night_wrtr
Non-human Protagonist
There are examples of hypervelocity stars that are already on their way out of the galaxy beyond escape velocity.
Giant exoplanet orbits smaller dwarf star
- Thread starter Brian G Turner
- Start date
You don't seem to understand the basic concepts of 'escape velocity', which is the simple physics you need: Escape velocity - Wikipedia
Basically when material attains or goes faster than this velocity it will never return to the original massive object. It is not about it being attracted to another massive object as if somehow it can only escape if it 'latches' onto another body. (Anyway, an object that has attained escape velocity for a solar system will still be bound to the galactic core and remain in its influence anyway, unless it has also attained the escape velocity for escaping that much bigger object.)
If the sun went Supernova, then you are clearly incorrect. The energy of the explosion would easily accelerate masses of the star well beyond any capture of the original system. i.e. by giving it a velocity well over the escape velocity of the original mass. You can actually observe this today in the Crab Nebula - the still expanding supernova remnant.
As this internet paper states (Expansion of the crab nebula): "Given that supernovae are highly explosive events, in the absence of some strong retarding force, the resulting supernova remnant should continue to expand uniformly into the surrounding interstellar medium" So, it will not return to the original point where the star blew up. See - I'm not making it up.
But there are also other different mechanisms for objects to be ejected from stars' systems by attaining escape velocity. Simple physics.
LordOfWizards
Well Known Rememberer
There are two reasons why I'm reasonably sure that planets can be hurled away from their stars.
1) What Lumens said.
2) Gravity assist.
And I'm reasonably certain that both of these things happen way more frequently early in the formation of a solar/star system. Then you have a ginormous amount of binary systems out there. Eighty percent of them according to this article. Binary systems have more potential than single star systems to throw things around since there can be plenty of "wobble" for lack of a better term.
The second item is probably the other most common mechanism for hurling planets around. Early in the star system's formation there is a lot of chaos, and orbits are not necessarily even close to being circular (in our system orbits are almost circular. Example: Earth's orbits' eccentricity is only 0.0167) We have already witnessed plenty of those. (Reference: "To date, astronomers have measured the orbital inclinations of 91 exoplanets and more than a third (36) move on orbits that are significantly misaligned, tilted by more than 20 degrees. Nine of them move on retrograde orbits." May 26, 2016 From here.) Two massive planets that closely approach but miss each other can yield a "slingshot" effect due to gravity. Or a planet flung toward it's own star can get gravity assisted from the star.
And yes, VB, I missed the fact that this was a red dwarf, and therefore still in it's main sequence so it could not be the remnant of a red giant.
What can be missed is that in this equation: F = (G * m1 * m2) / r^2 because the radius is in the denominator and it is squared, the force of gravity falls off rather quickly, making escape velocity easier to achieve than one might think. Twice the distance = one quarter of the gravitational force. (see graph below)
Lastly, with regard to the capture of a planet. I understand that due to the vastness of space, the likelihood may be small, but then again we have roughly three billion stars in the Milky Way. The Milky Way is a mere 100,000 light years across, and it is relatively flat. I could show the math, but it would be tortuous for some, so here: The true stellar density near the Sun is estimated as 0.004 stars per cubic light year.
I looked up rogue planets, and at a minimum, estimates say there are two Jupiter sized planets for every star. Statistically, over billions of years, thats still a lot of chances for planet capture. (given that approach velocities of the crossing paths would need to be nearly perfect).
Escape velocity starts at the surface, but drops off quickly.
Excellent post LoW, interested in your stats though.
My understanding is that the number of stars in the Milky way is a factor of 10 less - usually given as 300 million, or a range of 200-400 million because it's tricky finding exactly how many dim red dwarfs are actually out there. The 0.004 local star density value is stated explicitly in Wikipedia and corresponds to each star having a 'box' of space approximately 6 light years sided in length. (or if you want curves, a circle of space with a radius of 3.9 light years
)As for the estimates for rogue planets - as @night_wtr pointed out, a more recent, detailed survey using much more data downed this 2x planet per star limit to 0.75x and they stated that it was a high limit, as they there was a fair chance that they were also recording micro lensing from Jupiter-sized planets that were still bound to star systems.
Now it's true that the extent of the gravitationally bound extent of the solar system could be viewed as 1 light year in diameter, so that seems to fill that '6 light year box' reasonably well...but objects so far out (a big band of comet-like objects) are really easy to knock out of the system by perturbation - as escape velocity that far out is tiny.
To correctly estimate chances of planet capture you have to really match velocities, as you've stated and that's the thing I think that makes such capture so unlikely. On top of this also you have to provide a mechanism for the rogue planet to lose any excess angular momentum that it may bring etc. as the chances of it coming in exactly as it should to just take it's place in orbit are should be in practical terms zero.
