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.

And there's more wrong with them. Many lack the neat, stable orbits of our solar system. One planet, found around a star called 70 Virginis, has such a loop-shaped "eccentric" orbit that it spends extended periods in the deep-freeze, far from its sun, followed by brief jaunts of close-up overheating. Instead of four seasons, it would have two: frozen and boiled. Other extra-solar planets spin so far from their stars that they would simply always be below the freezing points of all liquids. The planet orbiting 51 Pegasi goes to the other extreme. It spins so close to its star--only five% as far from 51 Pegasi as the Earth is from the sun--that its surface would always be above the boiling point of all liquids, to say nothing of being constantly bathed in levels of radiation lethal to all forms of life we can imagine. If you could stand on this planet, the sun would not be a shiny dollar coin in the air. The entire sky would be intense, glaring yellow, with solar flares thousands of miles long rippling toward you.

Of course, it's possible there are forms of life capable of existing in conditions that seem incredible to us. (Our conditions may seem incredible to them: "What do you mean these Earth beings breathe oxygen? Nothing can live in oxygen!") It's possible that "wobble" measurements and other techniques make planets seem larger than they really are. Most of all, it stands to reason that since small planets would be harder to detect than big ones, there must be Earth-sized extra-solar worlds that we simply haven't spied yet.

But would an Earth-sized world in another solar system necessarily be an Earth-like world? Here enters one of the more disturbing calculations in astronomy, that of the "continuously habitable zone."

A few decades ago, an astronomer named Michael Hart calculated how far from a sun-like star a planet would have to be in order to have liquid water on its surface and the right temperatures and pressures for carbon-based or silicon-based life. What Hart found was that the planet would have to sit almost exactly where Earth sits, or be too hot or too cold. Venus and Mars seem evidence of this. Venus, the next planet closer to the sun than Earth, has a runaway greenhouse effect and 900°F surface temperatures much hotter than fire; Mars, the next planet out, is locked in what appears to be a permanent ice age. Hart called the star-distance relationship that allows for liquid water and carbon or silicon chemistry the "continuously habitable zone" and found that for a sun-like star, the zone is less than 1% of possible orbits. This means that other things being equal, an Earth-like planet has 99 chances out of 100 to form in such a way as to be uninhabitable.

Many astronomers have since tried to punch holes in Hart's work, for it implies that unless there are fundamentally different forms of existence, planets with life will be exceedingly rare. About a decade ago, a group of astronomers led by the late Carl Sagan offered a revision of Hart's calculations, suggesting that for various reasons (Earth-like planets may tend to form in the middle orbital bands, and different atmospheres may moderate temperature better than ours can) the continuously habitable zone will turn out to be 5% of possible orbits for a sun-like star, not 1%. But this still means the conditions for organic life will be highly improbable. Most planets won't be Earth-like to begin with (in this solar system, only three of nine are: Earth, Mars, and Venus), and of 20 Earth-like planets that do exist, 19 will fall outside the habitable zone.

Human understanding of distant space is in its infancy, so many of the assumptions made today may turn out to be wrong. And we know precious little about what causes life, since we still can't even answer whether the life of our own world is natural or divine in origin.

But what's known about other worlds so far, and what can be projected about their habitability, suggests that even in an immense cosmos, life may be rare. Certainly, the absence of artificial radio signals in our galaxy--SETI searches haven't found one yet--suggests intelligent life is atypical. That in turn makes our lives and the living creatures of our Earth all the more precious. If God placed us here, then we may indeed be the sole thinking beings of the cosmos, or one of just a few such species. If natural forces placed us here, our rarity may be the same, and the entire cosmic enterprise is dependent for its meaning on our survival and maturation to wisdom.

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