A rogue planet (or a rogue anything, for that matter: a celestial body other than a star) is something that's drifting through space without being attached by gravity to any star. They're just out there in the big black void. Is there any evidence that they actually exist? Or are they purely hypothetical?
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3Related: http://astronomy.stackexchange.com/questions/1715/how-are-rogue-planets-discovered – userLTK Nov 18 '15 at 10:12
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@userLTK: Barely. The OP actually asks how rogue planets are discovered; the answers treat of discovery methods, all hypothetical, and refer to some "claims." In other words, we're dealing with pure fantasy and wishful thinking. My question is more to the point: hard evidence. Does it exist or doesn't it? If the former is true, I could use some links. If not, a simple "no" would suffice. – Ricky Nov 18 '15 at 10:27
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2Why don't you follow the several links that are in my answer to that question and read what the evidence is? If you look at the more recent papers you will find references to other studies too. So, yes there is "hard evidence" for low-mass objects (much lower in mass than the minimum mass of a star) that are free-floating in the sense of not being bound to another star. Whether they could be called "rogue planets" depends on your definition of a "planet". – ProfRob Nov 18 '15 at 16:54
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2What is your problem? You have come here, seeking an answer. You've asked a question that has already been answered. I seek clarification about what it is about that answer you don't understand. If you already (think you) know the answer to your question, why bother to ask it. Indeed, why come to Astronomy SE to ask questions that you don't want an astrophysicist or astronomer to answer? I'm genuinely mystified. – ProfRob Nov 18 '15 at 21:18
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@RobJeffries: See what I mean? )) My question had not been answered when I posed it. I have my answer now. I always seek a neutral party's opinion first. If I asked an amateur of music which composers inspired verismo opera, they'd probably say 70% Wagner and 30% Verdi. A professional musician would probably explain why my question was wrong; and then divest himself of a long list of references discussing Bach's fugues, Berlioz's orchestration, Boito's Wagnerism, Giordano's "true verismo" (as opposed to Puccini's less-than-true verismo), etc, and never get around to mentioning Verdi. – Ricky Nov 18 '15 at 23:43
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@Ricky If your question hasn't been answered, rather than compare posters to the inquisition, it would be more effective to isolate what hasn't been answered and re-raise the question, either in a comment, perhaps an edit to clarify the question , or, if sufficiently different, a new question. – userLTK Nov 19 '15 at 00:07
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It occurred to me, after posting my brief answer, you might have been asking whether rogue planets are re-captured by a different solar-system, essentially not staying rogue planets for very long. That depends, essentially on relative velocity between the Rogue and Solar-System it passes close to vs the escape velocity at that distance. Mostly, at distances you find in deep space, escape velocity to the nearest star is much lower than relative velocity, so most rogue planets don't get immediately re-captured by another solar-system. I can try to run some numbers if you like. – userLTK Nov 19 '15 at 00:09
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@userLTK: Actually, I'm curious to know whether a celestial body with the mass of Jupiter or lower can even exist in interstellar space. Maybe they're all ground to dust by dark matter or something. How would I know. – Ricky Nov 19 '15 at 00:16
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@Ricky at risk of giving you an answer you won't like, yes it's possible. There's likely a gravitational minimum beyond which gas won't coalesce but it's smaller than Jupiter. Gas like hydrogen and helium tends to bounce off each other, while heavier gases tend to form ice in space and can stick together. You probably need roughly an earth mass object - give or take, to begin coalescing hydrogen and helium, the two most abundant elements and begin to form a Jupiter like object. Jupiter probably formed in a similar way, starting out with an Earth or a few Earth's of solid mass. – userLTK Nov 19 '15 at 00:26
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@userLTK: Thank you for that. I'm not looking for likable answers. I'll accept anything that makes scientific sense. Your suggestion does, so, thank you again. – Ricky Nov 19 '15 at 00:29
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Gas clouds small enough to only form into Jupiter mass objects might be rare though. Brown dwarf stars are less common than Red Dwarfs and that ratio may continue to shrink as you go smaller. Simply put, more mass is more likely to gravitationally coalesce, less mass has less attraction, so it's harder to happen. There may be a relative limit to that kind of formation of gas and dust in deep space into a new mini-solar-system. I realize I'm giving two contradictory answers, but there's aspects of truth to both. – userLTK Nov 19 '15 at 02:13
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Rogue planets have been discovered by infra-red imaging because planets are hot when they form. Here's a list of a few.
