How Come The Oort Cloud Isn't Torn Away From Our Sun By Nearby Stars?

If the Oort Cloud is three light years away from our Sun, then it’s closer to Alpha Centauri than our Sun, right? So how can it stay around our Sun if the mass of Alpha Centauri is 1.1 times the mass of our Sun - wouldn’t the gravity of Alpha Centauri rip it away?
An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. Image credit: NASA

An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. Image credit: NASA

Originally posted on Forbes!

The Oort cloud is an interesting feature of our solar system; a nebulous, spherical cloud of comets which marks the very outer limit of our solar system. The Oort cloud is also the source of our long period comets - those icy fragments of the early solar system which orbit our Sun very infrequently. To be classified as a long period comet, more than 200 years must pass between trips near the Sun. Hale-Bopp is probably the most well known of these, as it was visible to the naked eye for a long time in 1998. A more recent visitor was the Lovejoy comet, which swung near the Sun in 2011.

The Oort cloud is very far from the Sun. It is outside the bubble produced by our Sun’s solar wind and magnetic field by a considerable distance. While Voyager 1 has left this magnetic bubble, and entered what is called “interstellar space”, it has several hundred more years of traveling before it even reaches the inner edge of the Oort cloud. How is part of the solar system in interstellar space? Well, this means that the solar system at such a large distance from the Sun is not entirely ruled by our own star - the presence of other stars is mixing with the influence of our Sun.

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU. Image credit: NASA/JPL-Caltech

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU. Image credit: NASA/JPL-Caltech

The inner edge of the Oort cloud is typically quoted as beginning at somewhere between 1,000 and 5,000 au from the Sun. 5,000 au is about 0.08 light years away from the Sun, which is a little over four weeks of travel time for a beam of light, and considerably closer to our Sun than to Proxima Centauri, the closest star. These Oort cloud objects at the inner edge of their cloud are fairly reasonably more attached to our Sun than they are to anything else, and there are a lot of them here.

As we travel from the inner Oort cloud to the outer region, we should note that the Oort cloud is not an even assembly of objects, from some inner bound to a fixed outer bound. Instead, while there is something of an inner boundary, the outer boundary is more of a fizzling out, with objects getting fewer and farther between as you go farther and farther from the Sun. This means that the “outer boundary” is a very tricky thing to attach a number to. How many objects need to be out there to still count as part of the Oort cloud? Just one? Or do we need a higher density of objects before we’re dropping our delineation down? As a result of this fuzziness, plus the fact that it’s very hard to spot Oort cloud objects in the first place, estimates of the outer bound of the Oort cloud range from 50,000 to 200,000 au. It’s that 200,000 au that works out to 3.1 light years away from our Sun. NASA often quotes this outer edge as sitting at 100,000 au, which is about 1.6 light years, which means that this fuzzy “edge” is extending less than half the way out to Alpha Centauri.

Comet Lovejoy is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Image credit: NASA

Comet Lovejoy is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Image credit: NASA

All these numbers are for a sense of scale. In actual fact, the Oort cloud is incredibly sensitive to gravitational forces from objects other than our Sun. One of these is a very large-scale gravitational inequality; our solar system is not at the center of the Milky Way galaxy. The gravitational pull from our Galaxy is therefore stronger on one side of the solar system than it is on the other, and this galactic tide is enough to gradually jostle the Oort cloud. This kind of perturbation is part of how we think we get the long period comets, which can come blazing into the inner solar system, and, if they are unlucky, sometimes completely evaporated by the Sun.

The Oort cloud is also sensitive to the motions of other stars nearby in the Galaxy, and other extrasolar objects, like clouds of gas. As stars pass nearby (or through) the outer reaches of the Oort cloud, they will disturb the delicate gravitational balance that keeps these objects in their long, distant orbits. Stars aren’t likely to smash directly into a comet out there, but they might jostle it out of its orbit, and send it down into the inner solar system - another way of getting comets into the rest of the solar system.

Comet Hale-Bopp. Alex Krainov shot this image at Zabriskie Point in Death Valley in April 1997. Image credit: Alex Krainov, CC BY-SA 3.0

Comet Hale-Bopp. Alex Krainov shot this image at Zabriskie Point in Death Valley in April 1997. Image credit: Alex Krainov, CC BY-SA 3.0

But these perturbing stars are in motion too, and they will pass through relatively quickly, on an astronomical timescale. Alpha Centauri is still arriving into the solar neighborhood, and isn't yet close enough to do much influencing. With the combination of the fading density of objects, the short time frame with which a star will be close enough to really dramatically pull on the objects sitting out there, and the length of time between stellar close passes being quite long, we don't expect the Oort cloud to have been stripped away from our star. But it is absolutely influenced by the presence of those stars, and by the Galaxy at large, and our long, once-a-millennia comets like Hale-Bopp are the result.

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