Of course that's always possible. That's always possible even when we understand everything as far as we know. That's science.
But given that precision to which GR has been tested, something like to one part in ten to the eleventh, it's rather difficult for most physicists/astrophysicists to believe that we're so far off in our understanding of gravity. There are alternate models out there that seem to explain the observations without the dark matter, notably MOND.
But you're given a choice: you're best model of gravity -- which predicted gravitational radiation, black holes, and a host of other extremely counter-intuitive phenonomenon -- is wrong or a large fraction of the universe isn't visible to you. Which do you chose? To most of us, the latter seems more likely.
Remember, it wasn't just the perihelion shift of Mercury wasn't Einstein's only GR prediction. He made 3 of them, and it wasn't until the second was observed (the bending of light near the Sun observed during an eclipse in 1920, I believe, by Sir. Arthur Eddington) that people really bought into the theory. (The third was gravitational red-shift, or time dialation. This has been observed, but only significantly later.)
They've looked pretty hard and carefully for normal, baryonic matter to cause the effects. So far, little has turned up.
On the other hand, it's pretty clear at this point that dark matter in *some* form must exist. It's just a simple grasp of gravity coupled with some weird observations that lead to this conclusion. It is, in fact, very similar to the way Neptune was discovered. First, notice something odd about Uranus's orbit, then realize that another planet at position X could explain it. Just do it with galaxies and clusters, instead, and you start to suspect there's dark matter out there. Do some surveys and find that there doesn't appear to be enough brown dwarfs and black holes to make up the needed mass.
To be honest, while I'm a planetary scientist and thus obligated to make fun of cosmologists, I don't find dark matter, even heretofore undiscovered particles, that hard to believe. Not only is the evidence pretty good, it isn't difficult to imagine that we've only scratched the surface of what is out there. You suggest that we're just finding "what's out there" (a claim with which I might quibble). So why is hard to believe that we haven't found all of the particles in the subatomic zoo? Especially given that the ones we seek are, by definition, difficult to find.
And if you want "too convoluted to be natural", study quantum mechanics. It seems the universe doesn't care what we consider to be "natural", after all.
(And now, a few quibbles: the SNP was only recently really clinched with lab data, but people had speculated about the solution, neutrino oscillations, for quite a while before hand. The same is true of a lot of what HST and others have told us in the past decade: usually, they're helping refine our models and confirm our best guesses as to what's out there. So it isn't like astronomers a decade ago would be shocked at what we've learned.)
Yeah, it really was quite a feat. Although it's not that shocking: the folks at JPL have recovered missions from so many engineering SNAFUs that they deserve canonization. (The Cassini-Huyegens problem also springs to mind, along with Apollo 13, NEAR/Shoemaker, and others.)
Still, NASA doesn't often point this out, but we did lose out on a lot of data. In particular, the cloud observations were pretty much scrapped altogether. (Rather than look at the clouds during much of its Jovian orbit, Galileo had to spend that time reading back data from the recorder and beaming it back ever-so-slowly.)
But my real regret is that we'll never be able to retriever Galileo to take its worthy place in a museum. She was a fine ship and served us well.
If you want the planetary parameters, why not go to the obvious source: NASA. http://ssd.jpl.nasa.gov, Google not needed. Glad you're actually doing some research before speaking, anyway. And, yeah, yer right, Uranus and Neptune are nearer 14.7 and 17 Earth-masses. I was doing the numbers from memory, sorry. But all that means is that both ice giants are even more core than I asserted earlier, meaning you've just supported my point that what you were spouting about the planets' cores was total bull.
(The actual, mean core masses, in Wurtchel: Jupiter ~ 7, Saturn ~ 10, Uranus ~ 12, and Neptune ~ 14.)
It is NOT possible that this little hydrogen could cause an increase of a factor of 100 in the brightness. A star never exhausts its hydrogen, it just exhausts the stuff **in the core**. There is always plenty of hydrogen left over in the overlying layers. Which means throwing a giant planet in makes NO difference as far as the star is concerned. Even if you assume that the planet could make it into the shell-buring area, the SO COULD THE STAR'S OWN HYDROGEN. Therefore, it *doesn't* matter. Unless you think that the planet will actually punch through the entire overlying hydrogen layers, which would take a hell of a wrecking-ball collision, nothing like what a planet would be capable of. There is a whole lotta star to go through to get that deep in, and, even if the overlying layers are really tenuous, there's no way a planet could keep momentum up that long. (You can't invoke gravity, because the surrounding material quickly matches the planet's hydrogen density and buoyancy with do it's bit. The core will probably slowly down to the stellar core, but that's not a lot of power and there is only a limited amount of hydrogen fuel there anyway.)
It's worth noting that Alan Boss is, as far as I've seen, the only one who believes his theory. (The press will report any theory, no matter how speculative and poorly received by the general planetary community.) Even Alan refers to himself a heretic (in a light-hearted way) for espousing it. For one thing, Alan's mechanism would result in eccentric giant planets (zero eccentricity is as likely as anything else). But our giant planets are all in fairly circular orbits. This seems unlikely. Also, we KNOW that all of the giant planets, except Jupiter, have massive solid cores. The gravity data are pretty strong on that point. Jupiter is the only one that might lack a core, but it's quite conceivable that we've missed it due to the overlying layers of hydrogen and the fact that we don't really understand how metallic hydgoen behaves at those pressures. So no one seems to be moving to Alan's theory over the core-accretion model, especially give that the latter explains the terrestrial planets as well (and why we have the inner and outer planets). I'd say that the jury is still out on some of the ESP systems and Alan's idea, but the sentiment still seems strongly in favor of core-accretion, given everything else.
And do note that Jupiter isn't held together by its core, and no one is saying that it is. We're saying that you need that core as a nucleus on which to start accreting gas. Once you're as large as Jupiter, there's plenty of self-gravity holding things together.
Re:never underestimate gravitational potential ene
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Red giants are quite large. Even the Sun in that phase will probably be large enough to reach Earth's orbit. An F-class star is larger, so we can assume that these weren't hot Jupiters that got envelopped.
