the correct name to use would be planet, which is from the Greek for wanderer.
That sounds nice, but the problem is we've already redefined planet from "wanderer" to something different. This is the problem with words: they can, over time, change their meanings, and you have to go with what people are already using, you can't just arbitrarily change their definitions sweepingly to be more logical, though you can make adjustments as our knowledge of the universe increases.
(Score: 2) by HiThere on Wednesday March 14 2018, @11:13PM (1 child)
"rogue planet" would be ok, if they revise the definition of planet so it doesn't say "clears its orbit" ... which even Jupiter doesn't do. (See Trojan Asteroids, Lagrange, etc.)
If they are willing to attach a modifier to all technical usages, say "major planet", "rocky planet", etc. then I'm willing to accept "rogue planet" rather than "wandering planet". But the requirement that a planet "clear its orbit" is not viable. That would mean, e.g., that none of the things we normally call planets actually are planets.
P.S.: There are not only the various Lagrange positions, there are also the asteroids that have crossing orbits, etc. So even an exception for the Lagrange positions wouldn't suffice. You've basically got to redefine "clears its orbit" to only mean clears it of stuff that would never be there anyway, as you don't naturally get things into the same orbit without some sort of resonance driver.
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(Score: 2) by Grishnakh on Thursday March 15 2018, @12:17AM
"rogue planet" would be ok, if they revise the definition of planet so it doesn't say "clears its orbit"
Again, they don't need to do that: they already did that with "dwarf planet" (which everyone now knows isn't a "real" planet, because it isn't large enough to clear its orbit). As I said, it's like toy cars vs. cars. If you ask me what kind of car I have, and I say I have a Corvette, and you go "wow! I'd like to check that out!" and then I pull a Matchbox Corvette out of my pocket, you're going to be annoyed or amused, but you won't think it's a "real car", though it certainly is a "toy car", and then you're probably going to ask what kind of car I *really* have.
so it doesn't say "clears its orbit" ... which even Jupiter doesn't do. (See Trojan Asteroids, Lagrange, etc.)
This is wrong. Here's the Wikipedia page [wikipedia.org] about it. The trojans are in orbital resonance so they'll never collide with Jupiter. It's not about making sure there's no other bodies in that particular orbit, it's about making sure there's nothing nearby that's likely to collide with it. It's also not black-and-white, as it's impossible to completely clear the orbit, so they came up with some mathematical formulae to discriminate between them. The major planets score order of magnitude different by these measures than the dwarf planets, so it works out pretty well.
Of course, you're not alone in your disagreement: the guy who leads New Horizons also doesn't like the new definition (he also cites the Trojans), but he also helped come up with one of the algorithms to distinguish major and minor planets, so he also seems to recognize the need for discerning between larger planets like Jupiter and Mercury and tiny ones like Makemake and the various other TNOs and KBOs out there.
(Score: 2) by Osamabobama on Wednesday March 14 2018, @05:08PM (1 child)
It looks like you need to expand your definition of "orbit." Here's a good one from Wikipedia [wikipedia.org]:
An orbit doesn't have to be periodic; it could be parabolic or hyperbolic--one pass and it's gone. Of course, such an orbit wouldn't be cleared, so your point stands.
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(Score: 2) by HiThere on Wednesday March 14 2018, @11:16PM
I've always read of that as a geodesic rather than as an orbit. OTOH, while something following a hyperbolic path is bending under the control of a body at the focus, I can see generalizing the term orbit to include that. But not when it's past the point were the focal object is exerting dominant bending force.
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