The hunt for alien life
In universities and space agencies around the world, astrobiology is flourishing as never before. Clive Cookson investigates what scientists have uncovered so far
Over lunch at Los Alamos National Lab in 1950, several distinguished scientists were discussing the latest spate of implausible UFO sightings. “Where is everybody?” asked the Italian physicist Enrico Fermi. His point, which quickly became known as Fermi’s Paradox, was that in a universe billions of years old and presumed to contain billions of planets, at least one super-advanced civilisation would surely have evolved to colonise our galaxy — and we would have detected their presence by now. Yet there was no sign of alien life on Earth or, as far as astronomers could tell, out in space either.
The paradox has become ever more glaring. After a vast increase in observations, both from spacecraft exploring the solar system and from telescopes gazing out into the galaxy, we still have no evidence for creatures big or small, intelligent or otherwise, beyond Earth. Yet astrobiology, the study of extraterrestrial life, is flourishing as never before in universities and space agencies around the world. Though nothing has turned up so far, the scientific mood has not been so positive about the prospect of finding alien life since the late 19th century, when the US astronomer Percival Lowell convinced himself and many others that the surface of Mars was criss-crossed with artificial canals.
Publishers have long loved life beyond Earth, not only as a mainstay of science fiction but as a popular non-fiction subject too. The growing optimism is reflected in a new wave of books on the subject, four of which are reviewed here — all highly recommended. Jon Willis and Louisa Preston, both academic astrobiologists, have written solo accounts of the search for extraterrestrial life. The physicist Jim Al-Khalili has compiled a sharply edited volume with essays on aliens by 19 experts. And science journalist Charles Wohlforth has teamed up with planetary scientist Amanda Hendrix to take a different look at life beyond Earth: how humans could colonise space.
The search for extraterrestrial life has two broad strands. One is to send spacecraft out to investigate planets in our solar system and their moons. The other is to observe the more distant universe beyond the solar system — too far to explore directly with space missions in the foreseeable future — and look for signs of life, ranging from signals from an advanced civilisation to evidence of an atmosphere like Earth’s that could only be generated by biological activity.
The first extrasolar planet (orbiting a star other than our sun) was not detected till 1995. A trickle and then a torrent of other discoveries followed, as ground-based and orbital telescopes identified planets either from the slight dimming they induced when passing in front of their parent star or from the equally slight wobble their gravity caused in its movement.
At the latest count, several thousand extrasolar planets are known and statistical extrapolations show that “Earthlike worlds populate our Milky Way in their billions,” as Willis puts it in All These Worlds Are Yours. What is exciting is not just the sheer number of planets but their diversity. The ones discovered early on in the search were all too big and too hot to host any form of life imaginable by terrestrial scientists, but planets whose size and distance from their planet star make them potentially habitable are cropping up with increasing frequency.
“A perfect world does not need to be a complete replica of the Earth but should simply enjoy some of its finest features,” writes Preston in Goldilocks and the Water Bears. “The more exoplanetary worlds we see, the more we are starting to consider that the Earth may not even be the gold standard for life. There may conceivably be superhabitable worlds out there that are even better suited than the Earth to support life.”
Of course, as these authors emphasise, habitable (or even superhabitable) does not mean inhabited. Astronomers hope to use the next generation of telescopes with acronyms such as Cheops, Plato and Tess, as well as the James Webb space telescope that will replace the venerable Hubble, to characterise exoplanetary atmospheres through spectroscopic analysis.
Some atmospheric compositions would be signatures of life. For example, geochemists say biological processes would probably be needed to maintain high levels of oxygen, which is too reactive to survive for long without constant replenishment; the oxygen-rich atmosphere on Earth depends on photosynthesis by plants. The detection of certain organic molecules would also be exciting news for astrobiology.
If any planets with clear biochemical signs of life are detected, Seti — the search for extraterrestrial intelligence launched in 1960 when the astronomer Frank Drake pointed the Green Bank radio telescope at two nearby stars — will focus on them. Until now, Seti searchers have had no guidance about where to point their instruments, as they listen out for radio (and to a lesser extent optical or laser) signals from alien civilisations. So they have surveyed the whole sky with increasingly sophisticated telescopes linked to ever more powerful computers for data analysis. There have been a few heart-stopping false alarms but no clear detection of alien activity.
Perhaps most civilisations only use radio during a transient phase of development. A more recent strand of Seti is looking for stars showing strange energy and light patterns that might indicate megastructures built by far more advanced civilisations. Preston discusses a bizarre star called KIC 8462852 whose brightness oscillates by 20 per cent — possibly because a huge energy harvesting structure has been constructed around it. Astronomers have not detected any radio or light signals from this potential astro-engineering project but, as she says, KIC 8462852 will undoubtedly be the subject of Seti observation for years to come.