And, yes, distance DOES matter. If the planet never falls into the star (the star rises and meets it instead) you don't get to extract that gravitational potential energy. It's as simple as that.
And you're being pretty blithe invoking tides. Since tidal forces fall of like 1/r^3, you'd need a monster of a planet to induce significant tides in the star. (And if you did transfer that angular momentum, the planet would be moving away from the star, not towards it.)
So it isn't quite a red giant yet. Even your article stated it was probably on the way there, meaning it was getting quite large. Older data on this star claim it was a red giant already (see the AAVSO site I quoted elsewhere), so either way, this is a big mother of a star.
Finally, so that the planet does just fall into the star so it hits the surface and basically gives up all of its energy. What will you see? Yep, a brief flash of energy, then it'll calm down again because the energy has been transfered over. It won't last months.
I quite simply cannot see a reasonable way to get the energy that they require, either through fusion or energy from the engulfing. I need to see the actual paper to be absolutely sure, but the burden to convince is very much on the authors on this one. (And what happened to waiting to hold press conferences until the goddamn paper came out, anyway?)
Re:never underestimate gravitational potential ene
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You made a mistake, though. (The same one I made at first with this calculation.) The planet is being engulfed by the star, which is expanding out to it. Which means that the planet isn't moving inwards to the star, the star is moving out to *it*. So the star doesn't gain the PE from the planet's orbit, at least not much. If the planet does move inward into the star, most of that energy comes at the expense of moving more fo the star upwards against the star's gravity. (You'd basically have convection. The denser materials would allow for some gain in energy, but not much. That which would be released would do so deep in the interior of the star, taking (estimate ahead) thousands of years to reach the surface. At which point, the energy spike would be spread out over many years due to diffusion. (Note that energy from the Sun takes about a million years to leak out. However, I'll knock 3 orders of magnitude off to allow for a star that is less opaque.)
Re:A Couple Thoughts/questions
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Depends on the kind of star. The core of the star - that is, the "dense" part -
In this case, it's an F-class star. And you missed my point entirely, which was that if the star can convect the planet's hydrogen into the shell-burning zone, it can damn well convect its own hydrogren reserves down there, which are vastly in excess of what the planet could provide. So the star should never notice the miniscule addition of the planet's hydrogen.
This is all theory, of course, but unfortunately, theory doesn't quite bear out the "hydrogen compounds = gas giant planet cores".
See, that's where you're amazingly wrong. Let's review out giant planets, shall we? (If you want, I suggest you crack open Protostars and Planets IV; it's always good to actually do a bit of research.)
Jupiter May or may not have a core in the first place. If it does, it's at most around 10 Earth-masses (maybe as high as 15, but that's at the outer edge of the error bars). Mostly, it'll be hydrogen compounds with some rock and metal (real metals, not in the astrophysical sense). You need that core in standard formation models before you can accrete the hydrogen and helium gas. The metallic hydrogen is a layer right above the core, not the core itself.
Saturn Has a core, around 10-15 Earth-masses. Same as Jupiter in composition. This is easier to work out in theory because the equation of state is better understood for the interior pressures within Saturn. (Jupiter's higher pressures make things more dicey.)
Uranus and Neptune Definately have cores. Also icey with a bit of metals and rock thrown in. Again, need said core to hold on to the gas in the first place. Cores are pretty well constrained in size at around 15 Earth-masses in both planets. Given that both planets are around 18 Earth-masses in size, you bet your ass that this means that they are both mostly core. In fact, it's this that has lead some leading researchers to dub them "ice giants", in contrast to Jupiter and Saturn, the "gas giants."
I don't know where you got your "facts", but they're pretty much uniformly wrong. See Wuchterl et al. in P&P IV for more details on constraints on the present structures of these planets.
"The researchers say that V838 Monocerotis flared because it was fuelled as it engulfed three orbiting planets. It could be the first evidence for an event that had been predicted but not knowingly observed.
Support for this assessment, say the astronomers, is provided by the study of the shape of the light curve and comparison between the observed properties of the star and several theoretical studies.
In addition to the gravitational energy generated by the process, there may also have been a rapid release of nuclear energy as fresh hydrogen was driven into the hydrogen-burning region of the star."
Even the kinetic energy isn't that helpful, I'm pretty sure. The star is expanding out into the planets, so it's slowly engulfing them. The planets aren't smacking into the star. The result would be that the planets would spiral inwards under the increasing drag. They probably never really "hit" anything, the fluids just sort of merge. Without looking at the numbers, I'd suspect that most of the energy that they do add gets added rather far down into the star, so that it won't leak out as one, bright flash.
Re:A Couple Thoughts/questions
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To answer my own question (woo for Google!), the star has brightened from about magnitude 11 or 12 to about 6.5. That's around 5 magnitudes of brightening, or a factor of 100 in the overall luminosity. AAVSO's site talks about it: http://www.aavso.org/vstar/vsots/1202.shtml
I have to say, I'll be interested to see their paper when it hits press, but I'm really skeptical.
Cooler material is darker than hot stuff in stars. Not unless you want to actually burn the planet, but what will you use to oxidize it? All of the oxygen will already be in water molecules.
Without chemical reactions, spreading the planet's material on the surface of the star will *darken* the star by cooling off the surface. (This is, after all, why Sunspots are dark: the material is cooler.)
This group is positing that the planet's gas gets worked down into the fusing zone of the star. For a red giant (such as this), this isn't the just the core. Around the core there is a shell of hydrogen burning. This, you don't have to convect the planet down to the core to get the hydrogen-burning zone. But I'm still skeptical that they can work the planet down, but apparently this is a significant addition on top of all of the star's other hydrogen.
Re:A Couple Thoughts/questions
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1) See, I asked about how much it *brightened*. Not how bright it *got*. I noted that line, too., but knowing that it is now 600,000 L_sun isn't really helpful in telling us how much brighter it is now than before. We would need to also know what its starting point was. This makes a difference: if the star brightened by 0.1%, the possible mechanisms are quite different from the star brightening by a factor of 100.
2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.
