Earth’s comet apocalypse at the hands of Gliese 710

Every so often we’re told that a rogue star or planet known as Niblick or Niburu, or some similar gibberish, is about to cause the apocalypse. Earth’s going to be whacked in 1984, 1987, 1999, 2012, 2018, apparently, with all the effect of a nine-iron on a golf ball, and NASA are hiding the truth from us.

Personally I don’t take this rubbish seriously for a moment. I do science. But you know the weird thing? Real science is able to tell us when ‘intruding star doom’ will hit for real. The star likely to do it is Gliese 710. We also know when it will do it.

The Oort Cloud, which extends up to 1 light year beyond the Sun. NASA, public domain.
Our solar system’s Oort Cloud, which extends up to 1 light year beyond the Sun. NASA, public domain.

Gliese 710 is a K7vk class star, smaller than the Sun and, just now, 64 light years distant. But in 1.36 million years it’ll be closer. A lot closer – slashing past our solar system just 77 light-days distant, which is around 13,332 times the distance of Earth from the Sun (and about 1,994,000,000,000 km in everyday measure). This puts it on course to slam straight through our Oort Cloud, which is a residual outcome of our solar system’s formation, first identified by Jan Hendrick Oort (1900-1992), and which contains most of the comets. Because of error margins in the calculation, Gliese 710 might get as close as 40 light-days. And there’s a 1-in-10,000 chance that it’ll get within 5.77 light days (1,000 times Earth’s distance from the Sun), which is close enough to disrupt the Kuiper Belt.

This discovery is new. Gliese 710 was always known to be coming closer than any other star in the last few million years, but exactly how close had been debated. All stars move relative to each other – they’re in different orbits around the central galactic black hole. As viewed from Earth they move in two ways: they have a ‘proper motion’, which means left, right, up or down relative to other stars in the sky. And they also have a ‘radial velocity’, which is forward or back relative to Earth. However, the distances between stars are so vast by human standards, and the relative speeds so low against those distances, that against a human lifespan most stars never shift. The exception is Barnard’s Star, which has a huge proper motion of 10.3 arc seconds a year. To put that in perspective, the average width of the full moon, from Earth, is 1860 arc seconds. So it takes Barnard’s Star 180.5 years to move the equivalent distance across our sky as the distance between eastern and western edges of the Moon. That’s racing by stellar standards.

As you can imagine, measuring the proper motion of most stars becomes a painstaking exercise with delicate instruments. Think of it this way: if you’re standing in one lane of a multi-lane highway, vehicles in other lanes are going to whip past – left or right – because they’re moving sideways relative to you. That’s proper motion. However, a vehicle coming directly towards you won’t move much left or right, even though it’s perhaps moving as fast as the ones whipping past. But it will get bigger and bigger and – yah. The difficulty is that it’s harder to figure out the exact details of its vector than something that’s conveniently moving sideways. And that’s the problem with Gliese 710. It has almost no proper motion. But it has a high radial velocity of -13.8 km/second – and the negative value means it’s coming directly for us. The problem, as I say, is honing the exact details.

The closest stars to us just now. NASA, public domain.
The closest stars to us just now (remembering that astronomers keep buckets by their desks filled with zeroes). NASA, public domain.

Just how close it would get was never fully pinned down until last October when a paper by Filip Berski and Piotr Dybczyński in Astronomy and Astrophysics highlighted the latest calculations, based on data collected by the ESA’s Gaia-DR1 satellite, which has the task of mapping stars with unprecedented accuracy. Naturally that carries uncertainties – smaller than before, but still giving a range, which is why the figure for Gliese 710’s close pass is a range of probabilities. But that’s how science works. And we’re a lot more certain about what that range is than we were before. It isn’t great news for Earth from either end.

With a mass about 60 percent that of the Sun, Gliese 710 will likely have a visible brightness from Earth of -2.7 magnitude at closest approach, about the same as Jupiter. It won’t dislodge our major planets, and in all probability our Sun won’t dislodge the orbits of any planets orbiting Gliese 710. But the intruding star will do a handy job of knocking Oort cloud iceballs – comets – around. Gliese 710 won’t plunge through like a bullet, either – it and the Sun will be swung around their mutual centre of gravity, so it’ll curve around us and cut a swathe through the Oort cloud. About half the comets it dislodges will be accelerated and flung away. But about half will be slowed and fall towards our inner solar system. Up to 10 comets a year are expected to enter our planetary system for a period of three to four million years – long after Gliese 710 has ceased to have any direct effect.

That long shower occurs because, despite the proximity by usual stellar distances, the Oort Cloud is still way distant in human terms, and comets dislodged from their orbits can take hundreds of thousands of years, even millions, to fall towards the Sun. The swathe Gliese 710 cuts will throw comets into all sorts of orbits, meaning some will take a very long time indeed to arrive, others less so. Needless to say, our Sun will do the same thing to Gliese 710’s equivalent Oort Cloud, and that might send comets our way too. There’s also the possibility of Gliese 710’s Oort Cloud intersecting the planetary region of our solar system. No such cloud has been observed around it yet, but given the way stars form, it likely has one.

Some of the rain of comets that will follow have a chance of hitting Earth and devastating it – much as the Chixuclub impactor 65 million years ago did. But luckily for us, Gliese 710’s close-pass won’t be for 1.36 million years yet. I expect humans won’t be around to worry about it; and if we are, we’ll briefly look up from our caves in wonder at the new lights in the sky before returning to the endless task of finding enough food to eat.

Copyright © Matthew Wright 2017

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3 thoughts on “Earth’s comet apocalypse at the hands of Gliese 710

  1. Thanks for the warning. Headline writers do seem to take a lot of glee from saying ‘Comet Whatever will be the closest encounter in history’ before telling us in the small print it’ll be a trillion miles away.

    1. Yes, astronomical doom is always well heralded and isn’t quite as dramatic as the tabloids would like to think. It is also not on a tabloid timescale. Neither you nor I will be around when the comets kicked up by Gliese 710 arrive (though the rock band in your novels might be, being vampires… ☺)

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