Close encounters of the stellar kind

Close encounters of the stellar kind

The movements of more than 300 000 stars surveyed by ESA's Gaia satellite reveal that rare close encounters with our sun might disturb the cloud of comets at the far reaches of our solar system, sending some towards Earth in the distant future.

As the solar system moves through the galaxy, and as other move on their own paths, close encounters are inevitable – though 'close' still means many trillions of kilometres.

A star, depending on its mass and speed, would need to get within about 60 trillion kilometres before it starts to have an effect on the solar system's distant reservoir of comets, the Oort Cloud, which is thought to extend out to 15 trillion kilometres from the sun, 100 000 times the sun–Earth distance.

For comparison, the outermost planet Neptune orbits at an average distance of about 4.5 billion kilometres, or 30 sun–Earth distances.

The of stars that pass near the Oort Cloud could perturb the paths of comets residing there, jolting them onto orbits that bring them in to the inner solar system.

While this is thought to be responsible for some of the comets that appear in our skies every hundred to thousand , it also has the potential to put comets on a collision course with Earth or other planets.

Understanding the past and motions of stars is a key goal of Gaia as it collects precise data on stellar positions and motions over its five-year mission. After 14 months, the first catalogue of more than a billion stars was recently released, which included the distances and the motions across the sky for more than two million stars.

Waiting for a stellar encounter. Credit: ESA/Gaia/DPAC

By combining the new results with existing information, astronomers began a detailed, large-scale search for stars passing close to our sun.

So far, the motions relative to the sun of more than 300 000 stars have been traced through the galaxy and their closest approach determined for up to five million years in the past and future.

Of them, 97 stars were found that will pass within 150 trillion kilometres, while 16 come within about 60 trillion km.

While the 16 are considered reasonably near, a particularly close encounter of one star, Gliese 710, in 1.3 million years' time, stands out. It is predicted to pass within just 2.3 trillion km or about 16 000 Earth–sun distances, well within the Oort Cloud.

The star is already well-documented, and thanks to the Gaia data, the estimated encounter has recently been revised. Previously, there was a 90% degree of certainty that it would come within 3.1–13.6 trillion kilometres. Now, the more accurate data suggest that it will come within 1.5–3.2 trillion km, with 2.3 trillion km most likely.

Tracking stellar motions. Credit: ESA/Gaia/DPAC

Furthermore, although Gliese 710 has a mass of 60% that of our sun, it travels much slower than most stars: nearly 50 000 km/h at closest approach, compared with the average 100 000 km/h.

The speed of its passage means it will have plenty of time to exert its gravitational influence on bodies in the Oort Cloud, potentially sending showers of comets into the solar system.

Despite its slower pace, it will still appear as the brightest, fastest object in the night sky at .

Importantly, the latest study used Gaia measurements to make a general estimate of the rate of stellar encounters, taking into account uncertainties such as stars that might not have been observable in the existing catalogue.

For 5 million years in the past and into the future, the overall encounter rate is estimated to be around 550 stars per million years coming within 150 trillion km, of which about 20 would come closer than 30 trillion km.

That equates to about one potential 'close' every 50 000 years or so. It is important to note that it is not guaranteed that a star would actually perturb any comets such that they entered the inner regions of the solar system, and even if they did, if Earth would be in the firing line.

These estimates will be refined with future Gaia data releases. The second is scheduled for next April, containing the information for about 20 times as many stars, and many more distant stars as well, allowing reconstructions up to 25 million years into the past and future.

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Aug 31, 2017
"That equates to about one potential 'close' encounter every 50 000 years or so."

That is 20 stellar "close encounters" per million years each "closer than 30 trillion km," i.e., less than 3.2 light years away. It is hard to envision a scenario where the roughly 90,000 close encounters by stars over the last 4.5 billion years have not completely reworked the Oort Cloud many times over. It is time to toss the idea of the outer solar system being pristine out the window.

If this analysis is correct, it means that as close as the Alpha Centauri system is now, over geological timescales, there have been ~90,000 stars that were even closer. We are beginning to see just how dynamic the outer solar system really is.

Aug 31, 2017
One light year is about 9.5 trillion km. So 60 trillion km is about 7 light years. There are currently several star systems(alpha centauri, etc.) within that distance of the sun right now, and since most stars passing that close are on basically the same galactic orbits they will remain that close for potentially hundreds of thousands of years (they are traveling in the same direction and speed, basically, as the sun).

The numbers in this article just do not make sense to me. Can someone see anything wrong with my calculations?

Aug 31, 2017
Parsec, I think you have it basically correct, so I am not sure where the confusion lies. While some stars may linger nearby for tens of thousands of years or more, others approach and recede in a few thousand years or so. If you look at the chart under the "Future and Past" heading on Wikipedia entry for nearest stars, you will note that Alpha Centauri will be the closest star system on and off for another 80,000 years and that is actually on the long side compared to the other stars listed there. Barnard's Star, Ross 248 and Gliese 445 will pass by far more quickly during that time.


Obviously, we don't have the full picture yet, but some of the broad strokes are becoming more clear. If you are interested, you might want to read about the very close encounter with Scholz's Star at 0.8 light years away only 70,000 years ago.


Sep 01, 2017
Yeah, the Scholz's Star pass is interesting in several ways. Other than the Toba Supervolcano Caldera, which seems 'natural', there's a marked lack of known mega-craters from that era. So, Scholz's Star's Oort Cloud objects probably missed us. Of course, if the pass stirred our Oort Cloud, they'd still be in-bound...

A joker-- Simulation of Saturn's rings suggests they're a fairly recent phenomenon, rather than 'primordial'. What if simulation of Jupiter's Great Red Spot suggests that has only been around for, oh, 50~100k years ??

Sep 01, 2017
Parsec, I think you are assuming that stars in the same neighborhood would travel in the same direction and speed.

Instead of using the concept 'neighborhood'. Think about the stars contained within the Galactic Disk as a busy harbor.

With a multitude of boats. All traveling in different directions, at different speeds, for different purposes. These are all contained within the same body of water and there are traffic lanes and rules but a complete lack of coordination.

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