There's 2 types of Rogue planets. One is failed stars. condensing pockets of gas and dust that form similar to how our solar-system formed, but that are too small to form stars. Source.
The 2nd type is planets that escape from a star's orbit. This can happen by the star ejecting material and losing mass which causes the planets to expand in more distant orbits, some eventually escaping, or by gravitational assist either planet on planet or by two stars passing quite close to each other. It's statistically impossible for Rogue Planets not to, from time to time, get ejected from solar systems, so they have to exist at least in reasonably large numbers throughout the galaxy, though I don't think it's well known how common there are.
Our solar-system might have ejected a planet - see here. In general, larger planets can eject smaller ones but mostly not the other way around, but two stars that pass too close to each other can eject any planets, mostly ones with more distant orbits. A star that loses a lot of it's mass or a star that goes nova can push planets out of it's orbit too.
userLTK
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Thank you. Great stuff! Let me just make sure. 1. Type One, failed stars: at least one has been confirmed. 2. The existence of Type Two is hypothetical; it is probable according to today's paradigm, but none have been discovered. 3. Some calculations point to the ejection of a gas giant from the Solar System at some point in the past, but those are not as solid as, say, the calculations that suggested the positions of Neptune and Plato prior to their discovery. Does that sound about right? – Ricky Nov 18 '15 at 11:11
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@Ricky Sedna might be a captured rouge planet of the second type, formed by one star but ejected to another early on as several stars formed simultaneously in a crowd out of the same nebula. If so, it would be the first rouge (or rather now adopted) planet directly observed. Says something about how common they should be, I just don't know what. – LocalFluff Nov 18 '15 at 11:46
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@LocalFluff: Well, it does orbit the Sun, so it kind of doesn't count. Or does it? – Ricky Nov 18 '15 at 11:53
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@Ricky If it was an ejected planet, it would have been a rogue in the time prior to its capture by the Sun so, yes, it does count as evidence that rogue planets of the second type exist given that it is indeed an example of an ejected planet. – called2voyage Nov 18 '15 at 15:45
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@called2voyage: We can only assume it was captured by the Sun. Assumption isn't evidence. Not to mention that even if rogue planets did exist at some point in the past, it doesn't necessarily follow that they exist today except in the very special universe created by script writers to gratify the expectations of Star Trek fans. I kind of liked certain Voyager episodes, but the final series was downright boring. – Ricky Nov 18 '15 at 15:56
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@Ricky You are correct, that is why I said if it is an ejected planet. – called2voyage Nov 18 '15 at 15:57
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2@Ricky given the nature of 3 body or N greater than 2 body gravitational systems, saying rogue planets "hypothetically exist" is like saying some workers at a large company "hypothetically are looking for better jobs" the statistics make it impossible for ejected Rogue planets not to exist, even if none have been definitively observed. – userLTK Nov 18 '15 at 21:00
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Be careful here. Brown dwarfs - "Failed stars" - are not rogue planets. They are an entirely different class of objects. – HDE 226868 Nov 18 '15 at 21:50
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@HDE226868 click on the article I linked. It appears to be a bit of a grey area. Small brown dwarfs / failed stars / free floating planets / globulettes, They aren't escaped/ejected planets, but there's not much difference between a free forming planet and an ejected planet other than the situation where they formed. It would be difficult to tell one from the other without taking a close look. – userLTK Nov 18 '15 at 23:21
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@userLTK: One big difference is mass. Some icebergs can travel all the way to 40 degrees north (New York and Madrid's latitude) before they melt. Ice cubes from your freezer cannot do that. When it comes to science, statistical possibility is a very treacherous tool. Suppose those large company workers were all Jesuits serving a penance and you didn't know it when you made your assumption? Et voila. – Ricky Nov 18 '15 at 23:54
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@HDE226868 while that's mostly true, there's no reason why a super-Jupiter or brown dwarf couldn't escape from a very large solar-system or binary-star system and look very much like one that formed on it's own. Certainly smaller objects, rocky bodies, comets, etc are much more common, but brown dwarfs could be ejected on occasion too. (and I didn't love my workers example, should have thought of a better one), but I stand by the statistical "fact" that planets escape solar-systems from time to time. It's mathematically impossible for that not to be true. – userLTK Nov 19 '15 at 00:01
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@userLTK: In a Universe this big, pretty much nothing is mathematically impossible. Which is why a scientist should stick to the scientific method and obtain evidence before making claims. – Ricky Nov 19 '15 at 00:09
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@Ricky That's not how the mathematics of size works. If you have a very very small cluster of maybe 10 or 20 solar-systems, then it could be possible for rogue planets not to exist in that cluster, but it's not possible in a galaxy of our size for rogue planets not to exist. – userLTK Nov 19 '15 at 00:18
