Meeting the Neighbours

by Andrew Crumey

Review of All These Worlds Are Yours by Jon Willis. The Wall Street Journal, 9 September 2016.

Imagine that life is discovered beyond the Earth. How would you react? Would it bring about some profound change of attitude? Hold on, it's happened already. I remember the day, August 7 1996, when NASA announced the presence of what appeared to be microfossils in a piece of Martian rock. My jaw dropped at news I'd longed to hear since childhood: we were no longer alone in the universe. President Clinton stepped onto the South Lawn of the White House and told the assembled press, "If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered." Yet for all the fanfare, Martian meteorite ALH84001 soon slipped out of the limelight. Here were no little green men, only tiny mineral structures resembling bacteria. And twenty years on, the evidence is still considered inconclusive. It serves, Mr. Willis says in his book, as a warning that any search for extraterrestrial life "may not offer the clear answers that we perhaps naively expect."

A naive expectation on my part was that his book about the quest for alien life would give substantial coverage to SETI, the Search For Extraterrestrial Intelligence, which focuses on trying to pick up radio communications from distant planets. Leaving aside theoretical arguments against the likelihood of ever hearing anything, the decades of silence endured by SETI researchers are enough to convince Mr Willis that it does not merit public funding. NASA agree: they ceased support in 1995. The project now relies on the generosity of patrons such as billionaire Yuri Milner, and the roughly five million people who donate computer time to SETI@home.

Mr. Willis wishes them all the luck in this and every other world, but as a professional astronomer he is concerned with trying to identify areas with the best chance of actually finding something. There are, he says, two ways we might discover alien life: from a "biomarker" in a planetary atmosphere, or as actual organisms. The first requires powerful telescopes, the second involves robotic space missions. In either case we need to have some idea what we might be looking for.

NASA sets much store by both possibilities, and the anti-climax of ALH84001 has done little to dent hope or deflate hype. In April 2014 it was announced that the Kepler Space Telescope had detected an Earth-size extrasolar planet in the "habitable zone" of a star five hundred light years away. Dubbed Kepler-186f, the planet is at a distance from its parent star that would allow liquid water to exist on its surface, and the media were quick to take up the possibility of a "twin Earth" that could potentially support life. But we don't know if there is any water on Kepler-186f, let alone anything swimming there.

A better measure of life-bearing possibility is atmospheric composition. The idea originated with James Lovelock, famous for his Gaia hypothesis. Suppose an alien civilisation knew nothing about our planet except the chemistry revealed by sunlight glinting through air at the edge of Earth's tiny far-off disc. As Lovelock pointed out, the aliens would be very puzzled by the abundance of oxygen. The element was lacking on our planet until the appearance of photosynthesising organisms which produced it as a waste product, making possible the evolution of animals such as ourselves who breathe it, and it's life that keeps oxygen at a high and fairly constant level. If all life vanished from Earth then so too, over a relatively short geological time period, would the oxygen, consumed in fires, rusting and other reactions. One exoplanet that has yielded to the difficult challenge of having its atmosphere analysed is HAT-P-11b, a Neptune-like world a little over a hundred light years away, imaged by the Hubble Space Telescope and found to have water vapour in its clouds.

But is water necessary for life, and would oxygen be the surest biomarker? As Mr. Willis explains, extraterrestrial life could have evolved in very different conditions, and along very different chemical pathways, from our own. Aside from our oxygen-fed, glucose-burning metabolism there are other reactions that could potentially release energy into living cells: for example, inhaling acetylene and hydrogen, and releasing methane. Interestingly, there is an anomalous abundance of methane on Titan, the largest of Saturn's moons. Perhaps the gas is venting from reserves beneath the surface, but life is seriously being considered as a possible explanation. Measurements by the Cassini spacecraft have revealed that acetylene and hydrogen are somehow being used up near Titan's surface, in unknown chemical reactions. So, Mr. Willis asks, is this unambiguous evidence for acetylene-gulping microbes? No, he says; "a good astrobiologist should use life as the conclusion of last resort".

In 2005 the Huygens probe landed on Titan's surface, and through an orange fog it showed a terrain strewn with icy pebbles but devoid of any visible organisms. Scientists will need a lot of convincing before reaching for the last resort - as Carl Sagan liked to say, extraordinary claims require extraordinary evidence. Biomarkers detected from afar might never be enough to remove doubt, and if aliens never give us a call then perhaps the only hope of unequivocal proof will be when living goo is scooped from another world and brought back to Earth. That makes our own solar system the only possible search area, and Mr. Willis singles out the prime targets as Mars, Titan, and two of Jupiter's moons, Europa and Enceladus. His book largely concentrates on these worlds and the challenge of reaching them.

Mr Willis teaches astrobiology to students, and his book conveys great enthusiasm alongside necessary scientific scepticism. The technical details and problems are clearly laid out and discussed - even defining "life" is far from straightforward. All of this makes welcome reading for lovers of science-fact, though it might be disappointing for those raised on science-fiction, and perhaps hoping for a little more by way of historical perspective on a subject that people have been speculating about for centuries. And while the transfer from lecture hall to page generally works well enough, I did wish we could have had rather fewer rhetorical questions. At one point I counted thirteen in three paragraphs.

But while the prose may not always shine, the topic is certainly fascinating. If Mr. Willis could pick only one target out of his shortlist it would be Enceladus; not as far away as Titan, and with the convenient presence of water-spewing geysers whose outpourings could be collected by an orbiting craft. He puts the price tag at $4 billion, pointing out that this is how much the world's nations collectively spend on defense every single day.

All of which leaves the final rhetorical question: is that a price worth paying?