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Could a life-supporting planet have formed just anywhere in the universe?
According to astronomer Guillermo Gonzalez, “You just can’t form a habitable planet anywhere; there’s a large number of threats to life as you go from place to place to place.” 10 (Bold emphasis added.)
The Sun’s Safe Location
There are three basic kinds of galaxy in the universe: There are irregular ones with no discernible shape or pattern of movement; there are elliptical galaxies, shaped somewhat like an egg in space; and there are spiral galaxies, shaped rather like a pinwheel, with arms extending out into space away from their center.
Irregular galaxies are the worst possible supporters of lifeharboring planets. According to Gonzalez, irregular galaxies are “distorted and ripped apart, with supernovae going off throughout their volume. There are no safe places where there are fewer supernovae exploding, like we have between our spiral arms.” 11 In the estimation of many astronomers, irregular galaxies would be unlikely to have any stars with life-harboring planets.
In elliptical galaxies, the stars have “very random orbits, like bees swarming in a beehive. The problem for life in these galaxies is that the stars visit every region, which means they’ll occasionally visit the dangerous, dense inner regions, where a black hole may be active. In any event, you’re less likely to find Earth-like planets in elliptical galaxies because most of them lack the heavy elements needed to form them.
“Most elliptical galaxies are less massive and luminous than our galaxy,” which is “on the top one or two percent of the most massive and luminous. The bigger the galaxy, the more heavy elements it can have, because its stronger gravity can attract more hydrogen and helium and cycle them to build heavy elements. In the low-mass galaxies, which make up the vast majority, you can have whole galaxies without a single Earth-like planet. They just don’t have enough of the heavy elements to construct Earths.” 12
The spiral galaxy (such as our Milky Way) “optimizes habitability, because it provides safe zones … and Earth happens to be located in a safe area, which is why life has been able to flourish here…. Places with active star formation are very dangerous, because that’s where you have supernovae exploding at a fairly high rate. In our galaxy, these places are primarily in the spiral arms, where there are also hazardous giant molecular clouds. Fortunately, though, we happen to be situated safely between the Sagittarius and Perseus spiral arms.
“Also, we’re very far from the nucleus of the galaxy, which is also a dangerous place. We now know that there’s a massive black hole at the center of our galaxy. In fact, the Hubble telescope has found that nearly every large nearby galaxy has a giant black hole at its nucleus. And believe me—these are dangerous things!
“Now, put all this together—the inner region of the galaxy is much more dangerous from radiation and other threats; the outer part of the galaxy isn’t going to be able to form Earth-like planets because the heavy elements are not abundant enough; and I haven’t even mentioned how the thin disk of our galaxy helps our sun stay in its desirable circular orbit. A very eccentric orbit could cause it to cross spiral arms and visit the dangerous inner regions of the galaxy, but being circular it remains in the safe zone.” 13 (Bold emphasis added.)
Location, Location, Location
Gonzalez concludes, “In terms of habitability, I think we are in the best possible place. That’s because our location provides enough building blocks to yield an Earth, while providing a low level of threats to life. I really can’t come up with another place in the galaxy that is as friendly to life as our location.” 14 (Bold emphasis added.)
Not only is the sun located in the most optimal part of the universe for a life-supporting planet, but of all the planets in the solar system, only on Earth do we find the exact combination of conditions necessary for carbon-based life as we know it. Mercury is too hot. Venus has a toxic atmosphere. Mars is too cold and dry.
Gonzalez explains it this way: “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 ventually develop runaway glaciation…. 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.” 15