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@userLTK: That's right. By the same logic, it is also not possible for giant pineapples, each the size of a star, not to exist somewhere. Mathematics can't even square a circle after three thousand years of trying. Seriously, nothing in science should be taken on faith. I realize that many things are these days, with comical results, but that's not my cup of tea. My faith is with God. When I pose a question to science, I expect hard facts, a.k.a. empirical evidence and, if at all possible, proof. – Ricky Nov 19 '15 at 00:27
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@userLTK: I'm sorry. I'd like to, but I'm allergic to chats. I'm vain: I'm in favor of my written discussions' continuing existence and accessibility in cyberspace. Chats are too ephemeral. – Ricky Nov 19 '15 at 00:35
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@Ricky OK, I can live with the warnings, for a little while. Statistical improbability is real mathematics and good science. Some things are so statistically improbable as to be, well, you may as well say impossible. Solar Systems do eject planets from time to time because 3 body or n body gravitation is chaotic. That's a mathematical fact even if it's never been observed. Some percentage of solar-systems will eject the occasional planet. Maybe 1 in 3, or 1 in 5, 1 in 10 if you like, but with 100 billion or so solar systems in the milky-way, the rest is just math. – userLTK Nov 19 '15 at 01:03
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@userLTK: I hope you're enjoying this. I know I am. Math is a very useful tool. It is not God. It only works within ... well, a given frame of reference, if you will. Don't even get me started on the uncertainty principle. Math without observation is a pack of tarot cards. Each time a new observation comes along, it is math that needs to adjust itself, not the other way around. Math had no problem with the geocentric model; nor with the heliocentric model. When both were scrapped in favor of the center-less model, math made a few adjustments. Let me give you an example. – Ricky Nov 19 '15 at 02:13
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@userLTK: Observed with the naked eye, nature has a ridiculously small amount of truly spherical objects (the Sun and the Moon come off as flat disks). The amount of objects that are not sphere-shaped is infinitely greater. Statistically, then, Earth should be flat. But, behold, a man climbs a mountain, or a tree, and discovers he can see further from that angle, which can only be possible if the planet is, in fact, a sphere. That's observation trumping math. Should we ever establish that true rogue planets don't exist, math will inevitably adjust itself to fit that view, statistics and all. – Ricky Nov 19 '15 at 02:19
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@Ricky You and I see Math quite differently. Predictions aren't always right, but Math is a useful tool. It may have, in addition to observation, given us nearly 2,000 years of geocentrism, but once Tycho Brache used his astrolabe to track the planets 10 times more accurately than anyone had done before him, the geocentric model no longer worked and Kepler used Math to find a better one. Math found Neptune by studying Uranus and Math couldn't explain Mercury until Einstein unraveled Gravity. Math is a tool, only as useful as the skill of the person who uses it. – userLTK Nov 19 '15 at 02:21
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@userLTK: Exactly. It is a TOOL. Your spoon doesn't feed you: you use it when you're feeding yourself. Kepler started using math (as a tool) based on Brahe's observations. Math didn't study Uranus: Le Verrier did, based on OBSERVATIONS, other people's and his own. Without the telescope you could apply math till the cows came home without ever establishing the existence of Uranus or Pluto. Observations of objects beyond the reach of traditional triangulation are suspect to begin with. The ubiquitous use of Photoshop is the shame of science. – Ricky Nov 19 '15 at 02:36
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@Ricky I never said math studied things, that's an over-literal misinterpretation of what I wrote. By using math, we can know things. N-body gravitation of solar-systems means that there are rogue planets because some solar systems lose planets. Some orbits by interaction with more than one large body become unstable. That's a fact. It's a fact that some solar systems have lost planets. How often solar-systems lose planets, that's up for debate. That it happens is not up for debate and with 100 billion solar-systems in the Milky way. It happens a lot. – userLTK Nov 19 '15 at 03:01
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@userLTK: Charles Darwin (an honest man, as opposed to some of his followers) wondered (in print) why no evidence of transitional species (aka missing links) was ever found, since, according to his calculations (math) we should be practically tripping over it all the time. No loss of a planet has ever been observed. If in fact stars do lose planets, there's absolutely no guarantee that they even make it to interstellar space and not get destroyed by a phenomenon yet unknown to us. Should we ever discover this phenomenon, the math will adjust itself. – Ricky Nov 19 '15 at 03:07
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This is getting a bit chatty - though I am happy to see it's all friendly and on-topic. But please wrap it up. – Donald.McLean Nov 19 '15 at 12:00
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Giant planets when first formed are big and hot. They radiate their own light, mostly in the infrared. So young isolated planets can be seen directly.
