|MadSci Network: Astronomy|
I also remember being told that if Earth were just a bit closer to or farther from the sun then life could not exist. Actually, there is a range of distances within which the Earth could orbit the sun and still support life. I vaguely recall hearing about this on a television program, but I have been unable to find any reference to this "habital zone," as it was called, anywhere else. According to Cesare Emiliani, a geologist at the University of Miami, "given a planet like the Earth as a substrate, it is virtually impossible to prevent the evolution of live." We can take this idea and expand it a bit further. What does the Earth provide that allows life to form? Could other planets provide these same essentials but in different ways? Jonathan I. Lunine, a scientist at the Lunar and Planetary Laboratory in Arizona, writes that life needs a source of carbon and water. He does mention that oxygen is not necessary, because we know that cells can use an alternative compound such as carbon dioxide (as in plants) for cellular respiration. Emiliani adds that we need a source of energy, and for primitive life that was UV light from the sun, lightning, and volcanic heat. Recent evidence shows that these conditions might exist elsewhere in the solar system. Right now scientists are looking at Europa (a moon of Jupiter), Titan (a moon of Saturn), and Mars. Europa shows evidence of liquid water under its icy surface - it probably has a molten core that provides enough heat to keep water liquid. Titan has an atmosphere but probably only frozen water on its surface; however, meteor impacts may melt the ice and allow brief periods for liquid water to exist. Mars shows evidence that liquid water once flowed on its surface and that perhaps a thick atmosphere once covered the planet. So we can see that energy sources can come from places other than the sun. A planet's interior may provide thermal energy that can help life carry out its chemical reactions. Even if a planet were too far away to receive enough sunlight for life to exist, over time the greenhouse effect (given a thick enough atmosphere) would warm temperatures enough for liquid water to exist (as is theorized with Mars). And we know that carbon is fairly common in our solar system - a lot of it trapped in the form of carbon dioxide, carbon monoxide, or methane. But if we turn our attention back to Earth, evidence shows that the early sun, when life was just starting out 3.8 billion years ago, was much fainter than the sun we know today. So the Earth didn't receive nearly as much solar energy as it does now, yet life still evolved. Lunine writes that at that time the sun's luminosity was 75% of what it is today, and that it was 25% dimmer. This means that past temperatures were significantly lower than today. He goes on to say that at 75% of present luminosity, the mean temperature of the Earth would have been 268 K (that's -5.15 *C, below the freezing point of water!) had it not been for the greenhouse effect. So an atmosphere capable of trapping heat is important for planets too far from the sun to support liquid water. All this evidence can lead us to only one conclusion. The distance of a planet (or moon) from the sun may or may not plays a role in the formation of life. We Must consider other factors. Hope this helps. I consulted the following books: "Earth: Evolution of a Habitable World" by Jonathan I. Lunine. "Planet Earth: Cosmology, Geology, and the Evolution of Live and Environment" by Cesare Emiliani
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