Astrobiology – when it’s not all about us

Malcolm Gunn

 

The vaulted blackness of a moonless night where the Sonora Desert meets the Sea of Cortez is as good a place as any if you want to feel small. The sky there holds a staggering array of stars and the lack of trees, nearby hills and buildings that might otherwise anchor your perspective to Planet Earth, draws your consciousness to the heavens. The arid desert air doesn't scatter light so the smaller or more distant stars get to strut their stuff.

If you look at the sky long enough, you lose yourself among the galaxies and constellations and get a perspective that is hard to achieve when surrounded by earthly things. This, of course is the view our ancestors had. The rhythms of life seemed to be orchestrated by events in the skies. They signalled years, seasons and months, day and night. In Samoa, Palolo worms congregate in vast numbers, on the last quarter of the moon in October and November. Kina ripening and muttonbird nesting, the return of godwits and cuckoos could all be predicted by observing the heavens. The annual appearance of Matariki (Pleiades) on the eastern horizon in late May presages the start of a new year with the following new moon. Early civilisations built temples and pyramids dedicated to celestial bodies and the events that their movements heralded. Mayan pyramids and stoneworks recount the works of early astronomers, kings and their relationships with the heavens.

So for a long time we were at the centre of everything. All celestial bodies – the stars, sun and moon, revolved around us and that was enough to cement in the collective mind of mankind, our position at the hub of it all. The Sun, moon, comets and plants became gods, messengers and things to revere. With all these gods circling you, it's hard not to become a bit self absorbed; if we are at the centre of it all, maybe it is all about us. Aristotle and Ptolomy cemented this view in western minds and in the thirteenth century, this geocentric view became a cornerstone of Catholic theology through the work of Thomas Aquinas.

When Copernicus proposed an alternative, heliocentric model in his 1543 publication of De Revolutionibus Orbium Coelestium (On the Revolution of the Celestial Spheres), he dislodged us from the centre of the solar system. The corollary of that is if we are not at the centre of the solar system, then we are not the centre of our creator's magnum opus – Aristotle's immutable universe. Thus the astronomers, mathematicians and physicists who adhered to and developed this view were set on a collision course with the church.

Galileo of course believed the Copernican model to be the correct one and set about gathering evidence by both observation and experimentation. His experimental approach to astronomy set a benchmark for science and he established mathematical models for the movement of celestial bodies that were the foundation of Newton's laws. His doctrine of “Measure what is measurable and make measurable what is not so” goes to the heart of modern science. If you can measure it, so can others, and your work can be verified by others. The church was unimpressed. To challenge the accepted model of the universe was to challenge the teachings of the church. His books were banned and after publishing Starry Messenger in 1610, he was instructed by Papal edict to refrain from defending Copernicism. Despite the directive, Galileo considered that “In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual”. This sort of independent thinking got him into further trouble and when he appeared before the Inquisition for defending heliocentrism in defiance of the edict, he was sentenced to house arrest where he lived out the remainder of his days.

In the four hundred years since Galileo turned his telescope to the heavens and discovered Jupiter's moons, bigger and better telescopes have given us ever better understandings of the Universe. We have witnessed collapsing interstellar clouds in the act of forming new stars and we have seen them die in phenomenal acts of self destruction called supernovae. Through the eyes of astronauts, we have seen earthrise above the surface of our moon. That image of a blue planet against the deathly black void is one that should give us pause. Instead of occupying a very special place in the Universe we are starting to seem like a species that has flourished briefly, consuming resources and multiplying like a colony of bacteria in a petri dish. We do all this on a rocky planet that happens to have water and oxygen that hurtles through space like all other known objects. We might be top of the food chain, but the wet chunk of rock that we live on is hardly the center of it all.

Darwin too, confronted us with our ordinariness by providing a means (natural selection) by which we have evolved from our ancestors. He taught us humility in the presence of other species. Molecular geneticists have since shown that we share 97% of our DNA genome with chimpanzees. Thus they have acquired the status of distant cousins.

Yet for all the discoveries, plenty of big questions remain. Anyone who has spent any time contemplating the vast array of stars on a clear night will have wondered if there is any other life out there. The NASA Astrobiology Roadmap details the goals and objectives that underpin three big questions: how does life begin and evolve, does life exist elsewhere in the Universe, and what is the future of life on Earth and beyond? The roadmap draws on many branches of science to pursue specific objectives aimed at achieving seven science goals. Among them is the extent to which life can survive in extreme environments. Extremophiles – life forms that thrive in what we consider to be extremes of temperature, pressure or pH are perhaps the best indicators of the kind of life forms we might detect in the inhospitable parts of our solar system. Earth is the only planet where in what is termed the habitable zone of our solar system. Surface temperatures on other planets prohibit the existence of liquid water.

The existence of deep sub-surface biota several kilometers below the earth's rocky surface, however opens up possibilities for similar life to exist in similar environments beyond Earth. Jupiter's moons Europa, Ganymede and Callisto for example appear to have sub-surface brines maintained by internal tidal heating.