There have been various claims in the literature that objects as small as a few Jupiter masses have been identified in young star forming regions. See various papers by the IAC brown dwarf research group
http://adsabs.harvard.edu/abs/2000Sci...290..103Z
http://adsabs.harvard.edu/abs/2002ApJ...578..536Z
http://adsabs.harvard.edu/abs/2014A%26A...568A..77Z
http://adsabs.harvard.edu/abs/2013MmSAI..84..926Z
Another object that is part of the beta Pic moving group, recently discovered by Liu et al. (2013), has an estimated mass of about 8 Jupiter masses (Biller et al. 2015).
http://arxiv.org/abs/1510.07625
These claims are open to criticism - sometimes it is hard to tell whether a faint object really belongs to the star forming region observed, rather than being an unassociated background object. The claimed masses also depend heavily on models for the luminosity-mass relation as a function of age, and the ages of these objects are not easily constrained. The likelihood is that at least some of these objects are below 10 Jupiter masses and would rank as planets by some definitions; though none of the individual objects could be said to be proven beyond any doubt.
Nevertheless it would not be surprising if, in the maelstrom of the formation of a cluster of stars, some planetary systems were stripped from their parent stars by close encounters with other objects and indeed numerical simulations of planetary systems in dense star clusters show that this process occurs (e.g. Davies 2011).
http://adsabs.harvard.edu/abs/2011IAUS..276..304DD
The chances of seeing older, isolated, planetary mass objects are slim, but microlensing appears to be the only technique presently available. The microlensing signature of a free-floating planet is of course unrepeatable so a discovered planet could not be followed up in any way. However, surveys of microlensing events could be a way of saying something statistically about how common such objects are. See for example http://astrobites.org/2011/05/24/free-floating-planets-might-outnumber-stars/
It is also worth noting that the whether these things really are "planets" at all is disputed. They could either be genuine planets, formed in the same way that is hypothesised for most giant planets - that is by accretion onto a rocky core that formed around a star. They could then have been displaced from their parent star by dynamical interactions with other bodies in their system or with a third body. As I said above, N-body simulations do predict that this will happen (e.g. Liu et al. 2013).
On the other hand they could represent the very lowest mass gas fragments that are able to form during the collapse and fragmentation of a molecular cloud and that for some reason were unable to accrete further gas (i.e. they are really more like low-mass brown dwarfs). This so-called "fragmentation limit" is of order 10 Jupiter masses, but if it were a little lower it might explain the free-floating "planets" that have been seen so far.
ProfRob
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Thank you for your answer. It is everything I expected it to be and more: it is thoughtful, informative, and impeccably worded. The links are fascinating. I don't mean to be a nag, but I feel I must reiterate, that in my (admittedly very amateurish) view, claims and predictions do not constitute hard evidence, which means that the answer to my question is "No." Thanks again! I'll have to revisit the links a couple of times just to make sure. You are, in fact, a lot more objective and infinitely more courteous than most astrophysicists I know. – Ricky Nov 19 '15 at 02:47