David S. Pratt, age 13, of St. Paul, Minnesota, for his question:
Do we know that other stars have planet systems?
For a long time, astronomers suspected that our solar system was not unique in the universe. Then in 1949, a certain pair of twin stars was found to have a third invisible partner most likely a planet. In 1960, another star was found to have a massive, invisible planet. Three years later, a smaller planet belonging to still another star was discovered: We know that two of the star systems closest to our sun have at least one big planet apiece.
Our pinwheeling galaxy has at least 100 billion starry suns and 99 per cent of them are main sequence stars, that is, their dimensions and other features can be compared, more or less, with our sun. Modern astronomers assume that our sun and its starry relatives condensed from large clouds of cosmic gases, similar to the hazy nebulas strewn throughout the Milky Way. The generally accepted theory is that the planets of our solar system formed separate orbiting bodies as the sun condensed and eventually ignited its nuclear furnace.
If this feasible theory is a fact, we can assume that most of the average type stars repeated the same pattern. Chances are, planetary systems usually form during the birth of a normal star. If this is so, there are billions of other planetary systems in our Galaxy plus uncountable numbers within the uncounted galaxies beyond our Milky Way. Some experts estimate that our galaxy alone may have at least 600 million planets capable of supporting life more or less as we know it on earth.
Proving these possibilities is a tedious task. Planets are smaller than stars and shed no light of their own. The nearest stars are between four and 20 light years away and even our best telescopes cannot spot planets at such distances. Astronomers, however, use ingenious indirect methods of detection. For example, every orbiting planet has a gravitational effect on its starry sun, causing slight irregularities in its motions. For 30 years they studied such patterns in certain binaries twin stars that orbit about each other.
In 1949, it was announced to the world that the twin star system named Cygni 61 is affected by a third invisible partner. Its mass is eight times greater than our Jupiter's but it is not massive enough to be a star. In 1960, the star system Lelande 21185 was found to have a similar giant planet. It is only eight light years from us. In 1963, a planet was found for neighborly Barnard's star, at a distance of six light years. This third planet to be identified beyond our solar system is about 1 1/2 times more massive than our giant Jupiter.
We cannot at present probe for planets far out into deep space. But the stars sampled in our celestial neighborhood are very promising. We know that three have at least one whopping planet apiece. And where there is one, most likely there are planetary families. But the smaller planets are beyond our present range of vision.