At the other end of the continuum of alien sophistication are direct missions to explore the solar system, where any extraterrestrial life is likely to be microbial rather than superintelligent. Humans have sent robotic probes to investigate every planet as well as moons, asteroids and comets. Although astrobiology has not been their main purpose, they have discovered conditions where simple life forms might thrive — particularly beneath the planetary or lunar surface.
Potential life on Mars has received most publicity, as it is the body most like Earth and (after the Moon) the most easily visited. But these authors put more emphasis on prospects further out in the solar system. As Willis puts it: “In one hundred years, people may well question why astrobiologists began their search for life with dusty, dead Mars and not with the salty oceans of liquid water encased within the icy moons of Jupiter and Saturn.”
Satellite measurements suggest that the Jovian moon Europa contains twice as much liquid water as Earth’s oceans, beneath its frozen surface. Tidal heating of Europa’s rocky core through Jupiter’s gravitational pull may generate hot water vents like those on the floor of terrestrial oceans, where life thrives — and, according to some theories, may even have originated. “It is difficult to overstate the extent to which the discovery of deep-sea hydrothermal vents on Earth has transformed our view of the habitability of the outer solar system,” writes Willis.
Saturn’s Enceladus also has a liquid ocean but its frozen crust is less thick — thin enough, indeed, for some of the water to burst through cracks in the ice and erupt as geysers as far as 600km into space. Measurements by the Cassini spacecraft found that these plumes contain not only salty water but simple carbon compounds. The assumption, Preston says, “is that organic molecules used by and needed for life are present deep within Enceladus”.
Titan, Saturn’s largest moon, is a very different prospect. In some ways it is the most Earthlike body in the solar system. Cassini’s observations show a thick atmosphere, weather, lakes, seas and running rivers. But liquid hydrocarbons (methane and ethane) take over the role played by water on Earth. Titan is far too cold for water to remain liquid, with surface temperatures of about — 180C.
Even so, many astrobiologists see Titan as the best bet for alien life in the outer solar system. Wohlforth and Hendrix believe it is also the most hospitable place for people to establish a human colony beyond Earth — and their book outlines a path to achieving this. Beyond Earth alternates speculative passages about the future with accounts of present-day science and engineering; the imaginative scenario they present is rooted in today’s reality.
The indigenous fauna encountered by the Titanian colonists are “methane-based fish . . . tiny and strangely amorphous, their soft hydrocarbon membranes partially translucent as they flitted in the frigid methane medium”, Wohlforth and Hendrix write. “Examining them was difficult because of their delicacy and rapid movement. They disintegrated when touched by the slightest warmth.”
However, it may not be necessary to look or travel into space to find alien life. The physicist and astrobiologist Paul Davies, a contributor to Al-Khalili’s Aliens, takes the opportunity to advance his view that scientists should make more effort to look for a “shadow biosphere” — hidden creatures resulting from a second genesis of life here on Earth. All known terrestrial life follows essentially the same biochemistry, with genes made from nucleic acids and proteins from amino acids, and is presumed to have descended from a single origin still shrouded in mystery.
Yet, if conditions on the young Earth were hospitable for life to get going, why should it not have happened more than once? Success might be a long shot, Davies concedes, but “it is hard to imagine a discovery of greater significance to astrobiology than a shadow biosphere on Earth consisting of a different form of life descended from an independent origin . . . Because Earthlike planets are likely to be common, life could then be regarded as a truly cosmic phenomenon.”
For now, however, we are left with Fermi’s Paradox unsolved. Perhaps the origin and continuation of life depends on so many highly unlikely factors coming together that we really are alone. Most astrobiologists reject that conjecture. Perhaps intelligent life is out there, possibly even in plain view, but in forms that we are too primitive to appreciate. Then there is the possibility, favoured by Preston, that we are like animals in a galactic zoo. A super-advanced civilisation might regard striking up a conversation with humans as being as futile as us trying to talk to an ant colony.
“Perhaps more advanced civilisations are waiting until we are worthy,” she speculates. Or “they may assign us . . . to conservation and study in a galactic zoo with the aliens as our keepers. Perhaps they are reading this and laughing at us for not realising.”
Goldilocks and the Water Bears: The Search for Life in the Universe, by Louisa Preston, Bloomsbury, RRP£16.99/$27, 288 pages
All These Worlds Are Yours: The Scientific Search for Alien Life, by Jon Willis, Yale, RRP£18.99/$30, 232 pages
Aliens: Science Asks — Is Anyone Out There?, edited by Jim Al-Khalili, Profile, RRP£8.99, 232 pages
Beyond Earth: Our Path to a New Home in the Planets, by Charles Wohlforth and Amanda Hendrix, Pantheon, RRP$27.95, 320 pages
Clive Cookson is the FT’s science editor
Photographs: Dani Caxete; Carlos Fairbairn; Tom O’Donoghue; Pavel Pech; Nicolas Outter
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