Understanding Your Solar System 2.0
It’s not about planets anymore. In the past decade, astronomers have spotted hundreds of objects in the Kuiper belt, some nearly the size of Pluto; countless more await discovery. Even the distant and far-reaching Oort cloud is coming into scientific focus. Preserved in these nether realms are trillions of rocky, icebound insights into the early days of the sun and the formation of the solar system. In spatial scale and information potential, they are the solar system—past, present, and future.
Inner Solar System: 0–5.2 astronomical units, or AU
One astronomical unit (AU) equals the distance from the sun to the Earth, or about 93 million miles.
The inner solar system spans everything from the sun out to (but not including) Jupiter, 460 million miles away. The solar system as a whole formed roughly 4.5 billion years ago from a collapsing disk of dust and gas. Swarms of boulder-size objects called planetesimals slowly accreted to form the rocky planets—Mercury, Venus, Earth, and Mars—and their various and sundry satellites. The leftovers linger today in the asteroid belt. Farther out, well away from the sun’s warmth, gases like hydrogen and helium failed to vaporize and instead accumulated to form the gaseous outer planets: Jupiter, Saturn, Neptune, and Uranus.
The majority of asteroids occupy an orbit about 210 million miles from the sun, between the orbits of Mars and Jupiter. A smaller group orbits at about 370 million miles; a third group occupies the same orbit (though not the same spot) as Jupiter. At least a hundred asteroids are a mile or more in diameter, and a dozen or so are 200 miles wide. The largest, Ceres, is 600 miles wide.
Kuiper Belt: 30–50 AU
The Kuiper belt is a vast reservoir of icy rocks that extends from about 3 billion miles from the sun, beyond Neptune’s orbit, to about 5 billion miles out. Beyond that, the number of detected objects drops precipitously until one reaches the even more distant Oort cloud. There may be billions of Kuiper belt objects in all, including at least 35,000 that are more than 60 miles wide. Pluto, at 1,430 miles wide, is the largest known member and the only one known to have an atmosphere. Its possession of a moon, Charon, was long thought to set Pluto apart from other denizens of the frozen region, but a dozen Kuiper belt objects are now known to have partners.
Bodies in the Kuiper belt are believed to have formed roughly where they now reside, and many have stayed largely undisturbed in temperatures averaging –375°F. They come in a variety of colors, from gray to dark red, indicating their compositions, but all are thought to be half ice and half rock. On occasion, a gravitational bump from Neptune tosses a Kuiper belt object toward the sun. Then the heated ices sublime, producing a bright halo of glowing gas that trails behind the rock: the coma and the tail of a comet, which fade again as the object retreats past Neptune.
Oort Cloud: 10,000–100,000 AU
The entire solar system is enveloped by the Oort cloud, an enormous sphere of nascent comets extending more than 9 trillion miles into space. The cloud is thought to contain trillions of loose lumps of dust and volatile gases, near-perfect samples of the stuff from which the planets were made.
Dutch astronomer Jan Hendrik Oort first proposed the existence of the cloud in 1950 to explain the funny paths of comets that take more than 200 years to orbit the sun. This model suggests that the cloud’s contents started out closer by, near Jupiter and the other gas giants, until gravitational encounters with those planets knocked the primordial bits into the hinterlands. Barely 5 percent of that original material remains in the Oort cloud; the rest was ejected entirely from the solar system.
Over time, the gravitational yanks of passing stars jostled the orbits of the objects, leaving no hint of their formerly neat, flat paths around the sun. (However, the orbits of objects in the loosely defined inner Oort cloud, located a few thousand astronomical units from the sun, do retain some memory of their origins, so this part of the cloud is thought to be flatter.) The outer, round cloud would have taken about a billion years to form, making it the youngest structure in the solar system.