The Japanese National Institutes of Natural Sciences (NINS) are reporting about new research that throws a small wrinkle into the search for life on planets outside our solar system.
Such bodies, known as “exoplanets,” have emerged as one of the more exciting areas of astronomical study — an entire new field of research having essentially arisen in little more than two decades and now occupying many full-time researchers, several earth-bound telescopes, and even dedicated space missions. Early results have been impressive, with the improvements in sensor technology matched by the exponential increase in discovered bodies. After the first lone exoplanet was discovered around a main sequence star in 1995, a small trickle of additional exoplanets were discovered. Then the trickle became a stream. Then a flood. In terms of the sheer number of known planets, what we lost by Pluto’s ignominious demotion a decade later, has been exponentially made up for by the nearly 2,000 confirmed discoveries to date, with thousands more candidate planets still awaiting confirmation.
As the discoveries have expanded, so too have the goals and aspirations of exoplanet research. Initially, just the mere existence of an exoplanet was enough to spark the enthusiasm and imaginations of astronomers everywhere. (A wonderful recent article provides a glimpse into the initial excitement that first discovery sparked two decades ago.) Then efforts turned to finding additional exoplanets. Then to finding exoplanets within the “habitable zone” — a variable (but readily calculable) zone in which liquid water can exist at the surface. Then the hunt was on for Earth-like planets — with each advance in technology and each year of additional observation time leading inexorably toward the goal, the most exciting prospect to date being Kepler-452b, a planet some 1.5 times the diameter of Earth, within the habitable zone, and orbiting a Sun-like star.
But the ultimate goal, the Holy Grail, of exoplanet research, is finding life on a planet outside our solar system. Warp drive being unfortunately stalled at the theoretical stage and SETI’s radio telescopes thus far remaining obstinately silent, we are left to seek extraterrestrial life through indirect evidence, much like the archaeologist digging for broken pottery shards or the detective dusting for elusive fingerprints.
One of those potential fingerprints, it turns out, is indeed accessible to our current technological capabilities. For the few exoplanets that transit in front of their host star, as seen from Earth, it is possible to train our telescopes on the distant cosmic dance and tease out a signature, a fingerprint, if you will, of the planet’s atmospheric makeup. Or at least part of the atmospheric makeup: among others, water vapor, oxygen, and with recently-improving methods, nitrogen. A certain atmospheric makeup and, it is believed, we will have confirmation of life on a distant world. Oxygen is believed to be a key marker for life, with any meaningful atmospheric percentage standing as a tell-tale sign of the photosynthesis silently taking place on the surface below.
Thus the current wrinkle.
Norio Narita of the Astrobiology Center of NINS and Shigeyuki Masaoka of the Institute of Molecular Science of NINS now propose that it may be possible for planets to have large quantities of abiotic oxygen. The press article available at the National Astronomical Observatory of Japan website and at the primary exoplanet-tracking website, JPL’s Planetquest site, explains that “until now, it had been thought that if a planet has oxygen, that must mean that some form of plants are producing it through photosynthesis.” However, the researchers have argued that “abiotic oxygen produced by the photocatalytic reaction of titanium oxide, which is known to be abundant on the surfaces of terrestrial planets, meteorites, and the Moon” could be a source of significant atmospheric oxygen.
The sobering take home message is as follows:
They found that even in the least efficient production case of a low-temperature star, the photocatalytic reaction of the titanium oxide on about 3% of the planetary surface could maintain this level of atmospheric oxygen through abiotic processes. In other words, it is possible that a habitable extrasolar planet could maintain an atmosphere with Earth-like oxygen, even without organisms to perform photosynthesis.
This is but a preliminary analysis and it remains to be seen whether abiotic production of oxygen is indeed sufficient to create the kind of atmospheric signal the researchers suggest. If their analysis holds, the implications for detecting extraterrestrial life through indirect methods such as transit-based atmospheric spectrography are clear.
Implications for Abiogenesis
What is not explored in the article, but is also highly interesting and perhaps even more interesting to the present audience, is the possible implication for naturalistic abiogenesis scenarios. Specifically, if a dead, lifeless world were to produce a significant amount of pre-biotic oxygen through the kind of photocatalytic reaction proposed by Narita and Masaoka, how would that impact the likelihood of such a world producing a first living organism under typical abiogenesis scenarios?
Recall, after all, that in a 2003 interview with Astrobiology Magazine 50 years after his groundbreaking research with Harold Urey, Stanley Miller continued to affirm that a “reducing atmosphere” was necessary for pre-biotic life to arise. Oxygen throws a wrench into the works.
The existence of large amounts of pre-biotic oxygen would not, of course, in and of itself disprove abiogenesis. But it would constitute yet another nagging question and another problematic hurdle for the naturalistic origins storyline.
That is true whether on Kepler-452b or closer to home.