How many worlds are there out there? Are we alone in the universe? What were once speculative and philosophical questions are now being tackled with real data, generated by NASA's planet-hunting space telescope, Kepler.
Discussion of both questions - you could call them the practical and the spiritual aspects of the huge amounts of planetary data from Kepler - took up several seminars at the American Association for the Advancement of Science meeting in Washington DC.
First the hard data, which we heard about on Saturday. Launched in 2009, Kepler has been staring at a small patch of sky containing some 150,000 stars. By measuring the fluctuations in light from the stars, astronomers can deduce the existence of orbiting planets. So far some 1235 potential planets have been identified. Of these, 68 are Earth-sized, and 54 may be in the fabled Goldilocks zone, the habitable region of space not too close and not too far from its star.
Just 15 of these planet candidates have been confirmed as such by exhaustive checking of the mass estimates and of the radial velocity of the object. But it was one as-yet-unconfirmed planet, KOI 326.01, that generated particular excitement at the AAAS, because of its potentially Earth-like qualities.
Slightly smaller than Earth, KOI 326.01 lies in the habitable zone of its star, a red dwarf star dimmer than our sun. At around 100 light years away, it's "fairly close" to us, said William Borucki, of NASA's Ames Research Center, "but they're not places you can walk to on a Sunday".
Currently just a promising data point, KOI 326.01 (which stands for Kepler Object of Interest) might end up getting a better name if it does turn out to be a real Earth twin.
Extrapolation so far suggests that around one in two stars in the sample have planets associated with them, and 10.5 per cent have Earth-sized planets. Borucki was pleased that Kepler is detecting smaller planets because, he said, it had been feared that giant planets would destroy smaller ones. "It's a wonderful surprise that we've found lots of smaller planets," he said. "There may well be Earths out there."
The prospect of other worlds could have religious and societal implications, which were explored at a session of the AAAS meeting on Sunday. Jennifer Wiseman of AAAS Science and Policy Programs in Washington DC gave the case for Christianity, and Nidhal Guessoum of the American University of Sharjah, United Arab Emirates, the case for Islam.
Both their arguments amounted to the (to my mind) rather dubious claim that the discovery of extraterrestrial life would pose no challenge or crisis to terrestrial religion.
Then we had two speakers who took contrasting positions on the likelihood of our detecting intelligent aliens. Howard Smith, of the Harvard-Smithsonian Center for Astrophysics insisted that for all practical purposes we are alone in the universe, while Seth Shostak, director of the Search for Extraterrestrial Intelligence Institute in Mountain View, California, was relentlessly optimistic about the possibility of finding alien life.
Smith argued that while there might be bacteria or even plants on other planets, even in our solar system, these "primitive" forms of extraterrestrial life "don't count" - we can't learn from them or share things with them.
We should only care about a life form that can communicate between stars. But given the volume of the cosmos that is near enough to us to be accessible to potential communication with any extraterrestrial intelligent beings, it would take at least 100 human generations to get in touch, Smith said.
This means we are all alone, something he calls the "misanthropic principle". Rather than the Anthropic Principle - the idea that the universe is perfectly tuned for life - the misanthropic principle says that intelligent life is so unlikely to evolve that we might as well accept that we'll never know if we are unique or not.
By contrast, Shostak cited an argument that says increases in computer power mean we'll be able to analyse far more planetary data than has been possible until now. He was confident that within 24 years we would detect an alien civilisation.
"There are maybe 10 21 Earth-like planets out there," he said. "Believing there aren't ETs is believing in miracles." He bet the audience that we'd find ET within our lifetime or else he'd buy us a cup of Starbucks.
"Kepler could've been a showstopper: it could've turned out that only 1 in a million stars had a planet like Earth, but that doesn't seem to be the answer we're getting," said Shostak. "That's important for motivating the experiment."