No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.
And remember, the volume of a shell goes like the radius or the star squared, so the thickness of the shell has to be pretty thin to keep an appreciable fraction of the metals. Say we want to spread the metals out over a volume roughly equal to the volume of the original core. Uranus is mostly core, so let us use its radius as the radius of the core. (Note: much of Uranus's core is hygrogren compounds, as are all giant planet cores. This means that we're *over*estimating the volume of metals.) And lets spread it over a spherical shell on the Sun's surface. V_Uranus = 4/3 pi r_u^3 V_shell = 4 pi r_s^2 deltaR where r_u is Uranus's radius (2.62E9 cm) and r_s is the Sun's (6.9E10 cm), deltaR is the thickness of the spherical shell, and the Vs are volumes. Equating and cancelling, we get that deltaR = r_u^3/3 r_s^2. Plugging in numbers, that's a thickness of about 1.3E6 cm, or about 0.0018 % of the Sun's diameter. Which, when you consider that the Sun is fluid and convection does happen (although the most convective part is a bit lower down below the surface), isn't a whole lot. Confining the metals to that region would be very difficult.
This would probably be why current thinking tends more towards the "the clouds that formed star with planets were unusally rich in metals." Also, it makes sense: more metals, more stuff to actually *build* planets with.
1. They're positing that eating one or three giant planets is enough new fuel to make the star brighten significantly? (I wish the article had the details on how much it actually brightened.) A typical gas giant is around 1/1000 the mass of the parent star. That's not a lot of new fuel, particularly when you consider that the star has way more hydrogen than that left over from main sequence burning.
2. My most recent understanding (and I admit that I'm only half paying attention to this) is that the planets-contaminate-stars model for the heavy element enrichment probably doesn't explain the observed enrichment. (Probably because the planet's bits would have to stay right near the star's surface over the long run. See mass ratio, above.)
I'm not saying that this model doesn't work, but I'm skeptical. I'd really want to see their stellar models showing how addition of a giant planet's mass of hydrogen on the surface of the red giant affects the luminosity. I'd also like to see evidence that this star had planets before the brightening. (I wouldn't be shocked if the data didn't exist. But I still want to see it.:-)
Yeah, you get that from the fact that it orbits retrograde and its orbit is fairly inclined relative to Neptune's equator. That said, damn if we know how to capture such a huge chuck of ice and rock as Triton. Although it seems easier to do when the planet is very young and has a more extended atmosphere.
OK, this is my last post on this, because it's become clear that you really don't know the details.
First, the speed of light lag was only part of the landing on/orbit around Eros problem. I know, I've spoken with many of the mission scientists. And, no, it isn't trivial. You can't dismiss it that easily.
In the case of Deimos, is a postive SOB. You'll have to orbit within around 11 km of the average surface if you go retrograde, 5 if you go prograde. Given that you're orbiting a moon that is only 6 km in radius, that's pretty tight. (You get this from looking at the Hill radius for Deimos and Mars. It's a very small Hill sphere.) The fact that it is irregularly shaped makes you job that much harder.
Second, you don't know what the frost line is. The frost line is in place IN THE PROTOSOLAR NEBULA. It is there BEFORE THE NEBULA DISSIPATES. That's why we get icy bodies beyond 5 AU and little water inside that distance. Please, for the love of all that is good in the world, look some of this up?
And Earth did NOT lose its water from volcanic activity. Volcanoes can't give water enough heat to escape the Earth. I don't know *where* you heard that little gem, but you need to go back and smack that person. And small bodies have it WORSE. Their escape speed is much lower, so a little thermal energy makes you escape. Look at Mercury and the Moon. (If you're astute, you'll note that neither has any significant amount of water. Possibly theres a bit in the permanently shadowed craters near the poles, but that's the only place you'll find any.)
Next, SEDS needs to update their site. (Notice the date on the webpage? 5 years ago. A lot has happened with asteroid studies since then. And I have caught plenty of mistakes on the Nine Planets site in the past, so please don't take it as gospel. The best thing to do is to talk to someone who works in this field.) We've found that many asteroids have low densities, apparently because they are filled with a lot of vacuum. They very in how much this is the case, but we're pretty sure that low densities is a hallmark not of water (as in the outer solar system), but rather high porosity. Some asteroids show signs of being little more than loose rubble piles.
Where am I getting this from? Journals, talking to the other planetary scientists, etc. I hate to harp on this, but it's relevent: I am a planetary scientist myself.
And finally, as for screwing something in (to gravel?), how are you going to screw it in, pray tell? You need to push down to make it go in. In that gravity, you have no useful weight. No weight, no pushing in.
Look, you gain exactly jack for putting a base on a dinky rock like Deimos, and the trouble involved is nowhere near worth it. You might as well go with a station, where you have control over the orbital distance, you don't have to muck about with the difficulty of putting the base on the moon or the annoyances of the small, but omnipresent, gravity.
You also have to land the base onto the moon, and then secure it down. Do you know how hard it is to land in that kind of gravity? Really? It took a lot of careful effort to manuver NEAR/Shoemaker around Eros. And they didn't have to contend with a nearby Mars.
"Do you have any idea how much mass a 'few kilometer' body contains? Clearly not."
One might have assumed that, in order to calculate the surface gravity, I actually used that number. But that would ruin your attempt to take a swipe at me, wouldn't it? So go ahead and give logic a miss.
The rock is NOT 1% water. I can almost promise you that. It's an asteroid. Asteroids formed inside the "frost-line" in the protoplanetary disk. There would little or no water adhered to the rocky material in this region. Hell, EARTH isn't even 1% water, and we've had quite a bit delivered to us from the outer solar system via cometary impacts. (Deimos would have had far fewer of those and wouldn't easily hold on to any water on its surface anyway. You might think about what surface gravity/escape velocity has to do with that.) So you're making up a number, one that is unreasonably large. Which is jind of funny, because a little reflection tells you that if the water WERE mixed in through the entire volume, as you claim, you'd have to mine it out. Again, you're in microgravity here. That's hard, no matter what you think you learned from "Armeggedon".
