The Kepler Spacecraft surely ranks as one of NASA's most cost-effective and inspirational projects. It has hugely enriched our cosmic perspective by revealing that most stars in the Galaxy are orbited by systems of planets. The main surprise has been the huge variety of these systems. Few resemble the Solar System - in particular, there are many planets, even giant planets like Jupiter, orbiting so close to their parent star that a 'year' would last only a few days.
The variety is fascinating to astronomers. Until these recent discoveries, with only one planetary system known (our Solar System). It's been impossible to know which its features are generic and which are very special.
But special interest attaches to planets that are twins of our Earth - similar in size, and orbiting their star at a distance such that liquid water can exist. These are in what's called the 'Goldilocks Zone - neither too hot nor too cold for life to exist. And that's why Kepler 452 b has hit the headlines this week. It is the most Earthlike in these respects of the thousands of planets Kepler has identified. Its discovery strengthens the claim that there are literally billions of earth-like planets in our Milky Way galaxy with the size and temperature of our Earth.
But would these planets have developed biospheres? Or is our Earth unique, all others being sterile and lifeless? We know too little about how life began, and what evolutionary paths it might take, to answer this basic question. We know, at least in outline, the evolutionary steps whereby the first organisms 'seeded' the marvelous process of Darwinian evolution that led, about four billion years later, to the biosphere of which we humans are a part. But biochemists still don't understand the transition from complex molecules to the first replicating and metabolizing systems that we would deem to be 'alive'. We still don't know whether the emergence of the first life was a 'fluke', or whether it is near-inevitable in the kind of 'chemical soup' expected on any planet resembling the young Earth.
This great scientific challenge is now attracting more attention than ever before, and the answer may emerge within a decade or two. Such progress may also indicate whether there is something unique about the DNA/RNA structures that are the basis for all terrestrial life, or whether other 'chemistries' are possible.
Can astronomers somehow discern whether Earth-like exo-planets have biospheres, or whether they are sterile? At the moment, this is impossible - we infer the presence of these planets by detecting the slight dimming of their star's light when the planet passes in front of it. But within 10-15 years there will be a transformation. First, we will by then have found Earth-like planets at least 20 times closer (and 400 times less faint) than K 452 b. This is because Kepler has scanned only about a thousandth of the sky. Also, it detects only those planets whose orbital planes lie close to our line of sight, so that that they transit across the face of their star. There should be far more with orbits that don't lead to a transit.
But detecting the light from even the nearest Earthlike planet will be a challenge: it is swamped many millionfold by the light of its star - a firefly next to a searchlight. The best bet will be the next-generation of giant ground-based telescopes. For instance the European Extremely Large Telescope (the E-ELT), to be built on a mountaintop in Chile, recently leveled for the purpose, with a mosaic a mirror 39 metres across will be able to analyse the light from nearby stars precisely enough to pick out the tiny contribution from orbiting planets, and deduce what their atmospheres consist of. This will offer clues to simple life.
But there is a second still more fascinating question: Even if simple life exists, what are the odds that it evolves into something that we would recognise as intelligent? Even if primitive life were common, the emergence of 'advanced' life may not be - it may depend on many contingencies (like the event that wiped out dinosaurs 65 million years ago).
Searches for extraterrestrial intelligence (SETI) are a gamble. But they are surely a worthwhile one, even if there are heavy odds against success. A manifestly artificial signal - even if we couldn't decode it - would convey the momentous message that 'intelligence' wasn't unique to the Earth and had evolved elsewhere. But it could take a form very different from us. Seemingly artificial signals could come from super-intelligent (though not necessarily conscious) computers, created by a race of alien beings that had already died out.
Even if intelligence were widespread in the cosmos, we may only ever recognise a small and atypical fraction of it. Some 'brains' may package reality in a fashion that we can't conceive. Others could be living contemplative lives, perhaps deep under some planetary ocean, doing nothing to reveal their presence. The only type of intelligence we could detect would be the (perhaps small) subset that used a technology attuned to our own parochial concepts. It makes sense to focus searches first on Earth-like planets orbiting long-lived stars. But science fiction authors remind us that there are more exotic alternatives.
That's why we should surely acclaim the launch last Monday of the Breakthrough Initiative - a major commitment by the Russian investor Yuri Milner to buy time on the world's best radio telescopes and develop instruments to scan the sky in a far more comprehensive and sustained fashion than ever before. It's a gamble - the chances may be small but the stakes are so high that we'd surely all like to see searches begun in our lifetime.
Perhaps the Galaxy already teems with advanced life, and our descendents will 'plug in' to a galactic community - as rather 'junior members'. On the other hand, Earth's intricate biosphere may be unique and the searches may fail. This would disappoint the searchers.
But it would have an upside. Humans could then be less cosmically modest. Our tiny planet - this pale blue dot floating in space - could be the most important place in the entire cosmos. Either way, our cosmic habitat seems 'tuned' to be an abode for life. Even if we are now alone in the universe, we may not be the culmination of this 'drive' towards complexity and consciousness.
Thanks to the Breakthrough Initiative, there will be a serious scientific effort to probe these fundamental mysteries which all can follow with fascination. It makes sense to listen, rather than transmit. (Any two-way exchange would take decades, so there would be time to plan a measured response - and no scope for snappy repartee).
There are three reasons why an expanded search is timely.
First, technical advances in electronics and signal processing allow much more sensitive searches. Second, planets like Kepler 452 b offer obviously encouraging targets. And a third reason is that the advent of social media and citizen science would enable a global community of young and old to participate in this cosmic quest.
And there are two maxims that pertain in this field. First 'extraordinary claims will require extraordinary evidence' and second 'absence of evidence isn't evidence of absence'.
Even optimists wouldn't rate the chance of detecting a signal as more than a few percent - and most of us are more pessimistic, but thing the stakes are so high that it's worth a gamble.