The Case for a Creator (29 page)

“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. This is determined by the amount of light you get from the host star.

“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.”
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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 very bad, because it causes large temperature differences between the lit side and the unlit side. The lit side would be terribly dry and hot, while the unlit side would be prohibitively icy and cold. And there’s another problem—red dwarfs have flares.”

“But,” I said, “the sun has flares too.”

“That’s right. And the intensity of flares on red dwarfs is about the same as on our sun. The difference is that red dwarfs as a whole emit much less total light, so they’re much less luminous. That means in comparison to the luminosity of the star, the output of the flare is high.”

“Whoa!” I said, putting up my hand in protest. “You’ve lost me.”

Gonzalez regrouped. “Okay, let me get to the bottom line: for this kind of star, flares cause the star’s total luminosity to vary. In fact, astronomers call them ‘flare stars,’ and they watch as they get much brighter for a while and then dimmer again. We don’t pay too much attention to the solar flares of our sun, because the sun is so luminous that the flares are like a little blip. You barely notice them.”

“And remember we’re ninety-three million miles from the sun,” Richards said. “With a red dwarf, your planet would have to be much closer to the star.”

“Right,” said Gonzalez. “The luminosity increase would cause temperature spikes on the surface of an orbiting planet. But just as bad would be the increased particle radiation that would result from the flares. On Earth, we get a very mild effect called the aurora borealis. This is where there’s a flare on the sun, the particles eventually reach the Earth, they’re funneled down the magnetic field to the north and south poles, and we see the aurora borealis as these beautiful lights in the northern hemisphere.

“However, particle radiation has the effect of quickly stripping away the atmosphere, increasing the surface radiation levels, but most importantly, destroying the ozone layer, which we need to protect from radiation. All of this would be deadly for any life on a planet near a red dwarf.

“And then red dwarfs have one more problem: they don’t produce much ultraviolet light, which you need early on to build up oxygen in the atmosphere. Scientists believe that the oxygen in the Earth’s atmosphere was built up at first by the ultraviolet radiation that broke up water into oxygen and hydrogen. The oxygen was allowed to build up in the atmosphere, while the hydrogen escaped into space, because it’s lighter. But you get very little blue light from a red dwarf, so this phenomenon wouldn’t occur as rapidly and you wouldn’t get the build up of the oxygen you need to sustain life.

“Fortunately, our sun is not only the right mass, but it also emits the right colors—a balance of red and blue. As a matter of fact, if we were orbiting a more massive star, called an F dwarf, there would be much more blue radiation that would build up the oxygen and ozone layer even faster. But any momentary interruption of the ozone layer would subject the planet to an immediate flood of highly intense ultraviolet radiation, which would be disastrous to life.

“Also, the more massive stars don’t live as long—that’s the major problem. Stars that are even just a little more massive than the sun live only a few billion years. Our sun is expected to last a total of about ten billion years on its main sequence, burning hydrogen steadily, whereas stars just a few tens of percent more massive have considerably less lifetime on the main sequence. And while on the main sequence, they change luminosity much faster. Everything on their lifecycle happens faster.”

“Anything else that makes our sun unusual?” I asked.

“Yes, the sun is metal-rich; in other words, it has a higher abundance of heavy elements compared to other stars of its age in this region of the galaxy. As it turns out, the sun’s metallicity may be near the golden mean for building Earth-size habitable terrestrial planets.

“And the sun is highly stable, more so than most comparable stars. Its light output only varies by one-tenth of one percent over a full sunspot cycle, which is about eleven years. This prevents wild climate swings on Earth.

“Another way it’s anomalous is that the sun’s orbit is more nearly circular in the galaxy than most other stars of its age. That helps by keeping us away from the galaxy’s dangerous spiral arms. If the sun’s orbit were more eccentric, we could be exposed to the kind of galactic dangers I mentioned earlier, such as explosions of supernovae.”

I realized after Gonzalez’s comments that I would never look at the bejeweled night sky as I had in the past. I used to see stars as being fungible, which is a legal term meaning one is just as good as the other. But now I understood why the vast majority of stars would be automatically ruled out as being capable of supporting life-bearing planets.

It would take a star with the highly unusual properties of our sun—the right mass, the right light, the right composition, the right distance, the right orbit, the right galaxy, the right location—to nurture living organisms on a circling planet. That makes our sun, and our planet, rare indeed.

As much as I have been fascinated by the sun, I’ve also frequently stared in wonder at the other dominant celestial body in our sky—the moon. Curious to find out whether this barren, rocky satellite contributes anything to its host planet—other than inspiration for poets and other romantics—I proceeded to turn our discussion toward lunar issues.

OUR LIFE-SUPPORTING MOON

Centuries ago, the dark patches on the moon—low-lying areas that had been flooded with basaltic lava—were thought to be oceans that provided life-giving water to its unseen population. They were called
maria
, Latin for “seas.”
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The name has stuck; to this day, for example, we still refer to
Mare Tranquilitatis
, or the Sea of Tranquility.

Johannes Kepler, the seventeenth-century astronomer who fanned the flames of the Copernican Revolution, gazed at the moon and believed he discerned caves that were populated by moon people. He even wrote a book in which he fantasized about what their lives might be like.
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A century later, William Herschel, who gained fame by discovering Uranus, thought he made out cities, highways, and pyramids on the lunar landscape.”
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As scientific knowledge grew, dreams of finding lunar civilizations dissipated. Everyone came to agree that the moon cannot support life. Yet surprising discoveries in recent years have shown the opposite to be true: the moon really
does
support life—ours! Scientific evidence confirms how this parched, airless satellite actually contributes in unexpected ways to creating a lush and stable environment a quarter of a million miles away on Earth.

When I asked Gonzalez about how the moon helps support life on our planet, the first thing he brought up was a discovery that only dates back to 1993.

“There was a remarkable finding that the moon actually stabilizes the tilt of the Earth’s axis,” he said. “The tilt is responsible for our seasons. During the summer, in the northern hemisphere the north pole axis is pointed more toward the sun. Six months later, when the Earth is on the other side of the sun, then the south pole is more pointed toward the sun. With the Earth’s tilt at 23.5 degrees, this gives us very mild seasons. So in a very real way, the stability of our climate is attributable to the moon.”