Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe. Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
Every once in a while a new comet enters the inner solar system, cruising in from the unfathomable and uncharted depths of space. Typically a mile or two across of ice and dirt, it has so far lived a rather uneventful life, lazily orbiting the sun well beyond its planetary cousins. But now, as it screams inward toward the sun, the comet extends a million-mile-long tail of vented gas and dust as its body begins to rip itself apart from the unexpected forces.
If it's lucky, the comet will end its life quickly, plunging directly into the sun and disintegrating into dust. If it's unlucky, it will survive its first passage through the inner solar system, spreading a trail of debris behind it. And then it will come back again. And again. With every passage, each more torturous than the last, it loses a part of itself, diminishing orbit after orbit until it either evaporates or remains locked in orbit, inert and dead.
Comets live for billions of years in blissful isolation, and we get to see them only when they're close … which means we only track them in their final, tragic moments.
But where are these comets born? Where do they live? How do they find their way to a fiery doom in the heart of the solar system?
To figure it out, it helps that we've had a few millennia of observing comets to draw from. And beginning in the early 1700s we've known that some comets reappear on regular, reliable cycles — thanks to Sir Edmund Halley's genius applications of Newton's then-brand-spanking-new theory of universal gravity. After enough observations it's straightforward enough to assign orbits to those comets and discover their origins, a region we call the Scattered Disk, an unstable ring of debris just outside the orbit of Neptune.
But many comets — known as long-period comets — appear from basically nowhere, flare up as they cross into the inner solar system, then promptly die. Where do those come from?
The major difficulty with studying these comets is that whatever their origin might be, it's so far away that it's downright impossible to observe them in their home environment directly. So we can't rely on deep-space surveys to tell us about their homes. Instead, we have to infer the properties of their cometary birthplace from the behavior of the ill-fated messengers sent our way. And when we do, a few intriguing clues emerge.
First, these long-period comets appear from all directions of the sky. So wherever comets call home is distributed evenly, surrounding the solar system, and not locked into a disk like everybody else.
Second, comets die. They either crash directly into the sun or a planet, have an unlucky interaction with a giant world and get kicked out of the solar system altogether, or end up exhausting their ice, turning off their tails and rendering them essentially undetectable. They may make it for only a single orbit or persist for a few thousand, but either way that's far, far less than the billions of years that the solar system has been a system. So that means when a new long-period comet appears in our skies, it really is a new comet: There's a reservoir of comets well beyond the realm of the planets, and it only occasionally sends an emissary inward.
Lastly, these long-period comets have something in common. Through careful observations astronomers can reconstruct their entire orbits, and find their aphelion — their farthest distance from the sun. And many comets, as first noted by astronomer Jans Oort, share an aphelion around 20,000 AU, or 20,000 times the distance from the sun than the Earth.
A spherical arrangement with a definite thickness that occasionally sends one of its members inward. A shell. A cloud.
The Oort cloud: home to the comets.
Of course, we're not exactly sure how big the Oort cloud is or how many members call it home. To figure it out we rely on computer simulation after computer simulation, taking into account the orbits of the planets, models for the formation of the solar system, and the paths of known comets. Taken together this paints a picture of a tremendous, and tremendously empty, structure, spanning from 2,000 to 200,000 AU and containing upward of a trillion objects at least a mile wide, and countless more.
200,000 AU is quite a staggering distance — that's about 3 light-years away. At that level of remoteness, the comets are almost entirely aloof, just barely attached to our sun through a feeble invisible string of gravity. Because of that weak connection, they feel no need to settle into a ring or disk, and naturally arrange themselves into a shell.
What's more, with the sun's pull so minuscule, the comets are highly susceptible to other, foreign suggestions. A wandering passing star or a giant molecular cloud can exert an extra gravitational tug on them, destabilizing them and sending some scattering outward into the interstellar void … and others careening inward to their eventually doom.
But perhaps the biggest source of influence way out there is none other than the Milky Way galaxy itself. It's a matter of densities: The general arrangement of stars and nebulas on one side of the solar system is a little bit different than at the other. This is called the "galactic tide," because it's the exact same physics — differences in density from one side to the other — that give rise to the ocean tides. Here on Earth, deep within the sun's gravity well, those galactic density differences don't…well, make a difference. But in the Oort cloud they do.
As these comets make their way in their long, slow orbits, they can experience an extra gravitational tug from the galactic tide. When the comet is at aphelion, its farthest point from the sun, it might just get encouraged to move a tiny bit farther out than last time. And the way that orbits work, if the path gets stretched in one direction, it must shrink in the other; in this case, the extra tugging from the galaxy at aphelion ironically brings the comet even closer to the sun as it continues in its orbit.
Eventually the constant tugging will shape the comet's orbit to such extremes that it dips into the inner solar system, where the gravities of the sun and planets further alter its trajectory, sealing its fate.
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Learn more by listening to the episode "What happens when galaxies collide?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Marshall S. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom and on Facebook.