Will extraterrestrials have religion? This is one of the provocative questions people ask as they look outward to the stars. More basic is the question of whether there is or will be extraterrestrial life, and more basic to that, in turn, is whether there are other planets similar to Earth. On that final point, scientists have recently made a burst of discoveries with many implications for the question of whether there are other thinking beings in the universe.

As recently as six years ago, no "extra-solar" planet--meaning beyond our solar system--had been found. Because planets don't make any light and emit only slight amounts of heat, they produce little for astronomers to detect. Pluto, the outermost planet in this solar system, was not discovered until 1930, and then by accident, by an astronomer who was looking for something else. Today, even the Hubble Space Telescope has a hard time getting a decent view of Pluto, because the planet is so far away. And Pluto is only about five light-hours from Earth, making it a much more "nearby" object than extra-solar planets that are dozens, hundreds, or thousands of light-years distant.

Because today's telescopes cannot obtain a standard visual image of an extra-solar world, astronomers search for distant planets using two inferential methods. They examine the movements of other stars to determine if the stars "wobble" in a way that would indicate the gravitational tug of planets. And they watch some nearby stars to see if they can catch an extra-solar planet in "transit," passing across the disc of the star and causing a momentary, moving dim spot. Using these two techniques, astronomers have since 1995 located almost 50 extra-solar planets, including one orbiting Epsilon Eridani, the closest star that is similar to our sun. (Epsilon Eridani is about 10.5 light-years away.) Another planet has been found orbiting the sun-like star designated 51 Pegasi. A full solar system of several planets has been detected spinning around the star Upsilon Andromedae. And here's the common denominator in these discoveries: Every one of the planets found so far is utterly uninhabitable by any kind of life we can imagine.

Let's start by contemplating what kinds of life can be imagined. Based on what is known to us, at least, life must have access to liquid water or a similar liquid, and life must be either organic--that is, carbon-based--or silicon-based. Liquid as a necessity for any physical form of life seems an inescapable requirement. (All living things, including Homo sapiens, are primarily made of liquids, because liquids can engage in complex chemical reactions and yet hold an information-storing form: Life based entirely on solid materials wouldn't have metabolism, or if it did would evolve unimaginably slowly, while life based on gaseous substances might be too nebulous to have any information-storing equivalent of DNA.) Meanwhile, all living things we observe are carbon-based, because carbon is one of only a handful of elements capable of forming, breaking, and reforming chemical bonds with the kind of relatively low energy input available from sunlight. (This happens for a techno-reason concerning the number of electrons in its outermost "valence level.") Silicon, like carbon, also has the ability to form complex molecules with small energy inputs, which is why scientists speculate about silicon-based life. All other elements with this property are metals, which seems to rule them out.

Now how do these rules--liquid water, amenable to carbon or silicon chemistry--apply to the extra-solar planets found so far?

All are huge, roughly the mass of Saturn or larger. (Saturn's volume is about 766 times greater than Earth's.) This means the extra-solar planets have crushing gravity, which seems to rule out Earth-like biochemistry. Some of the discovered planets are spectacularly huge, much larger than Jupiter, the king of our solar system--one extra-solar world, spinning around a star designated HD162020 on charts, is 14 times the size of Jupiter. Complex carbon and silicon compounds might survive on such high-gravity worlds, but it wouldn't be in forms of life recognizable to us.

Heft, however, is just the beginning of what's wrong with the extra-solar planets, from the standpoint of the one form of life we know to be possible. Standard theories of how planets form hold that above a certain size (not too many times larger than Earth), planets cannot coalesce of solid materials, but must be frozen gas. Saturn and Jupiter are mainly frozen hydrogen. If the extra-solar worlds discovered so far really are as enormous as they appear, astronomers assume they must be "gas giants," again ruling out the form of biochemistry known to us. Some of these gas giants are sufficiently gigantic that they may be "brown dwarfs," or structures that are almost stars, but don't have quite enough gravitational pressure to ignite.