And, no, the densities do NOT tell you that they're made of ice. Who told you this? (Or are you making this up as you go?) They're densities are low because they are probably fairly porous. In order words, you're planning to mine empty space. We have plenty of that already, thanks for inquiring.
"Yes, probably true, although anchoring it or orbiting it around Deimos is probably indicated."
See above. You need to think this through to consult someone who actually knows about this stuff. Orbiting a small rock is hard as hell, espcially with Mars right there playing with your orbit all the time. On the other hand, how do you plan to anchor to Deimos? (I repeat, once again, "see low surface gravity") It's a not a very dense rock, so driving in an anchor (how would you do that, anyway?) would be tricky since it'd be anchored in lose dirt for the most part.
Look. Take an astronomy class. Talk to someone about this. Please. Your information is clearly inaccurate.
Not sure of the timescale on the second point. All of this depends on not just the planet's spin and mass, but also on the mass of the moon (bigger is actually better, as I recall), the moon's distance and the tidal reponse of the planet. Since Neptune's upper layers are pretty fluid, I'm guessing that they don't dissipate much and so Triton doesn't move as fast as the Moon. But I'd need to check up on that. (Earth-Moon face a lot of evolution, more now than is typical, because of ocean bulges trying to squeeze through narrow straights. (Gibralter, Magellen, etc.)
Perhaps he is suggesting that, but it's wrong. Angular momentum is transfer to the moon as a body force, not diffrentially. The moon can simultaneously run ahead of the bulge and behind it. In fact, at synchronous orbit, it does neither. It's perfectly aligned. Which is why the Earth-Moon system is heading for that right now. (It won't be stable for Earth, but that's because we feel the effects of the Sun pretty strongly, too. Pluto-Charon are already there, and are fairly happy. But they're really far from the Sun and tides vary as one over distance cubed.)
Walking is generally important, unless you want the astronouts to stay seated all the time. If there were in zero-g, they could float. But there's enough gravity to make that annoying, but not nearly enough to walk. Also, they'd still need to do daily exercises to keep their bones and muscles from atrophying.
You're making a pretty large leap from "probably water" to "send fuel back to Earth". There probably isn't that much water to start with, given that these guys are a few kilometers across.
It's not a good idea. You're better off parking a space station in orbit on its own. It'll be easier to handle, more flexible and you don't lose any advantages.
Er, what's your point? The moons are behaving as predicted by the theory, just like ours is. There's obviously some error in it, but there always is. (There's error in our orbital calculations for the Earth, another thing we can't actually integrate exactly. Do you not believe that we know pretty well where Earth will be in a month?)
Let's start with moons outside of synchronous orbit. These moons raise a tidal bulge on their planet. (The one on Earth is most apparent in the oceans. Or, rather, at their edges. But there's a bulge in the rock, too.) Now, the planet is spinning and it isn't a perfect fluid. So it will tend to carry the bulge forward with it, before the bulge can move back to under the moon where it wants to be. A balance is struck between these two competing forces where the bulge rides somewhere ahead of the moon.
The moon, then, feels a tug forward in its orbit. This tends to give it angular momentum, so that it drifts outward. (Angular momentum increases as you go out from the central object.) The planet, meanwhile, is being pulled backward so that its spin slows down. (As it must, to conserve angular momentum in the system.) This is why Earth's day in lengthening and why the Moon has drifted about 60 Earth radii from where it formed over the past 4.5 billion years.
What happens of the moon is *inside* synchronous orbit? The opposite happens: the moon moves ahead of the bulge and gets pulled back. So it drifts in.
I'll leave it as an exercise to the reader to work out what retrograde (backward orbiting) moons do. Triton is an example, by the way.
Right, there's a lot here that makes me dubious of the claim. First off, I should point out that I've worked on the capture problem for Mars's moons. (The results haven't been published, although the did land a grant.)
First off, why is synchronous orbit a hint as to their breakup? There's no reason that synchronous orbit is preferred, either as a capture point or as a point for breakup. In fact, synchronous orbit is an unstable equilibrium: a slight perturbation drives everything away from it. (Which is why Phobos is heading inward and Deimos outward.)
Also, he needs to explain why a larger moon orbited there happily (without perturbation!) for billions of years before breaking apart. In the very least, we're witnessing Mars's moons at a very unusal time, and such coincidence make me (and most astronomers) nervous.
Also, Phobos has drifted inward since any such breakup. Why isn't it breaking up more? Unless there's some internal strength (in which case, why did it break up then?), it should.
To be honest, I sort of question his background for this. Besides the fact that he's not an astronomer, he wants to put a base on Deimos? The surface gravity on those moons is virtually non-existant. (For Deimos, being smaller, it's under 1 cm/sec^2, I believe.) No one could even walk around properly. (Although, if he hollowed it out and made a colony ship out of it, we could launch it to Tau Ceti... But it might encounter some hostile, three-eyed aliens.*)
I'd be happy to hear him explain his idea to a group of dynamicists. Hell, I'll volunteer. But I'm very skeptical for now.
Depends on how large the wedge is. I'd have to work that out, but for a big stone, it'd have to be pretty large.
In any event, that isn't the issue. The issue is that you're now putting in all kinds of extra work to avoid crushing. And going up the ramp, you *aren't* able to use the rock's momentum to help you up, especially if you are constantly stopping. Going circular doesn't gain you anything on the ramp (it actually costs you) and isn't a huge gain over rollers on level ground.
But now you have guys dragging a large wedge up, adding to the toil. And they have to bee pretty coordinated to keep the wedge just behind the block, but not abutting it.
I'm not saying it is impossible, just that it's not that easy to do. Which is the issue here, really.
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Of course that's always possible. That's always possible even when we understand everything as far as we know. That's science.
But given that precision to which GR has been tested, something like to one part in ten to the eleventh, it's rather difficult for most physicists/astrophysicists to believe that we're so far off in our understanding of gravity. There are alternate models out there that seem to explain the observations without the dark matter, notably MOND.
