Now We Know When Stars Will Be Passing Through the Oort Cloud

To our Solar System, “close-encounters” with other stars happen regularly – the last occurring some 70,000 years ago and the next likely to take place 240,000 to 470,000 years from now. While this might sound like a “few and far between” kind of thing, it is quite regular in cosmological terms. Understanding when these encounters will happen is also important since they are known to cause disturbances in the Oort Cloud, sending comets towards Earth.

Thanks to a new study by Coryne Bailer-Jones, a researcher from the Max Planck Institute for Astronomy, astronomers now have refined estimates on when the next close-encounters will be happening. After consulting data from the ESA’s Gaia spacecraft, he concluded that over the course of the next 5 million years, that the Solar System can expect 16 close encounters, and one particularly close one!

For the sake of the study – which recently appeared in the journal Astronomy & Astrophysics under the title “The Completeness-Corrected Rate of Stellar Encounters with the Sun From the First Gaia Data Release” – Dr. Bailer Jones used Gaia data to track the movements of more than 300,000 stars in our galaxy to see if they would ever pass close enough to the Solar System to cause a disturbance.

As noted, these types of disturbances have happened many times throughout the history of the Solar System. In order to dislodge icy objects from their orbit in the Oort Cloud – which extends out to about 15 trillion km (100,000 AU) from our Sun – and send them hurling into the inner Solar System, it is estimated that a star would need to pass within 60 trillion km (37 trillion mi; 400,000 AU) of our Sun.

One of the goals of the Gaia mission, which launched back in 2013, was to collect precise data on stellar positions and motions over the course of its five-year mission. After 14 months in space, the first catalogue was released, which contained information on more than a billion stars. This catalogue also contained the distances and motions across the sky of over two million stars.

By combining this new data with existing information, Dr. Bailer-Jones was able to calculate the motions of some 300,000 stars relative to the Sun over a five million year period. From this, he determined that 97 of these stars will pass within 150 trillion km (93 trillion mi; 1 million AU), while 16 would come within 60 trillion km. While this would be close enough to disturb the Oort Cloud, only one star would get particularly close.

That star is Gliese 710, a K-type yellow dwarf located about 63 light years from Earth which is about half the size of our Sun. According to Dr. Bailer-Jones’ study, this star will pass by our Solar System in 1.3 million years, and at a distance of just 2.3 trillion km (1.4 trillion mi; 16 ,000AU). This will place it well within the Oort Cloud, and will likely turn many icy planetesimals into long-period comets that could head towards Earth.

What’s more, Gliese 570 has a relatively slow velocity compared to other stars in our galaxy. Basically, it moves at a speed of 50,000 km/h (31,000 mph), compared to the average speed of 100,000 km/h (62,000 mph). As a result, Gliese 570 will have plenty of time to exert its gravitational influence on the Oort Cloud, which could potentially send many, many comets towards Earth and the inner Solar System.

Over the past few decades, this star has been well-documented by astronomers, and they were already pretty certain that it would experience a close encounter with our Solar System in the future. However, previous calculations indicated that it would pass within 3.1 to 13.6 trillion km (1.9 to 8.45 trillion mi; 20,722 to 90,910 AU) from our star system – and with a 90% certainty.

Thanks to Dr. Bailer-Jones study, these estimates have been refined to 1.5–3.2 trillion km, with 2.3 trillion km being the most likely. His study has also allowed for a general estimate of the rate of stellar encounters over the past 5 million years, and for the next 5 million. He determined that the overall rate is about 550 stars per million years coming within 150 trillion km, and about 20 coming closer than 30 trillion km.

This works out to about one potential close encounter every 50,000 years or so. Again, while this might sound like a long time, in terms of the astronomical history, its a regular occurrence. And while not every close encounter is guaranteed to send comets hurling our way, understanding when and how these encounters have happened is intrinsic to understanding the history and evolution of our Solar System.

Understanding when a close encounters might happen next is also vital. Assuming we are still around when another  takes place, knowing when it is likely to happen could allow us to prepare for the worst – i.e. if a comets is set on a collision course with Earth! Failing that, humanity could use this information to prepare a scientific mission to study the comets that are sent our way.

The second release of Gaia data is scheduled for next April, and will contain information on an estimated 20 billion stars. That’s 20 times as many stars as the first catalogue, and between 5% and 10% the total number of stars within the Milky Way Galaxy. The second catalog will also include information on much more distant stars, will which allow for reconstructions of up to 25 million years into the past and future.

With every new release, estimates on the movements of the galaxy’s stars (and the potential for close encounters) will be refined further. It will also help us to chart when major comet activity took place within the Solar System, and how this might have played a role in the evolution of the planets and life itself.

Further Reading: ESA

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