StarDate: Distant Realms

FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M.

STARDATE 05/20/2014: We're surrounded. Hundreds of billions of icy leftovers from the birth of the solar system extend far beyond the realm of the planets. Close to the planets, they form a thick ring. Farther out, they form a shell that's light-years wide.

The planets probably formed as chunks of rock and ice — known as planetesimals — stuck together to form bigger and bigger bodies. When the outer planets grew big enough, their gravity kicked out most of the remaining planetesimals.

Some of them formed the ring, which is known as the Kuiper Belt. It begins just outside the orbit of Neptune, the most-distant planet, and extends outward for a couple of billion miles. It may contain hundreds of millions of objects; the best known is Pluto.

Beyond the Kuiper Belt is the Oort Cloud. Just outside the Kuiper Belt it forms a disk. As the distance from the Sun grows, though, it spreads out, eventually forming a spherical shell.

The Oort Cloud may contain a trillion planetesimals, but only a few have been seen. The biggest is Sedna, a dwarf planet that was discovered 10 years ago. But even bigger objects may await discovery; more about that tomorrow.

The Oort Cloud extends to perhaps one or two light-years from the Sun. At that distance, the Sun's gravitational grip is so weak that a passing star or gas cloud can scatter the planetesimals. A few plunge toward the Sun, where they eventually sprout long, glowing tails — the unmistakable signature of a comet.

Script by Damond Benningfield Copyright ©2014 The University of Texas McDonald Observatory