Essentially it means that the rogue planet coming in has to hit a very specific spot, hence extremely tiny, for the velocity that it's carrying, and also interact with a capture mechanism in situ at the correct place that will allow it to be bound. I don't know how one would go about statistically calculating this, but I'm sure this figure for 'normal' situations must be astronomically tiny. Possibly lots of chances of it perhaps happening (loads of 'fly-bys'), but virtually no chance of rogue planets actually getting captured. Even given billions of years.
There is one exception - planets should also be able to form on their own in nebula. So if they do so in the vicinity of star systems forming they will have the advantage of being relatively well matched velocity-wise, so they could be potential for more actual capture of rogues in these 'star nurseries'. Also very new planetary discs will be pretty hectic with large numbers of bodies, so again I think there should be more opportunity for these star systems to accommodate outsiders via gravitational mechanics.
Something else we might add to the discussion is brown dwarfs, which are proving to be surprisingly common - but difficult to detect.
In simple terms, they are somewhere between a gas giant planet and a small star, with the caveat that they don't have enough mass for fusion to take place in their core - which makes them very dim and very difficult to detect.
However, in the past few years we've been able to detect them more easily, including a binary brown dwarf just over 6 light years away - and now the 3rd nearest (detected!) star system to Earth: Luhman 16 - Wikipedia
The survey that looked for Rogues (the one giving 0.75x rogues per star in the galaxy, mentioned above) should have picked up brown dwarfs as part of the 'Jupiter-sized objects' (Brown dwarfs should be close in volume/radius to Jupiter), given that they used Gravitational Lensing to make the calculation and I'd guess that the larger the mass, the easier it would be to detect the lensing effect. However, I can't see if they made any comment on that, so perhaps they weren't looking at getting data on brown dwarfs??? I would have to research and read up!
There is actually quite a debate as to if brown dwarfs are common - surveys in 2012 stated that there were surprisingly few - maybe only one brown dwarf for each six main sequence stars: NASA - WISE Finds Few Brown Dwarfs Close To Home.
This is a bit counterintuitive for me, I'd think there should be more brown dwarfs lying about, but apparently in our vicinity it's just not the case!
However one should be careful with such a small sample - another more recent paper: How Many Brown Dwarfs in the Milky Way? suggests a bigger proportion of 2 MS stars to each BD. The other interesting point in this work is that the figure they arrive at is for 'high mass' brown dwarfs, so we still have to add in the BD's that are lower in mass than 0.03 stellar masses and higher than 13 Jupiter masses.
Could be masses of interesting stuff lying out there in the dark
.
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LordOfWizards
Well Known Rememberer
Quickly (I'm on my lunch break) This is from Wikipedia: "The Milky Way is a barred spiral galaxy with a diameter between 100,000 and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars." Here is the link: Milky Way
But if you ask google directly it says 250 Billion + or - 150 Billion.
Duh, <hits head> Of course! I had a bit of a nomenclature failure at one unfortunate spot in my old notes, out by a factor of 1000
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night_wrtr
Non-human Protagonist
Just wanted to step back into this comment because it is mind blowing all on its own when thinking on the vastness of space. Speaking on the power of gravity, even though two galaxies can merge without direct collisions, it will totally destroy their shape and structure. New stars form at a higher rate burning out its resources and will set other stars into a different motion, throwing many out of the galaxy altogether.
It would be spectacular - loads of gas and dust clouds coming together and being contorted and compressed, would start a wave of new star formation all over the place.
But then some parts would likely be shed, as you say. I was reading that some one had calculated there was a 12% probability* that the sun would be swept out of the resultant mega-galaxy completely. Not in a destructive way, just sort of slowly 'bumped' out. But there was also quite a strong chance we might be dragged into the centre...
Which would be bad, because it's the centre where the real 'light-show' would take place. Eventually, no matter what, the two giant black holes at the centres of each galaxy will eventually merge (possibly the only guaranteed 'collision'!) that will release a catastrophic amount of energy, possibly making the new galaxy a Quasar.
Of course this is 4 billion years in the future - by this stage isn't Sol expected to be a red giant and to have fried Earth to a crisp?
Should give us plenty of time to come up with a plan B.
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* The mind boggles at how one could come up with a firm figure for that scenario!
night_wrtr
Non-human Protagonist
The sun will already be too hot and will have evaporated all the oceans long before it gets there. So we probably only have a billion years left in the habitable zone to figure this out. Maybe we will have a nice set of condos on the shores of Pluto by then.
We might want to start working on this now. Someone put on a pot of coffee, please.
Interestingly enough, it may simply be that we haven't yet spotted them - the Bad Astronomy blog suggests that we may yet find one or more even closer than Proxima Centauri: Are we overlooking a lot of nearby brown dwarfs?
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