How often do we get close passages?

[Tom Mazanec]

Gliese 710 is now expected to miss the solar system by 13,000 AU about the year 1,352,000 AD https://en.wikipedia.org/wiki/Gliese_710
How often does a star come this close to the sun, per billion years?

[antoniseb]

I'm going to guess about once every million years.
If you want a more precise number, tell us what the lower bound for what you'd call a star (15 Jupiter masses?), and do you mean exactly 13,000 AU as the threshold?
Once we have those numbers, the number you ask for should be easy to calculate.

[Tom Mazanec]

Let's cut it at the "classical" red dwarf limit (0.08 solar masses) and have it 13,000 AU or closer.

[kzb]

There was a list of close approaches published just a few years ago. I remember linking to it on here.

However, the closest approaches in that article were in the light-year range, I think there is one due in about 35,000 years or so.

I looked at the Wikipedia link where it says Gliese 710 will approach to 0.2 light years.

But the reference given on that wiki page does not say this as far as I can see. So where has this information come from?

It also says there is a more-than-zero probability of it approaching to 1000 AU !

Anyhow, this GL 710 approach is much closer than in the list I referred to.

I have also linked in the past a paper which gave estimates for close stellar encounter rates in different regions of the galaxy and in globular clusters.

The upshot was that stellar encounters in the central few kpc of the galaxy and in globular clusters are too frequent to allow complex life to evolve in those regions.

[kzb]

Gliese 710 is now expected to miss the solar system by 13,000 AU about the year 1,352,000 AD https://en.wikipedia.org/wiki/Gliese_710
How often does a star come this close to the sun, per billion years?

It's about once every 1 or 2 billion years.

t = 3.3E+07 years * (100 pc^-3/rho) * (mean relative speed of objects km/s) * (1000 AU/r) * (Msun/Mtotal)

rho is the stellar density (0.12/pc^-3 in our locality)

r is the encounter distance, in this case 13000 AU.

The last term is the ratio of the sun's mass to the sum of the masses of the two stars.

The one question I have is the mean relative speed of objects. I have assumed 10 km/s as an order of mag but others may know better.

[grant hutchison]

Twenty years ago, based on Hipparcos data, Sanchez et al. set a lower bound of 4.2 * D^2.02/Myr, where D is the distance of closest approach in parsecs. Since 13000AU = 0.063pc, that comes to 0.016/Myr = 16 per billion years. They expected more because Hip was incomplete for low mass stars.

Grant Hutchison

[George]

Twenty years ago, based on Hipparcos data, Sanchez et al. set a lower bound of 4.2 * D^2.02/Myr, where D is the distance of closest approach in parsecs. Since 13000AU = 0.063pc, that comes to 0.016/Myr = 16 per billion years. They expected more because Hip was incomplete for low mass stars.

Nice find, Grant. [That paper shows a slightly different equation (3.5D^2.12/Myr).] I assume the extrapolation to a billion years would be fairly speculative, but the best available.

[grant hutchison]

[That paper shows a slightly different equation (3.5D^2.12/Myr).]

That's interesting. The preprint in my file drawer has the equation I gave. They must have responded to reviewer feedback before publication. The revision takes it down to 10 per billion.

Grant Hutchison

[Tom Mazanec]

This is another star I look forward to seeing in the GAIA release coming up. See if the estimated distance gets any closer.

[kzb]

<deleted post>

[kzb]

Whichever equation you believe, it does seem that encounters similar to this are not uncommon.

Travel between stellar systems that are 0.2 light years apart is less difficult than systems 5 light years apart.

As I've said before on here, this is an argument against that proposed solution to the Fermi paradox, that interstellar travel is just too difficult.

[selden]

FWIW, "Scholz’s star" (WISE J072003.20-084651.2) probably came within 52,000 AU about 70,000 years ago.
See http://www.rochester.edu/newscenter/scholz-star/ and

The Closest Known Flyby of a Star to the Solar System
Eric E. Mamajek, et al.
https://arxiv.org/abs/1502.04655

That paper estimates passages within 0.25 pc happen at a rate of about 0.1/Myr.

[George]

The Closest Known Flyby of a Star to the Solar System
Eric E. Mamajek, et al.
https://arxiv.org/abs/1502.04655

That paper estimates passages within 0.25 pc happen at a rate of about 0.1/Myr.

I think that estimate comes from the same folks (Garcia-Sanchez et al) Grant references, but in an updated 2001 paper. [I failed to snag it.]

[selden]

I think that estimate comes from the same folks (Garcia-Sanchez et al) Grant references, but in an updated 2001 paper. [I failed to snag it.]

You're right. Sorry for my sloppy attribution.

[George]

You're right. Sorry for my sloppy attribution.

Just a nit.

But it's interesting that there seems to be a significant change in their earlier equation. I'm not that great at finding articles and I had no luck in my novice attempt to find this newer one. Perhaps someone will find this 2001 paper so we can update the equation.

[selden]

The full text of the published paper is behind The American Astronomical Society's paywall, which probably is why you couldn't find it by a Web search.

If you have access to a library which subscribes to the journal, you should be able to get it for free from them.

STELLAR ENCOUNTERS WITH THE OORT CLOUD BASED ON HIPPARCOS DATA
JOAN GARCI�A-SA�NCHEZ, et.al.

(c) 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.
Received 1998 May 15 ; accepted 1998 September 4

However, its abstract is available for free at http://adsabs.harvard.edu/abs/1999AJ....117.1042G and says

We find that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from the Sun is given by N = 3.5D^2.12 Myr^-1, less than the number predicted by a simple stellar dynamics model.

I also found a couple of copies of the preprint. This one has handwritten figure numbers, so it's probably the oldest: http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

They say

We find that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from t h e Sun is given by N = 4.2 D^2.02M yr^-1, less than the numbers predicted by simple stellar dynamics models.

Note that "numbers" and "models" were changed, too, from plural to singular.