The Case for a Creator - Lee Strobel [92]
“However, we’re finding that the planets circling other stars are quite different from Jupiter. They orbit over a full range of distances, from just a tiny fraction of an Astronomical Unit—which is the distance between the Earth and the sun—out to several Astronomical Units. Most of their orbits are highly elliptical; very few are circular. These strongly non-circular orbits utterly surprised astronomers. Because they strongly subscribed to the Copernican Principle, they had expected that other planetary systems would be just like ours. And that expectation was basically dashed.”
“What’s wrong with an elliptical orbit for those kind of planets?” I asked.
“It poses a problem for the habitability of any terrestrial planets in their system, because it would make them less likely to have stable circular orbits,” Gonzalez replied. “For example, Earth’s orbit is almost a perfect circle. A planet with the mass of the Earth would be sensitive to any of the gas giant planets if they had more eccentric orbits. The Earthlike planet’s own orbit would be affected, making it less circular and therefore subjecting the planet to dangerous surface temperature variations.”
“So,” I said, “if our own Jupiter had a more elliptical orbit, the Earth wouldn’t be able to maintain as circular an orbit and have the steady temperature and predictable climate that come with that.”
“That’s right,” he said. “In fact, even small variations in our nearly circular orbit can cause ice ages, because of temperature shifts on the surface of the planet. We have to maintain a circular orbit as much as possible to maintain a relatively steady temperature. That’s only possible because Jupiter’s orbit isn’t very elliptical and therefore doesn’t threaten to distort our round orbit.”
TAKING HITS FOR EARTH
Now that we were discussing our solar system, I wanted to delve into other “local” factors that make our planet habitable. “What is it about our solar system that contributes to life on Earth?” I asked.
“A surprising amount,” said Gonzalez. “More and more, astronomers are learning how the other planets tie into the habitability of Earth. For example, George Wetherill of the Carnegie Institution showed in 1994 that Jupiter—which is huge, more than three hundred times the mass of the Earth—acts as a shield to protect us from too many comet impacts. It actually deflects comets and keeps many of them from coming into the inner solar system, where they could collide with Earth with life-extinguishing consequences.
“This was illustrated very nicely by the impact of Comet Shoemaker-Levy 9 into Jupiter in July, 1994. This comet was attracted by Jupiter’s tremendous gravitational pull and broke into fragments, with all of them hitting Jupiter. Even Saturn and Uranus participate in that kind of comet-catching.
“In addition, the other planets in our inner solar system protect us from getting bombarded by asteroids from the asteroid belt. The asteroids are mostly between the orbits of Mars and Jupiter. Our first line of defense is Mars, being at the edge of the asteroid belt. It takes a lot of hits for us. Venus does too. If you want to get an idea of the stuff that probably would have hit the Earth, look at the surface of the moon. The moon, unfortunately, has too little surface area to provide much protection, but it’s a nice record.”
“What about the Earth’s position in the solar system?” I asked. “How much does that contribute to its habitability?”
“There’s a concept invented by astrobiologists called the Circumstellar Habitable Zone. That’s the region around a star where you can have liquid water on the surface of a terrestrial planet.