SETH SHOSTAK

It bears mentioning that the Milky Way is only one of 150 billion galaxies visible to our telescopes - and each of these will have its own complement of planets.

A practical way to travel between the stars is a must-have for space opera, and a sine qua non for our frequently vaunted future as a galactic society.

Faith is a personal matter, and should never be a cudgel to stifle inquiry. We tried that approach about 1,200 years ago. The experiment was called the Dark Ages.

Any society that could come here could pick up the lights from New York. What should we do about that? Should we darken New York from now until the last human expires? Would we want to turn off all the radars at JFK airport?

Buying insurance is no one's idea of fun. And it's especially easy to berate something as funky-sounding as writing checks to defend our neighborhoods against apartment-size rocks from space. But this is one insurance pitch that makes perfect sense. Ask the dinos.

Hollywood usually guesses that extraterrestrials would only be interested in one of three things: (1) They want to breed with us, because their own reproductive machinery is on the blink; (2) They want Earth's resources; or (3) They want the Earth. All of it.

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.

Note to academics: Aristarchus' track record of astronomical research would probably have guaranteed him tenure somewhere, if tenure had been invented. His stack of reprints included measuring the distances of the Moon and Sun.

Our computers double in capability on time scales of only a few years. It's hardly outrageous to believe that we will successfully develop thinking machines within a handful of decades, or at most a century or two. If that happens, these artificial sentients will quickly leave us behind.

Astronomers still can't decide what the shape of our universe is. Is it closed and finite, which is to say, is there a countable tally of all the galaxies that exist, even beyond the ones we can see? Or is it infinite? The latter possibility is still on the table.

I still remember my dismay in the summer of 2007 when - for the first time in the history of planet Earth - America's share of auto production dropped below 50 percent.

Mountains aren't eternal: even the most imposing massifs are smoothed away by weathering in a few hundred million years or less. Plate tectonics makes new ones, and without it, our future would be flat.

The Earth has been lawned with life for something over 3.5 billion years. That's a span of time great enough to encompass some honest-to-goodness catastrophe. For example, 700 million years ago, Earth underwent a planet-wide deep freeze, with ice covering the oceans from the poles to the equator.

Estimates are that at least 70 per cent of all stars are accompanied by planets, and since the latter can occur in systems rather than as individuals (think of our own solar system), the number of planets in the Milky Way galaxy is of order one trillion.

Sure, nobody will make a fortune if we figure out why the Big Bang happened. But just about everyone would like to know.

Any beings advanced enough to traverse interstellar distances are at least a thousand years beyond our technical level. Spending gobs of time examining our missiles is equivalent to sending the Air Force back to the Middle Ages and insisting they examine the chain mail factories.

The thing to keep in mind is that we're still in the very early days when it comes to the search for extraterrestrial intelligence. Saying there's a silence is a bit like if Columbus, looking to discover a new continent, only sailed 10 miles off the coast of Spain before turning back to say, 'Nothing out there!'

Heads are a good deal, and I think they would be a common feature. It's hard to think of species that don't have heads, although there are some. It's good to have a head because it puts some of the sensory organs - eyes, ears, whiskers or whatever - next to the CPU, the brain.

The Moon is a ball of left-over debris from a cosmic collision that took place more than four billion years ago. A Mars-sized asteroid - one of the countless planetesimals that were frantically churning our solar system into existence - hit the infant Earth, bequeathing it a very large natural satellite.

We're interested in things that have big teeth, and you can see the evolutionary value of that, and you can also see the practical consequences by watching 'Animal Planet.' You notice they make very few programs about gerbils. It's mostly about things that have big teeth.

Is E.T. out there? Well, I work at the SETI Institute. That's almost my name. SETI: Search for Extraterrestrial Intelligence. In other words, I look for aliens, and when I tell people that at a cocktail party, they usually look at me with a mildly incredulous look on their face. I try to keep my own face somewhat dispassionate.

The idea of close encounters of the zero'th kind - which is to say, not a close encounter at all, but simply uncovering evidence that someone's out there - dates back to the Victorian era.

The total number of people that do a job that has the same description as mine in the entire world is fewer than 10. There's a lot of effort looking for life in space - that's a lot of what NASA does, but they're not necessarily looking for the kind of life that can hold up its side of a conversation.

Television is ephemeral, a fact that some will find reassuring. But earthlings will continue to pump the kilowatts into the ether. And eventually, when those signals have washed over a few hundred thousand star systems, someone may notice.

The ideas of science germinate in a matrix of established knowledge gained by experiment; they are not lonesome thoughts, born in a rarified realm where no researcher has ever gone before.