When a large asteroid hits Earth, such as it did 65 million years ago, material dislodged into space could conceivably hold such life forms. Capture of this material by another planet's gravitational field would be probable enough. Bacteria are known to be able to survive 5 years in space and if protected from solar radiation, many times that. Interplanetary transfer of life within our solar system is thus not regarded as wildly improbable. But where did life originate? The origin of life on Earth keeps getting pushed back so that the pre-life period of Earth's history is getting smaller, raising the possibility that life arrived her before flourishing, rather than evolved here. We simply do not know, but discovery of biosignatures on Mars would significantly lend weight to that theory.

Moving beyond our solar system, there are plenty of opportunities for planets to provide non-extreme habitats. The likelihood of that seems to grow each time we discover another "exoplanet" – a planet beyond our own solar system.

Finding exoplanets is not easy. One way to detect a distant planet is to detect the drop in light intensity from a star as the planet passes in front of (ie transits) a star. Naturally, the bigger the planet, the greater the change and hence the easier it is to detect. But the majority of exoplanets have been discovered by observing the movement of stars in response to the gravitational fields of their satellites. The most accurate method of measuring the extent of the 'wobble' that a revolving planet causes in a star is to observe the spectrum emitted from the star as it moves. Any movement away from the observer will result in a shift towards the red end of the spectrum, while any shift towards the observer will create a shift towards the blue end.

So far the vast majority of such exoplanets (around 350 have been discovered to date) are gas giants, dubbed "hot Jupiters" by astronomers. These vast gaseous planets seem hostile to the prospect of life as we know it. The reason these planets are found is because they are so big. They are big enough to influence the movement of the stars that they orbit, sometimes with very short 'year' spans.

Large planets will influence their stars' movement considerably if they are close, but smaller planets with a mass and density similar to Earth's are more elusive. A problem for astrobiologists searching for life outside the solar system is that if a small planet is close enough to exert a significant gravitational force on its star, it is likely to be in a very hot place – well outside the habitable zone. Small red dwarf stars provide the exception. Their habitable zones are closer in and their lower mass means they are influenced by gravity of smaller planets. Gliese 581 in the constellation Libra, around 20 light years away is one such star.

The instrument used to detect the light shift of wobbling stars is called a spectrograph. One of the best is the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph in Chile which can detect a star's wobble that is as slow as around 7km/hr. In September 2009, the HARPS spectrograph confirmed the existence of a planet just 1.9 Earth-masses. Gliese 581-e, orbits Gliese 581 in a little over three days and is the lightest exoplanet discovered to date. Being close to the star, it is not within the habitable zone, but its very existence as a rocky planet is encouraging. Its larger (at 7 Earth-masses) sister planet, Gliese 581-d appears to be within the habitable zone and it is tempting to speculate on the possibilities there.

The launch in 2013 of the successor to the Hubble Telescope – the James Webb Space Telescope will enable the atmospheric properties of rocky exoplanets as they transit. Meanwhile the European Space Agency (ESA) is considering the Darwin satellite launch – a mission to search for water and oxygen in the atmospheres of exoplanets.

But what will it mean to find life beyond Earth or at least some form of biosignature – some algal footprint equivalent, as it seems we will, one day? Will we feel some kind of primordial kinship with a chemosynthetic sub surface prokaryote? Will we feel superior? Threatened? What kind of self replicating complex of molecules will we be prepared to make room for in our collective psyche? What if the new life form is multicellular, has segments, even limbs, fins, who knows what else? Most of us probably consider that if we find some form of life within our solar system, it will be comfortably primitive. Nothing too challenging, too eucaryotic, multicellular or complex. But step outside our solar system and almost anything seems possible. Just the numbers make it so – as in Monty Python's Galaxy song says, "And our galaxy is only one of millions and billions in this amazing and expanding universe". The only thing we do know for sure is that we simply don't know. Scientists of course accept this lack of knowledge as their stock in trade. Not knowing is a key ingredient – the starting point of scientific research.

But how will the church react to the discovery of extraterrestrial life? Will the doctrine of “Intelligent Design” so popular in USA (where 40% of people do not believe in evolution) embrace extraterrestrial life in its ambit?

There was a time when it was easy to see the centre of the universe – you just needed a mirror. What you saw was the the image of God, the raison d'être for the universe itself. But not only is the universe unimaginably bigger than we imagined (and getting bigger), our place in it is by comparison becoming correspondingly smaller. Of course we still occupy the same place we always have as a species. What has changed is our perspective. For as Galileo himself observed, “The Sun with all the planets around it and depending on it, can still ripen a bunch of grapes as if it had nothing else in the Universe to do.” For that, and especially our capacity to appreciate it, we can all be grateful. Perhaps after all, one of the best views through the James Webb Space Telescope will be of ourselves.