Lots. As of May 2000, about 40 planets have been detected orbiting nearly as many nearby stars; in some cases there are two or more planets...making these the first true 'solar systems' detected. Out of about 150 or so stars, 35 have planets so that about 15% of the nearby stars within 200 light years have planets! During the last few decades, astronomers have put together circumstantial evidence that it may be very hard for a star NOT to form a planetary system of some kind as the star itself is formed. I think that a plausible estimate is that 1 in 5 stars has a planetary system of some kind. There must be an even larger number of stars than have smaller planets, and it is hoped that in the next decade, our instruments will be good enough to detect them.
NASA's 'Origins Program' will include building a set of telescopes that will detect ALL of the Earth-sized planets within 100 light years of the Sun, and determine spectroscopically what the chemistry of their atmospheres is. If we find any with free oxygen, that can only mean life exists there because only organic processes can produce free oxygen. If all life came to an end on Earth tomorrow, the Earth would loose all of its free oxygen in a matter of a few million years, leaving behind an atmosphere of Nitrogen which currently makes up 80 percent of it today.
Copyright 1997 Dr. Sten Odenwald
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