But you're given a choice: you're best model of gravity -- which predicted gravitational radiation, black holes, and a host of other extremely counter-intuitive phenonomenon -- is wrong or a large fraction of the universe isn't visible to you. Which do you chose? To most of us, the latter seems more likely.
Remember, it wasn't just the perihelion shift of Mercury wasn't Einstein's only GR prediction. He made 3 of them, and it wasn't until the second was observed (the bending of light near the Sun observed during an eclipse in 1920, I believe, by Sir. Arthur Eddington) that people really bought into the theory. (The third was gravitational red-shift, or time dialation. This has been observed, but only significantly later.)
They've looked pretty hard and carefully for normal, baryonic matter to cause the effects. So far, little has turned up.
On the other hand, it's pretty clear at this point that dark matter in *some* form must exist. It's just a simple grasp of gravity coupled with some weird observations that lead to this conclusion. It is, in fact, very similar to the way Neptune was discovered. First, notice something odd about Uranus's orbit, then realize that another planet at position X could explain it. Just do it with galaxies and clusters, instead, and you start to suspect there's dark matter out there. Do some surveys and find that there doesn't appear to be enough brown dwarfs and black holes to make up the needed mass.
To be honest, while I'm a planetary scientist and thus obligated to make fun of cosmologists, I don't find dark matter, even heretofore undiscovered particles, that hard to believe. Not only is the evidence pretty good, it isn't difficult to imagine that we've only scratched the surface of what is out there. You suggest that we're just finding "what's out there" (a claim with which I might quibble). So why is hard to believe that we haven't found all of the particles in the subatomic zoo? Especially given that the ones we seek are, by definition, difficult to find.
And if you want "too convoluted to be natural", study quantum mechanics. It seems the universe doesn't care what we consider to be "natural", after all.
(And now, a few quibbles: the SNP was only recently really clinched with lab data, but people had speculated about the solution, neutrino oscillations, for quite a while before hand. The same is true of a lot of what HST and others have told us in the past decade: usually, they're helping refine our models and confirm our best guesses as to what's out there. So it isn't like astronomers a decade ago would be shocked at what we've learned.)
And more to the point, how do they find the centers? (Given that there isn't such a creature and all.)
I suspect that they meant "centers of galaxies".
Yeah, it really was quite a feat. Although it's not that shocking: the folks at JPL have recovered missions from so many engineering SNAFUs that they deserve canonization. (The Cassini-Huyegens problem also springs to mind, along with Apollo 13, NEAR/Shoemaker, and others.)
Still, NASA doesn't often point this out, but we did lose out on a lot of data. In particular, the cloud observations were pretty much scrapped altogether. (Rather than look at the clouds during much of its Jovian orbit, Galileo had to spend that time reading back data from the recorder and beaming it back ever-so-slowly.)
But my real regret is that we'll never be able to retriever Galileo to take its worthy place in a museum. She was a fine ship and served us well.
If you want the planetary parameters, why not go to the obvious source: NASA. http://ssd.jpl.nasa.gov, Google not needed. Glad you're actually doing some research before speaking, anyway. And, yeah, yer right, Uranus and Neptune are nearer 14.7 and 17 Earth-masses. I was doing the numbers from memory, sorry. But all that means is that both ice giants are even more core than I asserted earlier, meaning you've just supported my point that what you were spouting about the planets' cores was total bull.
(The actual, mean core masses, in Wurtchel: Jupiter ~ 7, Saturn ~ 10, Uranus ~ 12, and Neptune ~ 14.)
It is NOT possible that this little hydrogen could cause an increase of a factor of 100 in the brightness. A star never exhausts its hydrogen, it just exhausts the stuff **in the core**. There is always plenty of hydrogen left over in the overlying layers. Which means throwing a giant planet in makes NO difference as far as the star is concerned. Even if you assume that the planet could make it into the shell-buring area, the SO COULD THE STAR'S OWN HYDROGEN. Therefore, it *doesn't* matter. Unless you think that the planet will actually punch through the entire overlying hydrogen layers, which would take a hell of a wrecking-ball collision, nothing like what a planet would be capable of. There is a whole lotta star to go through to get that deep in, and, even if the overlying layers are really tenuous, there's no way a planet could keep momentum up that long. (You can't invoke gravity, because the surrounding material quickly matches the planet's hydrogen density and buoyancy with do it's bit. The core will probably slowly down to the stellar core, but that's not a lot of power and there is only a limited amount of hydrogen fuel there anyway.)
It's worth noting that Alan Boss is, as far as I've seen, the only one who believes his theory. (The press will report any theory, no matter how speculative and poorly received by the general planetary community.) Even Alan refers to himself a heretic (in a light-hearted way) for espousing it. For one thing, Alan's mechanism would result in eccentric giant planets (zero eccentricity is as likely as anything else). But our giant planets are all in fairly circular orbits. This seems unlikely. Also, we KNOW that all of the giant planets, except Jupiter, have massive solid cores. The gravity data are pretty strong on that point. Jupiter is the only one that might lack a core, but it's quite conceivable that we've missed it due to the overlying layers of hydrogen and the fact that we don't really understand how metallic hydgoen behaves at those pressures. So no one seems to be moving to Alan's theory over the core-accretion model, especially give that the latter explains the terrestrial planets as well (and why we have the inner and outer planets). I'd say that the jury is still out on some of the ESP systems and Alan's idea, but the sentiment still seems strongly in favor of core-accretion, given everything else.
And do note that Jupiter isn't held together by its core, and no one is saying that it is. We're saying that you need that core as a nucleus on which to start accreting gas. Once you're as large as Jupiter, there's plenty of self-gravity holding things together.
Red giants are quite large. Even the Sun in that phase will probably be large enough to reach Earth's orbit. An F-class star is larger, so we can assume that these weren't hot Jupiters that got envelopped.
And, yes, distance DOES matter. If the planet never falls into the star (the star rises and meets it instead) you don't get to extract that gravitational potential energy. It's as simple as that.
And you're being pretty blithe invoking tides. Since tidal forces fall of like 1/r^3, you'd need a monster of a planet to induce significant tides in the star. (And if you did transfer that angular momentum, the planet would be moving away from the star, not towards it.)
