The Hale-Bopp comet streaking through the sky is not only a stargazer’s delight, but also a sign that trillions of little-known objects are floating in our local corner of the universe.
We can expect more such heavenly visitors to whiz by Earth, illustrating astronomers’ recent discoveries that the solar system is much bigger and more complicated than we once thought.
Instead of a snug little family of nine planets circling the sun, our neighborhood consists of three very different, concentric structures stretching almost a quarter of the way to the nearest star:
The familiar planets, from Mercury to Pluto, make up only the smallest and innermost ring, about 4 billion miles across.
Encircling them lies a much larger disk, perhaps 18 billion miles wide, called the Kuiper Belt. It contains thousands - perhaps millions - of icy objects, the first of which was detected just five years ago.
And enveloping that disk is an enormous sphere of dust and ice, up to 12 trillion miles in diameter. Known as the Oort Cloud, it is the home of major comets like Halley, Hyakutake and now Hale-Bopp.
For nearly half a century, astronomers suspected that vast, unexplored systems like these lurked beyond Pluto, the outermost planet then known.
Their conjectures were confirmed five years ago, when University of Hawaii astronomers David Jewitt and Janet Luu, using an 85-inch telescope on top of a volcano, spotted a baby planet, about the size of Vermont and New Hampshire combined, more than a billion miles beyond Pluto.
Dubbed 1992 QB1, the ice ball is 1/10th the size and 1/10,000th the brightness of Pluto, previously the littlest planet, which was discovered in 1930.
Since 1992, 50 more such “icy dwarfs” have been located in the Kuiper Belt - named for Dutch astronomer Gerald Kuiper, who proposed their existence in 1951.
“Our view of the outer solar system has changed dramatically in the last few years,” said Alan Stern, an astronomer at the Southwestern Research Institute in Boulder, Colo. “Pluto used to be the frontier of the solar system. Now the frontier has moved beyond Pluto. Pluto is just the tip of the iceberg.”
Some astronomers believe Pluto is a refugee from the Kuiper Belt that got snared by the massive gravity of Neptune, the eighth planet, and dragged into an orbit closer to the sun. Neptune’s icy moon Triton may also be a member of this class, Stern said.
In addition, the Kuiper Belt is home to a herd of “short-period” comets, which orbit the sun in less than 20 years. One of these was the Shoemaker-Levy comet that smashed into Jupiter two years ago.
Most comets, however, have lengthy orbits and dwell much farther out in the Oort Cloud, named for Jan Oort, another Dutch astronomer, who suggested its existence in 1950.
Oort theorized that there are as many as 10 trillion icy objects swarming inside an enormous shell with a radius of 3 trillion to 6 trillion miles. They are the debris left over from the time the sun condensed out of a cloud of gas 4.5 billion years ago.
From time to time, Oort surmised, a passing star or molecular cloud nudges one of these ice balls loose from the cloud and slings it into an elongated orbit looping around the sun.
One such object is Hale-Bopp, now visible in the evening and early morning sky. It was discovered in July 1995, when it was more than 600 million miles away, by amateur astronomers Alan Hale and Thomas Bopp.
The brilliant comet, one of the brightest of the century, reached its closest point to Earth, 123 million miles, last Saturday (March 22). It will whip around the sun at a distance of 85 million miles (closer than the Earth is to the sun) on March 31, before heading back out into the outer darkness.
It will return in 4,210 years.
Two other comets, too faint to be seen except with high-powered telescopes, are also passing through the vicinity this spring.
Comet Wirtanen swung by the sun on March 14, at a distance of 159 million miles. Comet Wild 2 will be 224 million miles from the sun on May 6.
According to Stern, the inner planets formed from small objects, like those remaining in the Kuiper Belt, that collided and stuck together.
“There probably were hundreds of thousands of objects out there in the early days that were later ejected and lost,” he said. “Pluto is probably the survivor of a lost population of ice dwarfs that inhabited the primeval solar system.”
MEMO: This sidebar appeared with the story: BEST VIEWING Comet Hale-Bopp can be seen in the northwestern sky in the early evening and in the north-northeast before dawn.