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On the verge of the world’s largest gathering of professional astronomers here in Vienna, it only seems appropriate to ask a few hot-button questions. The International Astronomical Union’s (IAU) XXXth General Assembly, which kicks off on Monday, will include discussions on everything from big bang cosmology to how galaxies first formed near the dawn of time.
But questions about the essence of life; how it starts; and where else it might be in the galaxy remain astronomy’s go-to topics these days. Thus, I turned to three prominent astrobiologists for perspective.
--- What remains the key puzzle about extraterrestrial life?
“We do not fully understand the origin of life; the transformation of non-living matter to living matter,” Manfred Cuntz, an astrobiologist at the University of Texas at Arlington, told me. “ This lack of knowledge is also limiting our ability to successfully identify habitable worlds beyond Earth .”
Our current lack of knowledge extends to such basics as the amount of oxygen in Earth’s current atmosphere, Ariel Anbar, an astrobiologist at Arizona State University in Tempe, told me.
“We want to use [molecular] oxygen (O2) in planetary atmospheres as a signature of life,” said Anbar. “But we don’t really understand what controls the amount of O2 in Earth’s atmosphere and what drove its rise about 2.4 billion years ago.”
--- In 50 years time, will we look back and say life on Earth was inevitable?
As Anbar notes, that also depends on whether a future flyby mission of Saturn’s moon of Enceladus detects microbial life in one of those bodies volcanic plumes. If so, the argument could easily be made that microbial life at least must be very common if it occurred twice in one solar system.
“I would expect that at least microbial life should be widespread in the universe as it occurred in the first 85% of earth’s history,” Vladimir Airapetian, an astrobiologist at NASA’s Goddard Space Flight Center in Maryland, told me. He says it’s still not clear how complex chemistry evolves to first replicate itself biologically, as a living system. Partly as a result, Airapetian says complex life could be a luxury for most exoplanets.
Even so, Anbar says most his colleagues think that life inevitably emerges as a way for planets to dissipate energy “trapped” in the form of chemical disequilibrium. That is, sort of like the way convection currents arise in a hot pot, or in Earth’s mantle, to help heat escape, says Anbar. But he says the fact that this is what life seems to do does not mean that the emergence of “life” is inevitable.
--- What are we missing in our current research?
“We are probably thinking too narrowly about how to detect life,” said Anbar. Ideas about the laws of biology will be tested against what we find out there, he says. That tends to get lost in the obsession over “finding life” as though it is its own reward, says Anbar.
Anbar says finding extraterrestrial life should not be treated as if it were a quest for a holy grail . Instead, he says astrobiology’s purpose is to describe and understand the distribution of life in the universe so that we better understand what we are and how we fit into the cosmos.
The late evolutionary biologist Stephen Jay Gould famously wrote that if we were to rewind the tape of life, its chances of evolving in exactly the same way on Earth would be akin to the chances that a Boeing 747 aircraft could self-assemble in a hurricane.
--- Does life’s evolution follow universal vectors?
“Once you have living things feeding off trapped energy, and a mechanism for evolving-through-competition-and-selection, the laws we seem to understand tell us that the machinery will grow more sophisticated at doing that job,” said Anbar. That is at least up to a point, he says.
“So, there is some arrow pointing toward metabolic complexity of a sort,” said Anbar. “I suspect there is also an arrow pointing to the development of multicellular life and intelligence. But that’s harder to prove.”
--- Where do you think extraterrestrial life beyond our solar system is most likely?
Based on research done with Villanova University astronomer Edward Guinan, Cuntz still thinks advanced life forms have the highest likelihood to exist in the environments of certain orange dwarf stars that are still in their main sequence hydrogen-burning phases.
These types of stars are relatively numerous, have decent-sized habitable zones, and relatively long lifetimes on the hydrogen-burning main sequence, that is 15 to 30 billion years, longer than the present age of the universe. That’s some 5 to 15 billion years longer than the hydrogen burning life of our own Sun.
Cuntz also notes for much of their lives, such orange dwarfs have limited stellar activity.
But are we making progress?
Arguably, astronomers have made great progress in the last 50 years in quantifying our existence in this part of the galaxy. After all, we now know that thousands of planets circle other sun-like stars. And we are on the verge of detecting spectra from these planets that may be indicative of life.
Yet there has been progress in the search. After all, as Anbar points out, in the last 25 years the field of astrobiology has evolved from science fiction to science.
The search for extraterrestrial intelligence, on the other hand, has thus far come up with nothing. That is, except to rule out the possibility that intelligent aliens around many nearby stars are not sending out radio or optical laser signals that we can detect. Nor are they producing any electromagnetic leakage that we are capable of detecting.
Thus, we as a species remain mired in the possibility that we may be the lone intelligent technological species in this part of the galaxy. A world adrift in a sea of potentially habitable extrasolar planets that may or may not harbor microbial life.
Are we alone?
As Cuntz reminds me, “absence of evidence” is not “evidence of absence.” And in a universe like ours, proof that we are truly an island of intelligence in this one galactic sea will likely never come.
