For most of this century, alien planets were an astronomer's speculation and a sci-fi author's gimmick. Now they're entering the catalogue of known objects at a record rate. It's a foretaste of the exploration of other star systems that will burgeon in the 21st century.
We now know that our solar system is not alone. But is it typical? We'll find out as the new century unfolds.
The recent report by American, Australian, and European observers that a Jupiter class planet orbits the star Gliese 876 just 15 light years from Earth makes an even dozen of such worlds reported in the past 2-1/2 years. A Swiss-led team is expected to announce two more planets any day now. All were discovered by a long-known search technique that only became practical in this decade.
No one has actually seen any of these planets. They reveal their presence by the way they tug at their parent stars. This makes the stars wobble. Astronomers have looked for that characteristic wobble for decades. But in the few cases they found, the data were frustratingly fuzzy. New instruments and powerful computer analysis have sharpened the view. Astronomers now can pin down the wobble precisely enough to estimate the mass of an unseen planet, its distance from the star, and the shape of its orbit.
Right now, this technique only picks up large planets. Within a decade or less, it should be able to pick up earth-size worlds as well. Advanced ground-based and space-based telescope systems now in the planning will do better. We should actually see planets in some of the nearer star systems within the next few decades.
At the dawn of this century, the only alien planets we knew were in our own solar system. And they were just bright lights in the sky. Now we explore them as we once explored Earth and look for signs of life. At the dawn of the 21st century, we will know more than a dozen other planetary systems. Their planets will just be unseen bodies that wiggle their stars. But many astronomers expect that we will have images of those and other planets and perhaps even have seen signs of life on some of them by the end of the coming century.