Not as we know it
Last week saw the successful launch of the Kepler Space Telescope, designed to search our region of the Milky Way for extrasolar planets. One of the goals of the mission is to discover the frequency of “terrestrial” planets, rocky worlds between half and twice the size of Earth, and especially those in the habitable zone where life might arise. What would such life look like, were we able to visit?
The logic of restricting the search for life to the habitable zone is clear: we have a single example of life, and it arose right here on Earth. Our planet lies in a low-eccentricity orbit around a hydrostatically stable, main sequence, Population I star, at a distance where water can remain liquid for billions of years.
There’s little point in pointing Kepler towards stars like Eta Carinae, as interesting as it might be in itself, because it is of relatively short lifespan and is “prone to violent outbursts” that will have obliterated any planets in its orbit. Nor is there much point in looking towards stars like Vega, whose spectral classification leads scientists to believe it is particularly poor in metallic elements, which are important in planet formation.
Moreover, high-metallicity may indicate that the star formed in an accretion disk rich in elements heavier than iron, which cannot easily form in stellar nucleosynthesis. Simple life-like chemicals like those postulated for the “primordial soup” may have been composed entirely of carbon, hydrogen, nitrogen, oxygen and a smattering of phosphorous. But many of the elements essential for life on Earth are heavier than iron, so it would be helpful if our candidate stars were formed from the debris of supernovae.
Despite these restrictions, we are fortunate enough to have a “few dozen” sun-like stars within 30 light years of our own, and perhaps half of these will have rocky planets in their orbit, according to Dr Alan Boss of the Carnegie Institution for Science.
So Kepler is presumably looking in the right direction. But, having identified planets where life might arise, what might we expect to find if we could visit?
Again, since we only have one example to work with, we may as well take a look around on Earth. The IUCN Red List gives the following numbers of described species in the major divisions of life:
- Invertebrates: 1,232,384
- Plants: 298,506
- Vertebrates: 61,259
- Lichens, fungi and brown algae: 50,040
- Microbes: 100,000 – 1,000,000
The relative number of species is to say nothing of their relative abundance, of course. So it’s instructive to note that, phylogenetically, we have more in common with a dromedary, or a stegosaurus, or a coelacanth, or even a sea squirt, than any of the myriad creatures you might scoop up in a handful of soil.
This makes the panoply of “aliens” common to space opera seem depressingly unimaginative. Vulcans, Klingons, Romulans, Cardassians, Wookies, Ewoks, and whatever the hell Yoda is supposed to be are all virtually indistinguishable from humans apart from the addition of a prosthetic nose or a skin complaint.
A couple of early Star Trek episodes at least nodded towards the possibilities, with parasitic flying omlettes that drive the crew of the Enterprise to the brink of insanity, and the Horta: a corrosive subterranean silicate blob which clashes with miners before it is subdued, etching its pleas for mercy in the floor of a cave.
The Horta shows that silicon enjoyed some popularity as a theoretical basis for alien life in past years, since lies in the same group as Carbon on the periodic table and has four valences. Unfortunately, however, there are no known conditions in which Si forms the long-chain polymers common to organic chemistry, and thus life.
In science fiction proper, meanwhile, Stanislaw Lem’s Solaris presents us with an entire planet that seems to be a single, sentient organism, presaging Lovelock’s Gaia hypothesis a decade ahead of its publication.
Given the extraordinary variety of life on Earth, the probability of finding bipedal aliens of about our stature are vanishingly small. The question of size was memorably examined by Douglas Adams, when an entire alien invasion fleet is devoured by a small dog after a “terrible miscalculation of scale”.
At the other extreme, discounting for the moment planet-sized organisms, we might reasonably expect to meet gigantic creatures. Elephants remind us we are by no means the largest terrestrial organisms, to say nothing of the blue whale.
Indeed, among animals, natural selection seems to favour ever larger specimens as a result of competition among males for mating rights. Komodo dragons presumably reached their monstrous proportions through generations of males establishing dominance in wrestling matches. And even today’s terrestrial vertebrates are dwarfed by many specimens in the natural history museums of the world.
Which brings us neatly to the fourth dimension. Breaking down the 4.5 billion years of Earth’s history, we have:
- 1 billion years as a lifeless rock
- 2.5 billion years of single-celled organisms
- 250 million years of multi-cellular organisms
- 50 million years of cephalopods and arthropods
- 200 million years of fish
- 140 million years of reptiles
- 65 million years of mammals, including 10 million years of large primates
- 100,000 years of anatomically modern humans
It is disheartening to look at this list, and think that any aliens capable of visiting Earth to make contact may have already been and gone, with only Eusthenopteron around to tell the tale.
And yet, according to some researchers, the best place to look for “alien” life may be right here on Earth after all. Paul Davies of Arizona State University in Tempe postulates “shadow life” in an article in Astrobiology.
All life on Earth is generally understood to be monophyletic. This does not preclude the possibility of life having arisen multiple times, only to be obliterated time and again by the late heavy bombardment, or simply out competed by our ancestors.
But Davies suggests a “Second Genesis” of a kind of life unlike ours may have led to lifeforms hanging on in inhospitable niches, such as the now-famous hydrothermal vents on the sea floor, or the vast sub-glacial lakes of Antarctica.
Discerning shadow life from the ordinary variety will be tricky, but one way would be to look for unusual uses of chemicals. Ordinary life almost exclusively uses right-handed isomers of sugars, and left-handed isomers of amino acids. Detecting either of the reverse might indicate life of a different ancestry, though recent studies suggest water may have a role in explaining the enantiomeric excess: asteriods contain 15% more left-handed amino acids. Organisms that use arsenic in place of phosphorous for energy transport (adenosine triarcenate?) would be weirder still. Arsenic’s very toxicity in ordinary life is due to its chemical similarity to phosphorous.
Tantalisingly, traces of shadow life may already have been discovered. The manganese-rich patina called “desert varnish” has no known source, and strongly resembles the structure of the stromatolites found in western Australia.
As Davies says, “shadow life may be right under our noses. Or even in our noses”. Think about that next time you reach for the tissues.