As you can imagine, some astronomers have tried to answer such questions. Now the only example of life we have to learn from is our own. So the answers must come from an understanding of our own experience. What is it about our situation that gave rise to the incredible flowering of life we see around us on Earth? If we can answer that, we can look for similar conditions elsewhere in the universe, hoping that life will have similarly flowered there too.
The assumption, of course, is a big one: that life elsewhere will have characteristics like on Earth. But since we don't know about other kinds of life, we have no way to look for them. So we have no choice. We can only search for life as we know it.
So start with the Sun, without which the Earth would not exist, far less any life on it.
As stars go, our Sun is a pretty run-of-the-mill one. Yet several factors work in its favour:
So if we are searching for life elsewhere, we must look for stars that fulfil these conditions. The majority, it turns out, fail one or more.
For example, take stars much more luminous (and generally, hotter) than the Sun. Sirius, for example, the brightest star in the sky, is 25 times more luminous than the Sun. Its ecosphere is about 5 times larger than the Sun's. If Sirius had planets, it would be very likely that at least a few of them would lie in its ecosphere. Is Sirius a possible candidate as a life-giver?
No, because hot, luminous stars live short lives. A star like Sirius burns itself out in a few hundred million years. From geological records, we know that it was about 4 billion years after the Sun was formed that life appeared on Earth. Long before life can appear on one of its planets, Sirius will itself have died.
What about much cooler, less luminous stars? They do last long enough, sure. Their problem is that their ecospheres are small. So it is unlikely that any planets they have will be in that critical zone.
So we need to look at stars that are both luminous enough and live long enough for the evolution of life. Such stars form about 8 per cent of all the stars in our galaxy. Disheartened by that small number? Don't be, that's still 8 billion stars.
But of course, finding an appropriate star is just a first step. Next we have to find planets orbiting such a star. That's a hard task, because they emit no light themselves. We can only see them by the light they reflect from their parent stars: which is why we see Venus and Jupiter, Saturn and Mars.
Trouble is, planets of more distant stars would reflect too little light for us to detect on Earth, and that light gets drowned out by the light from their stars. So we need more sophisticated methods to detect planets than simply searching for reflected light.
One way is to look for stars whose position in the sky seems to wobble, or move about a point. One or more planets orbiting around a star, even if we can't see them, cause a wobble that we can detect. Imagine looking at our Sun from a few light years away. We would not see Venus and Jupiter, ringed Saturn and red Mars, nor even familiar Earth. But we would see that the Sun appears to wobble in the sky. That motion says that the Sun has something orbiting around it.
And by carefully analysing such motion, astronomers can estimate the size of the orbiting object. A good example is Barnard's star, the second closest star to us in the sky, almost 6 light years away. In 1963, Peter van de Kamp announced that the size of the wobble in Barnard's star could be explained by a planet about as large as Jupiter. (Though later, he decided that a better explanation would be two planets, one about 80 per cent Jupiter's size, the other 10 per cent larger than Jupiter).
Other wobbling stars have been found. In 1974, van de Kamp concluded that Epsilon Eridani, about 11 light years away, may have a massive planet six times as large as Jupiter orbiting it. Later, Helmut Abt and Saul Levy searched 123 Sun-like stars within about 85 light years from us, and concluded that 20 or 25 of them probably had planets.
So we can identify stars with planets. That's a start in our search for life.
And what of intelligent life? If it's out there, I wonder if it really wants to be discovered and written about, even obliquely. Maybe it will decide to shut down this blog as a retalia