So it isn't quite a red giant yet. Even your article stated it was probably on the way there, meaning it was getting quite large. Older data on this star claim it was a red giant already (see the AAVSO site I quoted elsewhere), so either way, this is a big mother of a star.
Finally, so that the planet does just fall into the star so it hits the surface and basically gives up all of its energy. What will you see? Yep, a brief flash of energy, then it'll calm down again because the energy has been transfered over. It won't last months.
I quite simply cannot see a reasonable way to get the energy that they require, either through fusion or energy from the engulfing. I need to see the actual paper to be absolutely sure, but the burden to convince is very much on the authors on this one. (And what happened to waiting to hold press conferences until the goddamn paper came out, anyway?)
You made a mistake, though. (The same one I made at first with this calculation.) The planet is being engulfed by the star, which is expanding out to it. Which means that the planet isn't moving inwards to the star, the star is moving out to *it*. So the star doesn't gain the PE from the planet's orbit, at least not much. If the planet does move inward into the star, most of that energy comes at the expense of moving more fo the star upwards against the star's gravity. (You'd basically have convection. The denser materials would allow for some gain in energy, but not much. That which would be released would do so deep in the interior of the star, taking (estimate ahead) thousands of years to reach the surface. At which point, the energy spike would be spread out over many years due to diffusion. (Note that energy from the Sun takes about a million years to leak out. However, I'll knock 3 orders of magnitude off to allow for a star that is less opaque.)
Depends on the kind of star. The core of the star - that is, the "dense" part -
In this case, it's an F-class star. And you missed my point entirely, which was that if the star can convect the planet's hydrogen into the shell-burning zone, it can damn well convect its own hydrogren reserves down there, which are vastly in excess of what the planet could provide. So the star should never notice the miniscule addition of the planet's hydrogen.
This is all theory, of course, but unfortunately, theory doesn't quite bear out the "hydrogen compounds = gas giant planet cores".
See, that's where you're amazingly wrong. Let's review out giant planets, shall we? (If you want, I suggest you crack open Protostars and Planets IV; it's always good to actually do a bit of research.)
Jupiter May or may not have a core in the first place. If it does, it's at most around 10 Earth-masses (maybe as high as 15, but that's at the outer edge of the error bars). Mostly, it'll be hydrogen compounds with some rock and metal (real metals, not in the astrophysical sense). You need that core in standard formation models before you can accrete the hydrogen and helium gas. The metallic hydrogen is a layer right above the core, not the core itself.
Saturn Has a core, around 10-15 Earth-masses. Same as Jupiter in composition. This is easier to work out in theory because the equation of state is better understood for the interior pressures within Saturn. (Jupiter's higher pressures make things more dicey.)
Uranus and Neptune Definately have cores. Also icey with a bit of metals and rock thrown in. Again, need said core to hold on to the gas in the first place. Cores are pretty well constrained in size at around 15 Earth-masses in both planets. Given that both planets are around 18 Earth-masses in size, you bet your ass that this means that they are both mostly core. In fact, it's this that has lead some leading researchers to dub them "ice giants", in contrast to Jupiter and Saturn, the "gas giants."
I don't know where you got your "facts", but they're pretty much uniformly wrong. See Wuchterl et al. in P&P IV for more details on constraints on the present structures of these planets.
"The researchers say that V838 Monocerotis flared because it was fuelled as it engulfed three orbiting planets. It could be the first evidence for an event that had been predicted but not knowingly observed.
Support for this assessment, say the astronomers, is provided by the study of the shape of the light curve and comparison between the observed properties of the star and several theoretical studies.
In addition to the gravitational energy generated by the process, there may also have been a rapid release of nuclear energy as fresh hydrogen was driven into the hydrogen-burning region of the star."
Even the kinetic energy isn't that helpful, I'm pretty sure. The star is expanding out into the planets, so it's slowly engulfing them. The planets aren't smacking into the star. The result would be that the planets would spiral inwards under the increasing drag. They probably never really "hit" anything, the fluids just sort of merge. Without looking at the numbers, I'd suspect that most of the energy that they do add gets added rather far down into the star, so that it won't leak out as one, bright flash.
To answer my own question (woo for Google!), the star has brightened from about magnitude 11 or 12 to about 6.5. That's around 5 magnitudes of brightening, or a factor of 100 in the overall luminosity. AAVSO's site talks about it: http://www.aavso.org/vstar/vsots/1202.shtml
I have to say, I'll be interested to see their paper when it hits press, but I'm really skeptical.
Cooler material is darker than hot stuff in stars. Not unless you want to actually burn the planet, but what will you use to oxidize it? All of the oxygen will already be in water molecules.
Without chemical reactions, spreading the planet's material on the surface of the star will *darken* the star by cooling off the surface. (This is, after all, why Sunspots are dark: the material is cooler.)
This group is positing that the planet's gas gets worked down into the fusing zone of the star. For a red giant (such as this), this isn't the just the core. Around the core there is a shell of hydrogen burning. This, you don't have to convect the planet down to the core to get the hydrogen-burning zone. But I'm still skeptical that they can work the planet down, but apparently this is a significant addition on top of all of the star's other hydrogen.
1) See, I asked about how much it *brightened*. Not how bright it *got*. I noted that line, too., but knowing that it is now 600,000 L_sun isn't really helpful in telling us how much brighter it is now than before. We would need to also know what its starting point was. This makes a difference: if the star brightened by 0.1%, the possible mechanisms are quite different from the star brightening by a factor of 100.
2) Um, no. When a star gets to Fe (and only very large stars do), it makes a nice little explosion adn we enrich the interstellar medium. Which is where pretty much all of the "metals" (anything heavier than helium, according to astrophysicsists) in your body, Earth, the Sun, etc. come from. So the question about metal-rich stars isn't "are they producing the metals", they would have had to leave the main sequence for one thing. The question is did the cloud that formed them have an more metals than the average, or did the metals get preferentially introduced by, say, planets smacking in to them.
