Does simple life count?
It’s important to remember that the Rare Earth hypothesis only applies to the emergence of complex life. Ward and Brownlee believe that simple life, such as bacteria, is widespread in the universe — after all, even the harshest habitats on Earth harbor microbes. However, the pair feel that complex life, metazoans like animals and us, are exceptionally rare.
“If you find life elsewhere, it’s likely to be microbial,” said Brownlee. “You know, Earth will have a lifetime of about 12 billion years, but [compared to bacteria], metazoans have a much more restricted range of environmental criteria that they can survive in.” That means that a planet’s environment is conducive to simple life for much longer than it is conducive to complex life.
“The period of time when we have oxygen in the atmosphere — carbon dioxide to go to plants and oxygen for metazoans — is probably only like 10 or 20 percent of [Earth’s lifespan]. So, if you just landed on our planet randomly throughout its entire history, you would not have anything to see.”
Counter-evidence welcome
Just because Ward and Brownlee don’t believe complex life is common throughout the universe, that doesn’t mean they don’t want it found. The duo welcome new data from cutting-edge observatories, like the James Webb Space Telescope (JWST), which seek to reveal the atmospheres of exoplanets in detail. And there are certain atmospheric signatures that would be more revealing than others.
“I think is way more important to try to look for oxygen atmospheres, but also look for reflections that indicate chlorophyll. You’re only going to have a number of ways to build specific molecules,” said Ward. “It really does come back to the fact that, as [University of Washington planetary scientist] David Catling has said, any animal equivalent is going to have to need oxygen — a lot of it. You cannot have really rapidly moving creatures and rapidly thinking creatures, which is a form of movement, and not have oxygen in the atmosphere to do it. You’re not going to have people living on carbon dioxide out there,” he added.
While compelling, the Rare Earth hypothesis still has its detractors; many of the environmental factors Ward and Brownlee identified in their book have come under fire over the past 20 years. Among the most frequently attacked proposed conditions for complex life is that a large planet like Jupiter is required to keep the inner solar system relatively free of dangerous debris. Some researchers argue such planets could actually increase the frequency of planetary impacts. Other critics have taken issues with the proposed requirements of a global magnetic field and plate tectonics.
With regard to these criticisms, Ward is understanding, encouraging challenges to his ideas. “Good science does a couple of things,” he says,”but the most important thing it does is it stimulates other science; good science makes people angry. It makes some people angry enough that they go out and do something about it.”
The Rare Earth hypothesis remains unproven, but it is hard to ignore the plethora of data that Ward and Brownlee have compiled to support their case. The barren and stark surfaces of Mercury, Venus, and Mars all serve as nearby reminders of what a lucky paradise Earth is by comparison. And rare or not, it’s the only home we have.
Doug Adler is the co-host of The Right Stuff Companion podcast and the co-author of the book: From The Earth to the Moon: The Miniseries Companion
Rare Earth hypothesis: Why we might really be alone in the universe
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