Is Earth one of a kind? In a universe of at least 40 billion galaxies, each containing perhaps 100 billion suns, it seems there would be many Earth-like planets. Yet so far science hasn't found a single one. The small number of deep-space or "extra-solar" planets that have been located by astronomers appear so drastically different from Earth that life on them is difficult to imagine. Researchers hoping to find other Earth-like worlds have been getting glum.
This isn't just a pure-science question, but one bearing on faith. If Earth were truly alone as a possible home for life, our religions--and our survival--might be all that matters in the enormity of the cosmos. On the other hand, if there are many worlds where life like ours might arise, the theological questions that trouble us here--like 'What does it mean to be made in the image of God?" and 'How does God want his creation to live?'--might become even more vexing. If other life were discovered, would humans see that as proof that life began as a random swirl of physical reactions in the universe, or would they instead say these new worlds reflect the multifaceted power and majesty of God?
Recently the outlook regarding other worlds took a big swing toward the positive. In the Big Dipper constellation, astronomers found the first distant solar system that looks more or less like ours. This hardly guarantees other life. But it increases the chance there are other Earth-like planets, and that the living part of creation may be wondrously vast.
The discovery, by astronomer Debra Fischer of the University of California at Berkeley, concerns a star called 47 Ursae Majoris, about 45 light-years away and visible to the eye in the bottom of the cup of the Big Dipper. This star has long interested astronomers, because physically it is a sister to our sun - about the same size, age and power, and most important, a "main sequence" star that, like our sun, burns at fairly steady temperatures for billions of years. (Some stars burn rapidly, erratically or with pulsating power, all presumably bad for life.) Now it turns out that spinning around 47 Ursae Majoris are two gas-giant planets similar in size and orbital location to the gas giants of our solar system, Jupiter and Saturn. And the orbit of the planets is circular, just like the orbit of planets in our solar system. That's a vital clue.
Once astronomers assumed that because our solar system is circular, others would be too. But the circular solar system, essential for life, is turning out to be the exception rather than the rule. (Now you can admonish the kids, "You didn't finish your vegetables and you don't act grateful for living in a circular solar system!") Almost every distant solar system that has been found - the count is up to about 60 so far - is "eccentric."
This means planets loop in elongated racetrack-shaped orbits that take them far from their suns for extended periods, then very close for short periods, then far out again. These worlds in eccentric orbits would experience months or years of planetary deep freeze, perhaps even close to absolute zero, followed by brief periods of boiling, sterilizing heat. For life to arise under such conditions seems inconceivable. But since planets detected around 47 Ursae Majoris move in circular paths, they will be exposed to consistent amounts of heat and light, as is Earth.
It's also important that the two gas giant planets of 47 Ursae Majoris orbit this star at approximately the same distance that Jupiter and Saturn orbit the sun. This augurs well for an Earth-like world. All planets in our solar system take approximately circular orbits because Jupiter's overwhelming gravity forces them to. The large gas giant at 47 Ursae Majoris may force any other planets in that system into circular orbits, by the same means.
Having gas giant planets where they are in our solar system and at 47 Ursae Majoris favors life in another way. Strikes by large asteroids and comets can cause mass extinctions, as we know from the ancient comet that appears to have killed off the dinosaurs. The reason Earth has been struck by large asteroids and comets relatively infrequently in its past - mass extinctions are separated by millions of years -- is that the much stronger gravity of Jupiter and Saturn "vacuum up" most of what falls inward toward the sun from the outer solar system and from deep space. Telescopes show us that asteroid and comets are constantly smashing into Saturn and, particularly, Jupiter. This gravity 'vacuum cleaner' protects the inner plants of our solar system, including Earth, from space cataclysms that would otherwise devastate life. Presumably, the gas giants of 47 Ursae Majoris would protect any planets existing in its inner solar system, too.
The final intriguing finding about the solar system of 47 Ursae Majoris is that its protected inner area is about the same breadth as that of our solar system. Calculations suggest there is only a comparatively slender "continuously habitable zone" around stars, the band in which a planet would be warm enough to have liquid water, but not so hot the water boiled. Maybe there will turn out to be remarkable living things for whom water doesn't matter, but liquid water is essential for the only form of life we are certain is possible. And just inside the protective gas giants of 47 Ursae Majoris appears to lie a habitable zone similar to the one in which Earth spins.
What is do not know is whether there are any small worlds in the inner solar system of this fascinating star. Small worlds are what planet-hunting astronomers most avidly seek, for three reasons. First, because Earth is a comparatively small world. Second, because theory about planets formation says that only small worlds coalesce from solid material; large planets form of gas, and though perhaps there will turn out to be gas-based life, for the moment it defies imagining. Third, astronomers seek small planets because even if there were a giant rock-formed world, its gravity and atmospheric pressure would be so tremendous that our sort of life seems ruled out.
Unfortunately, small planets in distant solar systems are impossible to see with current telescopes. Even the best current instrument, the Hubble Space Telescope, has trouble getting a good view of Pluto, at the boundary of our own solar system; and 47 Ursae Majoris is about 80,000 times farther away than Pluto. We can "find" planets in deep space only inferentially, by calculating the "wobble" their gravity causes suns. The gas giants of 47 Ursae Majoris are so huge that their gravity produces a noticeable stellar wobble. But if - let's just speculate - an Earth-like planet were present in this solar system, its gravitational influence would be too small to produce any effect visible from here.
So astronomers are now watching 47 Ursae Majoris closely, hoping to detect an Earth-like planet in the second possible way - if one should make a transit directly across the face of its star, allowing researchers to glimpse a small darkened disk against the glow.
And if there is a living Earth-like world at 47 Ursae Majoris, the spiritual questions leap off the page. Would the faiths of its sentient beings be similar to ours, their creation stories similar and their scriptures and divine messages similar? In that case, many doubts we humans have might be erased. What if their faiths were utterly different? What if they considered faith nonsense, and said they had existed for eons without ever being contacted by any God?
In any case, when we look up into the ocean of space, we see an almost incomprehensible magnitude of inanimate structure. Surely its purpose cannot be to support this Earth solely, but to support a vast richness of life that may be evolving on its own, or to which human beings may someday contribute by spreading life from star to star. In space we see physical resources abundant enough to sustain trillions upon trillions of self-aware beings, each grateful for the gift of life. We can only gaze upward with wonder--and wonder if right now someone is looking through a telescope back toward us from 47 Ursae Majoris, eager to believe that his world is not alone.