Discussing Gliese 710's 86% Chance of Hitting the Solar System

[Andre]

Searching did not produce an older thread about this interesting star, so maybe we can discuss it a bit.

Here we read:

Based on proper motion and more recent radial velocity data from the HIPPARCOS space satellite mission, Gliese 710 is expected to come within 1.1 light-years (0.34 pc) of Sol in less than 1.4 million years, but astronomers do not expect it to "perturb the Solar System's Oort Cloud sufficiently to create a substantial increase in the long-period comet flux at Earth's orbit" (García-Sánchez et al, 1999; and Weissman et al, 1997).

Here however we read:

In fact, this could produce a swarm of comets heading towards the inner Solar System. It might even be pretty for any inhabitants of the Solar System then, as you might expect a spectacular comet every couple years. There might even be almost always a naked eye comet somewhere in the sky. Very pretty indeed. Until one of them runs into us.

and then we have http://www.technologyreview.com/blog/arxiv/24917/

What the new data has allowed Bobylev to do is calculate the probability of Gliese 710 smashing into the Solar System. What he's found is a shock.

He says there is 86 percent chance that Gliese 710 will plough through the Oort Cloud of frozen stuff that extends some 0.5 parsecs into space.

That may sound like a graze but it is likely to have serious consequences. Such an approach would send an almighty shower of comets into the Solar System which will force us to keep our heads down for a while. And a probability of 86 percent is about as close to certainty as this kind of data can get.

So my questions:
What is hype and what is realistic?

Would such an event give reason to revisit the Nemesis hypothesis of Richard Muller about extinction events from the geologic past?

 

[Vanadium 50]

From http://arxiv.org/abs/1003.2160" : "This star also has a nonzero probability, 0.0001, of falling into the region d<1000 AU, where its influence on Kuiper Belt objects becomes possible. "

 

[eachus]

From http://arxiv.org/abs/1003.2160" : "This star also has a nonzero probability, 0.0001, of falling into the region d<1000 AU, where its influence on Kuiper Belt objects becomes possible. "

First, let me point out what the paper did NOT do. It ignored the gravitational attraction of the sun and the target star, and in fact, the gravitational effects of every star in the vicinity of the solar system.

This is where the probabilities come in. The motions of the stars are not random, but doing a proper simulation requires a lot of data that doesn't exist yet (dark matter in the solar neighborhood for one), and a lot of computer time. (At a minimum, hundreds of objects and millions of years.) Even with a good oct-tree implementation, and good initial data, you are probably looking at thousands of CPU-hours.

What is the probability that given better data and a good simulation, Gliese 710 will not pass close to the solar system? Pretty good. Of course, there are other stars moving in our direction that could come even closer. :-(

Is the modelling exercise worth doing? Sure, but it is probably a lifetime project. The first iteration will probably be little better than rolling dice. (Dark matter again.) But in 20-30 years, the data should be good enough to answer questions like: Was Sedna captured from another solar system? Will it be lost to some other star? I have to suspect that most Kupier belt objects are home grown, but that Öort cloud residents are very likely to swap stars over a period of say 100 million years.

 

[ideasrule]

I tried doing a back-of-the-envelope calculation of how much the Sun would perturb Gliese 710's path. It turned out to be 0.02 light years. Not much if we're talking about coming within 1.1 light years.

 

[eachus]

I tried doing a back-of-the-envelope calculation of how much the Sun would perturb Gliese 710's path. It turned out to be 0.02 light years. Not much if we're talking about coming within 1.1 light years.

Not the problem. Gliese 710 is currently 1.6 light years away from Eta Serpentis and 3.2 light years from BD-01 3500. Will it get closer to either one? Don't know, I'd have to do the math. Are there other stars that will come close to Gliese 710 before it reaches the sun? Sirius? Alpha Centuri? You need the simulation for that.

What about dark matter? We know that there is some in the area, there is a huge project you can sign up for: MilkyWay@home, http://milkyway.cs.rpi.edu/milkyway/ which is doing modeling and regression to get a map of the dark matter in the sun's general area. (Technically for the whole Milky Way, but the model will be more precise near the solar system.)

There may be more dark matter within the 100 parsecs of the solar system than the mass of all the stars in the same volume. Not sure, but that's the way to bet. Are some of the dense star forming areas pulling in dark matter gravitationally, or is it hotter than that? can't even guess at this point.

