Seth Shostak

The bottom line is, like, one in five stars has at least one planet where life might spring up. That's a fantastically large percentage. That means in our galaxy, there's on the order of tens of billions of Earth-like worlds.

The overwhelming bulk of the cosmos is deathly quiet. But here and there - on worlds where matter is thick and conditions are right - noises are commonplace. And in some cases, these noisy worlds may ring with the sounds of life - the bleats and bellows of creatures we have never seen, but may someday discover.

Even if the Moon didn't exist - even if it had been vaporized billions of years ago by cantankerous Klingons - there would still be (somewhat lower) tides raised by the Sun. For creatures dependent on the oceans' ebb and flow, life could go on.

If this is the only planet on which not only life, but intelligent life, has arisen, that would be very unusual.

Diminutive worlds are more likely to be rocky, and lapped by oceans and atmospheres. In the vernacular of 'Star Trek,' these would be M-class planets: life-friendly oases where biology could begin and bumpy-faced Klingons might exist.

Planets that don't currently sport plate tectonics, such as Venus and Mars, are scarcely habitable. Tectonics might be a requirement of any world that aspires to a rich diversity of life.

Once typecast as the indispensable altarpiece of a well-appointed living room, TVs have infected every human environment. The average American household has more television sets than people.

Since the Renaissance, a concept called 'progress' has been baked into our society. Progress - founded on an accumulation of knowledge through experience (and in the case of science, through experiment). To build on the past rather than endlessly relive it. That's what separates us from the beasts.

When I was a kid, which was just after Edison invented moving pictures, there were films that involved aliens coming to Earth for bad purposes.

There's no doubt that the Moon is more than a handy night light and a hair restorer for werewolves. It's responsible for the substantial amplitude of earthly ocean tides. These are of obvious influence if you're a geoduck, a type of clam that people dig up at low tide.

The most attractive habitats for synthetic sentience might be the vicinities of exceptional sources of energy - for example black holes, or even the neighbourhoods of large stars, which routinely boil off the energy of ten thousand suns. These are the destinations they may seek.

Thanks to the fact that the Earth isn't a perfect sphere, and invoking a bunch of Newtonian physics, you can deduce that our planet wobbles, too, taking roughly 26,000 years to trace out a small circle on the sky, a phenomenon known as precession.