No, see, as star is WAY bigger than a planet. (By definition, almost.) So a planet, particular a gas giant which is in large part hydrogen and helium (10s of percent and up, by mass) smack into the star, unless the material stays right near the surface, all of those metals will basically be so thinly spread throughout the volume of the star that you'll never see a real enrichment to within error bars.
And remember, the volume of a shell goes like the radius or the star squared, so the thickness of the shell has to be pretty thin to keep an appreciable fraction of the metals. Say we want to spread the metals out over a volume roughly equal to the volume of the original core. Uranus is mostly core, so let us use its radius as the radius of the core. (Note: much of Uranus's core is hygrogren compounds, as are all giant planet cores. This means that we're *over*estimating the volume of metals.) And lets spread it over a spherical shell on the Sun's surface.
V_Uranus = 4/3 pi r_u^3
V_shell = 4 pi r_s^2 deltaR
where r_u is Uranus's radius (2.62E9 cm) and r_s is the Sun's (6.9E10 cm), deltaR is the thickness of the spherical shell, and the Vs are volumes. Equating and cancelling, we get that deltaR = r_u^3/3 r_s^2. Plugging in numbers, that's a thickness of about 1.3E6 cm, or about 0.0018 % of the Sun's diameter. Which, when you consider that the Sun is fluid and convection does happen (although the most convective part is a bit lower down below the surface), isn't a whole lot. Confining the metals to that region would be very difficult.
This would probably be why current thinking tends more towards the "the clouds that formed star with planets were unusally rich in metals." Also, it makes sense: more metals, more stuff to actually *build* planets with.
1. They're positing that eating one or three giant planets is enough new fuel to make the star brighten significantly? (I wish the article had the details on how much it actually brightened.) A typical gas giant is around 1/1000 the mass of the parent star. That's not a lot of new fuel, particularly when you consider that the star has way more hydrogen than that left over from main sequence burning.
:-)
2. My most recent understanding (and I admit that I'm only half paying attention to this) is that the planets-contaminate-stars model for the heavy element enrichment probably doesn't explain the observed enrichment. (Probably because the planet's bits would have to stay right near the star's surface over the long run. See mass ratio, above.)
I'm not saying that this model doesn't work, but I'm skeptical. I'd really want to see their stellar models showing how addition of a giant planet's mass of hydrogen on the surface of the red giant affects the luminosity. I'd also like to see evidence that this star had planets before the brightening. (I wouldn't be shocked if the data didn't exist. But I still want to see it.
Yeah, you get that from the fact that it orbits retrograde and its orbit is fairly inclined relative to Neptune's equator. That said, damn if we know how to capture such a huge chuck of ice and rock as Triton. Although it seems easier to do when the planet is very young and has a more extended atmosphere.
OK, this is my last post on this, because it's become clear that you really don't know the details.
First, the speed of light lag was only part of the landing on/orbit around Eros problem. I know, I've spoken with many of the mission scientists. And, no, it isn't trivial. You can't dismiss it that easily.
In the case of Deimos, is a postive SOB. You'll have to orbit within around 11 km of the average surface if you go retrograde, 5 if you go prograde. Given that you're orbiting a moon that is only 6 km in radius, that's pretty tight. (You get this from looking at the Hill radius for Deimos and Mars. It's a very small Hill sphere.) The fact that it is irregularly shaped makes you job that much harder.
Second, you don't know what the frost line is. The frost line is in place IN THE PROTOSOLAR NEBULA. It is there BEFORE THE NEBULA DISSIPATES. That's why we get icy bodies beyond 5 AU and little water inside that distance. Please, for the love of all that is good in the world, look some of this up?
And Earth did NOT lose its water from volcanic activity. Volcanoes can't give water enough heat to escape the Earth. I don't know *where* you heard that little gem, but you need to go back and smack that person. And small bodies have it WORSE. Their escape speed is much lower, so a little thermal energy makes you escape. Look at Mercury and the Moon. (If you're astute, you'll note that neither has any significant amount of water. Possibly theres a bit in the permanently shadowed craters near the poles, but that's the only place you'll find any.)
Next, SEDS needs to update their site. (Notice the date on the webpage? 5 years ago. A lot has happened with asteroid studies since then. And I have caught plenty of mistakes on the Nine Planets site in the past, so please don't take it as gospel. The best thing to do is to talk to someone who works in this field.) We've found that many asteroids have low densities, apparently because they are filled with a lot of vacuum. They very in how much this is the case, but we're pretty sure that low densities is a hallmark not of water (as in the outer solar system), but rather high porosity. Some asteroids show signs of being little more than loose rubble piles.
Where am I getting this from? Journals, talking to the other planetary scientists, etc. I hate to harp on this, but it's relevent: I am a planetary scientist myself.
And finally, as for screwing something in (to gravel?), how are you going to screw it in, pray tell? You need to push down to make it go in. In that gravity, you have no useful weight. No weight, no pushing in.
Look, you gain exactly jack for putting a base on a dinky rock like Deimos, and the trouble involved is nowhere near worth it. You might as well go with a station, where you have control over the orbital distance, you don't have to muck about with the difficulty of putting the base on the moon or the annoyances of the small, but omnipresent, gravity.
You also have to land the base onto the moon, and then secure it down. Do you know how hard it is to land in that kind of gravity? Really? It took a lot of careful effort to manuver NEAR/Shoemaker around Eros. And they didn't have to contend with a nearby Mars.
"Do you have any idea how much mass a 'few kilometer' body contains? Clearly not."
One might have assumed that, in order to calculate the surface gravity, I actually used that number. But that would ruin your attempt to take a swipe at me, wouldn't it? So go ahead and give logic a miss.
The rock is NOT 1% water. I can almost promise you that. It's an asteroid. Asteroids formed inside the "frost-line" in the protoplanetary disk. There would little or no water adhered to the rocky material in this region. Hell, EARTH isn't even 1% water, and we've had quite a bit delivered to us from the outer solar system via cometary impacts. (Deimos would have had far fewer of those and wouldn't easily hold on to any water on its surface anyway. You might think about what surface gravity/escape velocity has to do with that.) So you're making up a number, one that is unreasonably large. Which is jind of funny, because a little reflection tells you that if the water WERE mixed in through the entire volume, as you claim, you'd have to mine it out. Again, you're in microgravity here. That's hard, no matter what you think you learned from "Armeggedon".