That's why I think that you may need to wait a few years to get good input data to the simulation. Of course, assuming that the dark matter is diffuse but not homogenous, even oct-trees won't help much in the calculations. In a decade or two though, we should all have access to much faster processors. ;-)

 

[Jon Richfield]

Thanks for the discussion folks, but one item nonplusses me just one damned big bit. What is all this about the gravitational effect of dark matter? Why should it be significant? As I understand it, DM is pretty diffuse and we and G710 both are largely within it anyway. It is not as though the dark matter occurred in the form of lot of large black holes, right? As I see it, its gravitational effect on G710 and the whole solar system should be pretty well identical. Not that I have been out measuring it, nor even spending nights calculating it, you understand, but it seems to me that to steer one of us at the other, would require relatively strong tidal effects, by which I mean that one of us would be affected much more strongly than the other. After all, at a separation of a few tens of parsecs there would be very little difference in the effect on our trajectories.

Why am I wrong to assume that, even of all the speculation about the existence of dark matter turns out to be worst-case correct, the effect on our collision probabilities would be trivial?
This field is far from my competencies, so I am neither denying nor asserting anything, but all contributions gratefully...
Cheers,

Jon

 

[eachus]

Thanks for the discussion folks, but one item nonplusses me just one damned big bit. What is all this about the gravitational effect of dark matter? Why should it be significant? As I understand it, DM is pretty diffuse and we and G710 both are largely within it anyway. It is not as though the dark matter occurred in the form of lot of large black holes, right?

If you know that much, you know more than anyone studying it.

I'll try not to get too deep here, but the whole subject is pretty complicated. The only way currently to "see" dark matter is through its gravitational effect on stars and galaxies. So trying to extract a map of dark matter concentrations from observational data requires knowing the non-dark matter gravitational effects pretty well--and for the galaxy modeling even a small portion, and putting in all the known stars (and black holes) is beyond the reach of most supercomputers.

Enough work has been done to show that dark matter is not smoothly distributed, it is lumpy. How lumpy? And more important are there compact objects (dark stars) with stellar masses and sizes? Good bet, but little or no confirming data. Similarly, we know--at least if general relativity is true, that some of the primordial dark matter is now in the black holes at the center of galaxies.* So (at least partially) dark matter black holes exist. More to the point, the way we detect supermassive black holes is from the normal matter falling into them. The way we detect stellar mass black holes is usually due to their having a normal matter stellar companion. No normal matter, no detection.

How can lumps of dark matter in our own galaxy be detected? In theory, analyzing the star field for weak lensing is possible. But you would get a statistical answer--that the distribution of stars and galaxies that we see is consistent--or inconsistent with stellar mass lumps of dark matter in our galaxy. A better, but as I indicated long-term research effort would be to look at the change in lensing as the stars and dark matter move.

Hmmm... If I were not retired, I might write a grant proposal to look at the shifting of the background stellar field as the Earth moves around the sun. The signature of any local stellar mass, and size, dark matter objects should be detectable. Local as within a few thousand light years of the solar system. The distant background stars would be displaced relative to foreground stars, just the opposite of gravitational searches for planets. Anyone who wants to pick up the idea for a PhD thesis, feel free. Just be warned that it will probably turn into a career.


* An interesting question, but not relevant here, is whether there are supermassive black holes with no surrounding regular matter galaxy. There are some known galaxies where the dark matter outweighs the normal matter by a twenty to one or more. Are there some galaxies with no normal matter? Good question, and one that can only be answered by looking at gravitational lensing.

 

[Jon Richfield]

Thanks Eachus.

I was never much of a dark matter enthusiast, but your discussion certainly livened it up. I have not the slightest basis for any informed opinion on the matter, but I have bad feelings about the various WIMPs axions and the like that people have proposed as the major components of dark matter. Accordingly, it is ironic that I should have considered the exotic elementary particle in preference to my own, more unimaginative idea that a simpler (to me) idea would be that the main component could be far more familiar black lumps of solid material with a size range mainly in the range of metres to thousands of kilometres in diameter. That should not affect interstellar light emission much, but there would be room for a lot of gravitationally active material out there and it would explain why most of it was concentrated inside the galaxies.

Oh well, all I have to do is wait a few decades till the powers that pay authorise a bit of hands-on research. (I should live so long!)

Cheers
Jon