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: 2) by PartTimeZombie on Wednesday March 14 2018, @01:07AM (4 children)
Considering things as small as 243 Ida [wikipedia.org] can have a moon, maybe moons are really common.
The problem with poor old astronomers is that the observations they need to make are of tiny things a long way away, or they need to observe stuff over really long periods.
They must all dream of a telescope outside Pluto's orbit, which I imagine would be a help.
(Score: 3, Interesting) by Grishnakh on Wednesday March 14 2018, @01:20AM (3 children)
They must all dream of a telescope outside Pluto's orbit, which I imagine would be a help.
I seriously doubt it. The distance from Earth to Pluto's orbit is a tiny fraction of the distance from Earth to the nearest star system (Alpha Centauri), and to further star systems it's insignificant I'm sure. A telescope out there isn't going to help. A radio telescope on the dark side of the Moon would be helpful, though, for getting away from our radio noise here. And of course, a telescope in space, outside of Earth's atmosphere, is a big help, which is why they built Hubble and now are working on the JWST, but sticking a space telescope farther away isn't going to do much good, unless of course you could get it *really* far away, but that isn't feasible since it'd take forever to move it there (assuming it even still works), and radio communication would be ridiculously slow to get the data from it.
What we need to do is figure out how to do FTL communications, and also how to open a wormhole and send inanimate objects through it, so we can build a fleet of space telescopes and transport them around the galaxy and then operate them remotely. Of course, if we could do that, we could just build ships and go there ourselves to take a look (and better yet, start a new colony in a far-away star system, so we can get away from the idiots here!).
(Score: 3, Interesting) by takyon on Wednesday March 14 2018, @01:41AM (2 children)
Actually, a telescope *past* Pluto can help. But it has nothing to do with Pluto and it actually needs to be over 550 AU away from the Sun:
https://en.wikipedia.org/wiki/FOCAL_(spacecraft) [wikipedia.org]
https://www.technologyreview.com/s/601331/a-space-mission-to-the-gravitational-focus-of-the-sun/ [technologyreview.com]
https://www.centauri-dreams.org/2006/08/18/the-focal-mission-to-the-suns-gravity-lens/ [centauri-dreams.org]
https://www.newyorker.com/tech/elements/the-seventy-billion-mile-telescope [newyorker.com]
Proxima Centauri is about 268,331 AU away, so 550 AU is about 0.2% of that distance. If Planet Nine exists in its hypothesized orbit, it would probably be around 700-1000 AU away. So that's two potential missions to around that distance that we may see attempted in our lifetimes. In order to get there in less than 20 years instead of 100 years, we could use newer propulsion technologies, an expendable BFR or similar rocket to achieve a lot of delta-v (and the rocket could also be refueled in orbit at least once), and a gravitational assist using the Sun instead of Jupiter.
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(Score: 3, Informative) by Grishnakh on Wednesday March 14 2018, @03:38AM (1 child)
Yeah, we talked about this FOCAL idea a couple days ago here. But I don't think that's what the OP was talking about, I think he was talking about just getting out there to get a better look at stuff outside our system in general. The problem with FOCAL is that it's only useful for looking in one very particular direction (towards the Sun, to use it as a gravitational lens). That's great and all if you just want to look at one thing (like Proxima Centauri), but it does mean you have a very limited-use telescope.
Also, according to your article at technologyreview.com, 550AU won't work anyway: the telescope has to sit at 2000AU and will have a ridiculously hard time actually locking onto something and imaging it. Honestly, I don't see how the whole thing could possibly work, after reading that article. Maybe a few centuries from now, once we have settlements elsewhere in the solar system and the technology to attempt this.
(Score: 2) by toddestan on Thursday March 15 2018, @11:46PM
One benefit of a regular telescope way out there would be to take parallax measurements to determine the distance of other objects in our galaxy with precision. Basically take a picture of something from the telescope and from the Earth, and measure how much the object shifts from the background between the two shots. You can then use your geometry skills to determine how far away the object is. This technique is already done on Earth - take two shots 6 months apart and the base of the triangle you can make is 2 AU. With a telescope that far out, the base of your triangle could be in the hundred's of AU.
(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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(Score: 3, Insightful) by PinkyGigglebrain on Wednesday March 14 2018, @05:52PM (2 children)
We used to think Interstellar space was pretty much empty, at most with some thin Hydrogen gas floating around.
Now its cluttered with rogue moons, planets, comets, asteroids, brown dwarfs, neutron stars, black holes, worm holes, and possibly the occasional alien mega-structure.
Going to make the job of navigator much more interesting for future star ship crews.
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(Score: 2) by ese002 on Wednesday March 14 2018, @07:09PM (1 child)
It still pretty much empty. These objects are very very sparsely distributed. What makes rogue moons and planets interesting is that you may encounter a sizable natural object that isn't emitting any light. In system, asteroids, planets, dwarf planets, and moons are illuminated by the sun. But in the vast spaces between stars, there is no significant light to reflect. The odds of encountering one is remote, but if you do, you won't get much warning.
(Score: 2) by PinkyGigglebrain on Thursday March 15 2018, @08:01PM
I was mostly commenting on how Humanities view of interstellar space has changed over the years. I know that the space really is mostly empty, and the odds that you will encounter something bigger that a dust grain is really, really, really, small, but it is non-zero and if precautions are not taken there will be a collision someday, and it wouldn't take much to ruin everyone's day..
I expect that before Humans actually launch a ship fast enough for it to matter that they won't have years or decades to make course corrections, they would have done something to detect the junk in the path of a fast star ship. Probably be quite the array of imaging devices and probably some kind of radar, I could see most of the ship's width might just be the dish antennae needed to detect the return signal.
I just had an interesting thought that the first non Human signal SETI picks up might be the "ping" from a star ship's navigation radar. I wonder what the blue shift of a radar pulse would be for ship moving at a good % of c? It might push the frequency outside something Earth's SETI program can currently pickup. The ship also wouldn't need to ping very often too.
Well, Humanity has plenty of time to figure all that out, if they don't screw things up and wipe out their civilization first.
"Beware those who would deny you Knowledge, For in their hearts they dream themselves your Master."