And, no, the densities do NOT tell you that they're made of ice. Who told you this? (Or are you making this up as you go?) They're densities are low because they are probably fairly porous. In order words, you're planning to mine empty space. We have plenty of that already, thanks for inquiring.
"Yes, probably true, although anchoring it or orbiting it around Deimos is probably indicated."
See above. You need to think this through to consult someone who actually knows about this stuff. Orbiting a small rock is hard as hell, espcially with Mars right there playing with your orbit all the time. On the other hand, how do you plan to anchor to Deimos? (I repeat, once again, "see low surface gravity") It's a not a very dense rock, so driving in an anchor (how would you do that, anyway?) would be tricky since it'd be anchored in lose dirt for the most part.
Look. Take an astronomy class. Talk to someone about this. Please. Your information is clearly inaccurate.
Exactly on the first point.
Not sure of the timescale on the second point. All of this depends on not just the planet's spin and mass, but also on the mass of the moon (bigger is actually better, as I recall), the moon's distance and the tidal reponse of the planet. Since Neptune's upper layers are pretty fluid, I'm guessing that they don't dissipate much and so Triton doesn't move as fast as the Moon. But I'd need to check up on that. (Earth-Moon face a lot of evolution, more now than is typical, because of ocean bulges trying to squeeze through narrow straights. (Gibralter, Magellen, etc.)
Perhaps he is suggesting that, but it's wrong. Angular momentum is transfer to the moon as a body force, not diffrentially. The moon can simultaneously run ahead of the bulge and behind it. In fact, at synchronous orbit, it does neither. It's perfectly aligned. Which is why the Earth-Moon system is heading for that right now. (It won't be stable for Earth, but that's because we feel the effects of the Sun pretty strongly, too. Pluto-Charon are already there, and are fairly happy. But they're really far from the Sun and tides vary as one over distance cubed.)
Walking is generally important, unless you want the astronouts to stay seated all the time. If there were in zero-g, they could float. But there's enough gravity to make that annoying, but not nearly enough to walk. Also, they'd still need to do daily exercises to keep their bones and muscles from atrophying.
You're making a pretty large leap from "probably water" to "send fuel back to Earth". There probably isn't that much water to start with, given that these guys are a few kilometers across.
It's not a good idea. You're better off parking a space station in orbit on its own. It'll be easier to handle, more flexible and you don't lose any advantages.
Er, what's your point? The moons are behaving as predicted by the theory, just like ours is. There's obviously some error in it, but there always is. (There's error in our orbital calculations for the Earth, another thing we can't actually integrate exactly. Do you not believe that we know pretty well where Earth will be in a month?)
Glad you asked!
Let's start with moons outside of synchronous orbit. These moons raise a tidal bulge on their planet. (The one on Earth is most apparent in the oceans. Or, rather, at their edges. But there's a bulge in the rock, too.) Now, the planet is spinning and it isn't a perfect fluid. So it will tend to carry the bulge forward with it, before the bulge can move back to under the moon where it wants to be. A balance is struck between these two competing forces where the bulge rides somewhere ahead of the moon.
The moon, then, feels a tug forward in its orbit. This tends to give it angular momentum, so that it drifts outward. (Angular momentum increases as you go out from the central object.) The planet, meanwhile, is being pulled backward so that its spin slows down. (As it must, to conserve angular momentum in the system.) This is why Earth's day in lengthening and why the Moon has drifted about 60 Earth radii from where it formed over the past 4.5 billion years.
What happens of the moon is *inside* synchronous orbit? The opposite happens: the moon moves ahead of the bulge and gets pulled back. So it drifts in.
I'll leave it as an exercise to the reader to work out what retrograde (backward orbiting) moons do. Triton is an example, by the way.
Right, there's a lot here that makes me dubious of the claim. First off, I should point out that I've worked on the capture problem for Mars's moons. (The results haven't been published, although the did land a grant.)
First off, why is synchronous orbit a hint as to their breakup? There's no reason that synchronous orbit is preferred, either as a capture point or as a point for breakup. In fact, synchronous orbit is an unstable equilibrium: a slight perturbation drives everything away from it. (Which is why Phobos is heading inward and Deimos outward.)
Also, he needs to explain why a larger moon orbited there happily (without perturbation!) for billions of years before breaking apart. In the very least, we're witnessing Mars's moons at a very unusal time, and such coincidence make me (and most astronomers) nervous.
Also, Phobos has drifted inward since any such breakup. Why isn't it breaking up more? Unless there's some internal strength (in which case, why did it break up then?), it should.
To be honest, I sort of question his background for this. Besides the fact that he's not an astronomer, he wants to put a base on Deimos? The surface gravity on those moons is virtually non-existant. (For Deimos, being smaller, it's under 1 cm/sec^2, I believe.) No one could even walk around properly. (Although, if he hollowed it out and made a colony ship out of it, we could launch it to Tau Ceti... But it might encounter some hostile, three-eyed aliens.*)
I'd be happy to hear him explain his idea to a group of dynamicists. Hell, I'll volunteer. But I'm very skeptical for now.
(* Kudos to anyone who catches *that* reference.)
Depends on how large the wedge is. I'd have to work that out, but for a big stone, it'd have to be pretty large.
In any event, that isn't the issue. The issue is that you're now putting in all kinds of extra work to avoid crushing. And going up the ramp, you *aren't* able to use the rock's momentum to help you up, especially if you are constantly stopping. Going circular doesn't gain you anything on the ramp (it actually costs you) and isn't a huge gain over rollers on level ground.
But now you have guys dragging a large wedge up, adding to the toil. And they have to bee pretty coordinated to keep the wedge just behind the block, but not abutting it.
I'm not saying it is impossible, just that it's not that easy to do. Which is the issue here, really.