There is a part of the solar system that no astronomer has ever seen, but that nearly every astronomer is sure is there. Last month, three Soviet astronomers offered arguments that the invisible portion is much larger and more important than had been supposed.
Their theory started with comets. There are always comets streaking through the planetary system. Where are they all coming from?
Back in 1950, a Dutch astronomer, Jan Hendrik Oort, suggested that way out beyond the farthest known planet there is a vast cloud of small icy objects. Each, he theorized, slowly circles about the sun in an orbit that takes millions of years to complete, and there may be billions of these objects all together.
Every once in a while, something--a collision with another piece of icy debris, or the gravitational pull of a nearby star--will cause an icy object to slow and fall toward the sun. It makes its way among the planets and as it approaches the sun, its ice vaporizes and the rocky dust within the ice lifts off the surface and forms a fog about the object. This fog is swept back by the solar wind into a huge tail, and the object has become what we call a comet. It sweeps around the sun and then out toward the far-off cloud again.
Every once in a while, though, one of these comets is caught by the gravitational pull of a planet and then, like comet Halley, it remains among the planets forever. It becomes a “short-period comet,” returning to the neighborhood of the sun every few years or decades.
How large is this “Oort cloud” of distant comets? To estimate that, we must have some idea how large a typical comet is. Last year, rocket probes were sent to pass near a comet for the first time during Halley’s close approach, and they took certain measurements. It turned out that comet Halley was considerably larger than had been suspected. It is an irregular object, but its average diameter is about 7 1/2 miles, and it contains about 140 cubic miles of ice. That amounts to about 30 billion tons of ice--quite a snowball.
The Soviet astronomers presented reasons for supposing that comet Halley is a typical comet and that the Oort cloud is made up of objects averaging 30 billion tons in weight.
Recent estimates indicate that the thickest part of the Oort cloud lies at a distance of 2 trillion to 4 trillion miles from the sun. That’s roughly 1,000 to 2,000 times as far away as the most distant known planet (which is why the objects can’t be seen--they are too far off). The most recent estimates as to how many cometary objects may exist in this cloud is about 2 trillion (2,000,000,000,000).
If there are that many objects, each one with a mass equal to comet Halley, the total mass of the Oort cloud is equal to about 100 times that of the Earth. This means that the total mass is roughly equal to that of Saturn, the second largest planet. This is about 1,000 times as great as had been previously estimated, which makes the cloud a considerably more important portion of the solar system than had been thought.
Here is something else:
Every object in the solar system rotates on its axis, and every object, except for the sun itself, revolves about the sun. All this turning of an object about itself and other objects is measured as “angular momentum,” an important property of all objects from stars to electrons.
There are two factors that determine how large angular momentum might be. First, there is the mass of the object, and second, there is the distance of the object from the center about which it turns.
The sun is 1,000 times as massive as all the planets and other objects that circle it put together, so you might think the sun has almost all the angular momentum in the solar system. However, the sun just turns about itself. Its various parts are not very far from its center, only 430,000 miles or so at most.
The planets, though much lighter than the sun, move in grand sweeps that place them hundreds of millions of miles from the sun. The distance more than makes up for the lightness of the planets. The result is that the sun has only 2% of the angular momentum of the solar system. The planets have the other 98%.
In fact, Jupiter--which, although the largest planet, has only a thousandth the mass of the sun--has about 30 times as much angular momentum as the sun.
But what about those comets, which are individually tiny but that turn at a distance of trillions of miles from the sun? The Soviet astronomers calculate that the comets have 10 times as much angular momentum as all the rest of the solar system combined. That means 90% of angular momentum is in the comets, 9.8% in the planets and 0.2% in the sun. If this is so, we may have to rethink our notions as to how the solar system had its beginnings.
For the last 40 years, scientists have worked out how angular momentum was transferred from the sun to the small planets when the solar system was formed. It wasn’t easy to do, and if they have to figure out how all that angular momentum was transferred to the distant Oort cloud, it will make things much harder.