Among the countless Earths that, we are told, sail the galaxies (an estimate released last week offers 8.8 billion “just-right planets”), one or two may indeed be researchable, even reachable. And what if we find no life there?
Ah. Recall that the Copernican Principle is not evidence. It is an assertion: Earth cannot be a rare planet because that would make us special, which is philosophically unacceptable today.
So invoking the Principle, many of these planets must be inhabited. As Fox News puts it,
As for what [the 8.8 billion estimate] says about the odds that there is life somewhere out there, it means “just in our Milky Way galaxy alone, that’s 8.8 billion throws of the biological dice,” said study co-author Geoff Marcy, a longtime planet hunter from the University of California at Berkeley.
Caution: Biology is an awkward science to play dice with. One may as well argue that there must be several species of reasoning animals like humans on Earth because there are just so many species. But there aren’t several, just one.
Undeterred by such cavils, enthusiasts of the Principle press on. Two diverging themes emerge: On the one hand, scientists seeking planets that are just like Earth are “tantalizingly close to hitting pay dirt,” in terms of finding a specific planet rather than just making estimates. On the other hand, their critics continue to deplore the emphasis on animals like us, calling it “hopelessly parochial and unimaginative”: Life we would not recognize could live on planets nothing like Earth.
The discovery of Gliese 581g (a rocky planet much larger than Earth) in a habitable zone prompted this response from astronomer Steven Vogt:
Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent.
If grey is the new black, subjective certainty is the new evidence.
And so are faint hopes. Consider the hunt for fossils of simple micro-organisms on good old, dependable Mars, a planet we know a fair bit about. Despite severe budget cutbacks, NASA is inundated by ideas. Life on Mars, we are told, makes even more sense than life on Earth because as biochemist Steven Benner puts it, Mars is dry. And “How is it possible that the chemicals that we now have supporting modern life, which is so unstable in water, could have arisen in water?”
Listen carefully, for you will seldom hear that point raised when enthusiasts are explaining how life could have arisen by chance on Earth. But we mustn’t get ahead of our story.
In 2010, hope flared: “Samples of dirt collected by NASA’s Viking Mars landers in the 1970s may have contained carbon-based chemical, study finds.” But setbacks happened too, like the 2009 finding that methane, hailed as a possible sign of life on Mars, “disappears from the planet’s atmosphere hundreds of times faster than expected,” making life unlikely there (the absence of methane was confirmed in 2013). And some researchers found that the carbon from Mars is not biological.
There was indeed water on Mars a long time ago there but then, we are told, a 600 million year drought ensued, so some now continue the search underground. Others look for life in the Martian pits. Or Mars’ volcanic glass. Still others search asteroid craters on Earth for clues to Mars “hiding life”. And some, impatient with Mars perhaps, hope to find life in undersea caves elsewhere in our solar system.
Channels of life-giving Martian water turned out to be frozen lava. Doesn’t matter; lava freezes but the dream never dies. Maybe the water, like the life, is underground. And when the oldest (sulfur-based) fossils to date were found in Australia (3.4 bya), investigator Martin Brasier of Oxford said, “Could these sorts of things exist on Mars? It’s just about conceivable.”
Meanwhile, manned U.S. Mars exploration, perhaps the only sure way to find out, has been put off till the 2030s and, hype aside, there is a question whether Europe can afford it either. So in the end, $2.5 billion dollars later, the Curiosity rover has shown only that “at least in one place and time on ancient Mars, life as we know it could have survived,” but provides no clue whether it did or not.
In the Washington Post, Joel Achenbach explains that life on Mars is awfully cryptic: “Time and again, scientists have detected signatures of Martian life, only to discover that they were written in vanishing ink.”
They used to say that about ghosts.
But it doesn’t really matter. The Copernican Principle features assertions, not assessments. If nearby Mars proves stubborn, the exoplanets beckon. No one has so far shown that they do not teem with life, perhaps intelligent life. We need only come up with a convincing explanation of why we do not hear from them more often.
Image credit: Surface of Mars, Viking Project, M. Dale-Bannister WU StL, NASA.