April 04, 2006

Life on my mind

One recent night in a spot surrounded by trees and animals, I looked up in wonder at the sky. Something about this -- the stars, the blackness, the quiet -- inevitably brings some kinds of questions to my mind, as I'm sure it has done to countless millions before me. So it was, that night: Anybody out there? Who? Where? Are we alone? Can we on earth really be the unique example of life in an entire universe?

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:

  • the Sun has planets.

  • the Sun has a large enough "ecosphere" -- a zone around it where conditions can support life. Close to the Sun -- like on Mercury -- temperatures are too hot for life to exist. Far from the Sun -- as on Jupiter -- it becomes too cold. It is only in the zone in between, the ecosphere where we live, that conditions are conducive for life.

  • three of the Sun's planets are in its ecosphere.

  • the Sun has been in existence long enough for life to evolve on one of those planets -- Earth.

    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

    Shruthi said...

    Enjoyed reading this - loved the last line!
    Coincidentally I have just started reading Bill Bryson's "A short history of nearly everything", and in the first couple of chapters, he attempts to explore this very topic.

    Anonymous said...

    There is one problem with your premise - that life could "evolve" at all from non-living chemicals. This has never been observed nor indicated in any scientific study, so how can life come from non-life? That is the problem with life existing elsewhere in the universe.

    Anonymous said...

    Great last line. Reminds of this David Quammen essay that ended with "This essay would simply stop."

    Anonymous said...

    Anonymous, I would recommend you read GEB by douglas hofstadter.

    zap said...

    sorry, could not resist the last comment, after your last line...

    Arthur Quiller Couch said...

    Shruthi took away my comment. Gah.

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    Anonymous said...

    Good Post.


    Wild Reeds said...

    Of course there is life elsewhere. Watch the BRILLIANT Jody Foster film "Contact", on the subject, based on the book by the same name. There is a beautiful dialog in the film: "If we were indeed the only living beings in the universe... seems like an awful waste of space!"

    Dilip D'Souza said...

    Shruthi, I read Bryson's book a couple of years ago and enjoyed it, though perhaps his stops at various points in science are a little too brief. It remains the only Bryson book I've managed to read -- I got mildly irritated with one he wrote about travelling around the US.

    WildR, I haven't seen the film, but Contact (Carl Sagan) is a terrific book too. A fine read, but also thought-provoking.

    To whoever is interested, GEB is a classic.

    Umesh Patil said...


    Here is one very interesting article related to what you discuss here (it is by an expert, worth spending the time):

    Prof. Kaku is the boss in what they call eschtology (hope I am correct here) and his web site is:

    Very cool stuff.


    Anonymous said...

    Richard dawkins explores this in the selfish gene and various other books. The basic question is "what is life", and a possible answer is a self-replicating mechanism that maintains a high degree of fidelity but has enough variation to evolve. Your hypothesis of stars, planets etc are for a hydrocarbon based system, but may not be necessary for other possible types 9you are generalising based on a sample size of 1). Possible candidate: silicon based (think computer viruses).

    Anonymous said...

    Can u give more details on GEB. Contact was good, but I prefer Sagan's other works - I think he touched on this topic in Dragons of Eden or Broca's Brain. do not remember. Both the above books are great.

    Anonymous said...

    Regards Intelligent life check out SETI@Home. You too can be part of the SETI fraternity thru a simple Screen saver.

    Dilip D'Souza said...

    Nikhil, GEB is "Godel Escher Bach", by Doug Hofstadter. Endlessly fascinating book about the recursive and contrapuntal nature of the work of these three men. (I won't say more). I read it at one go, over a few days one December soon after it first appeared. I couldn't put it down.

    "Contact" is fiction, a novel, and thus different from Broca's Brain and Dragons of Eden. Just to clarify.