The Case for a Creator - Lee Strobel [93]
“You can’t be too close, otherwise too much water evaporates into the atmosphere and it causes a runaway greenhouse effect, and you boil off the oceans. We think that might be what happened to Venus. But if you get too far out, it gets too cold. Water and carbon dioxide freeze and you eventually develop runaway glaciation.
“The main point is that as you go further out from the sun, you have to increase the carbon dioxide content of the planet’s atmosphere. This is necessary in order to trap the sun’s radiation and keep water liquid. The problem is that there wouldn’t be enough oxygen to have mammal-like organisms. It’s only in the very inner edge of the Circumstellar Habitable Zone where you can have low enough carbon dioxide and high enough oxygen to sustain complex animal life. And that’s where we are.”
“So if the Earth’s distance from the sun were moved by, say, five percent either way, what would happen?” I asked.
“Disaster,” came his quick reply. “Animal life would be impossible. The zone for animal life in the solar system is much narrower than most people think.”
“And that’s why you need a circular orbit like the one Earth has,” Richards added. “You don’t just want to be in the Circumstellar Habitable Zone part of the time; you want to be in it continuously. It doesn’t do you any good to have melted water for four months and then have the whole planet freeze up again.”
OUR OVERACHIEVING SUN
Obviously, the key to continued life on Earth is the sun, whose nuclear fusion, taking place at twenty-seven million degrees Fahrenheit at its core, provides us with consistent warmth and energy ninety-three million miles away. Ever since witnessing a solar eclipse as a child, carefully protecting my eyes by observing the phenomenon through a projected image inside a cardboard box, I have been fascinated by this fiery behemoth, whose mass is an incomprehensible three hundred thousand times greater than the Earth’s.
However, I had always been told that there was nothing out of the ordinary about the sun. As one text says flatly: “The sun is a common fixed star.” 33 And if the sun is truly so average, so typical, so undistinguished, then the logical implication would be that lots of life-bearing Earths must be orbiting around lots of similar suns throughout the universe.
“Today, astronomers know a lot more about stars than they did when I was growing up,” I said to Gonzalez. “Is the consensus still that the sun is just a common star?”
“No, not at all,” Gonzalez replied. “It’s just recently that some new astronomy textbooks are finally starting to say that, well, the sun really is unusual after all. For instance, it’s among the ten percent most massive stars in the galaxy. In fact, if you pick a star at random, you’re likely to pick one that’s far less massive than the sun, usually red dwarfs, which make up about eighty percent of stars. Another eight or nine percent are called G dwarfs, most of which also are less massive than the sun. The sun is a yellow dwarf; technically, it has a G2 Spectral Type.”
His comment about the ubiquity of red dwarfs piqued my curiosity. “Since red dwarfs dominate the universe, let’s talk about them for a moment. Are they conducive to having life-bearing planets orbiting them?” I asked.
“I don’t think they are,” Gonzalez said.
“Why not?”
“Several reasons. First, red dwarfs emit most of their radiation in the red part of the spectrum, which makes photosynthesis less efficient. To work well, photosynthesis requires blue and red light. But a much greater problem is that as you decrease the mass of a star, you also decrease its luminosity. A planet would have to orbit this kind of star much closer in order to have sufficient heat to maintain liquid water on its surface.
“The problem is the tidal force between the star and the planet gets stronger as you move in, so the planet will spin down and eventually end up in what’s called a tidally locked state. This means it always presents the same face towards the star. That’s