Astronomy: the long quest for alien life
K.S. RAJGOPAL
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Planets most conducive to life should be solid bodies
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— Photo: A.P.
THE DISCOVERERS: Swiss astrophysicist Michel Mayor (left), and Swiss astrophysicist Stephane Udry have discovered a planet outside our solar system that is potentially as habitable as Earth.
FINDING EXTRATERRESTRIAL life, leave alone intelligent extraterrestrial life, has been the cherished goal of astronomers, astrophysicists and other space scientists since man first ventured into outer space. Recently came a report (The Hindu, 26-04-07) of the discovery of an earth-like planet that could potentially harbour life.
Let us look at some of the steps taken in the past nearly decade and half that culminated in this finding. Until the 1990s, the nine planetary members of our own solar system were the only known planets.
Sun's environment
Astronomers did not believe that our sun's environment was particularly unique to be the only planet producer in the universe. However, there had not been evidence of other planets outside our solar system.
At that time state-of-the-art optical and spectroscopic instruments were limited in spectral and spatial resolution. But things changed rapidly from 1991 when radio astronomers detected the first extrasolar planets orbiting none other than a dying pulsar star. It was the first example of a star other than our sun producing planets.
In 1995 Swiss astronomers found another extra-solar planetary candidate. What about finding earth-like planets?
Such planets, the most conducive to producing life, are required to be solid bodies (unlike gaseous planets) with masses roughly between 0.5-10 earth masses.
These planets need to be found at such a distance from their parent star, which enables temperature and atmospheric pressure supportive of the existence of liquid water.
In 2001, it was revealed by a stellar survey that the rocky material for earth-like planets is orbiting billions of stars in our galactic neighbourhood.
The new data showed that many stars are rich in iron, having swallowed large numbers of asteroids and meteorites. These are made from the same material that forms earth-like planets.
If the material is as plentiful as it appears around these stars, it is highly likely to clump together into planets. And in February 2001, New Scientist reported that 53 planets had been identified orbiting other stars but all were gas giants.
In 2002, Serge Tabachnik and Kristen Menou of Princeton University modelled all the planetary systems known till then to work out which could be hiding habitable planets.
Narrowing the search
To narrow the search, they created computer simulations of the 85 systems known to estimate which might have habitable planets. The work appeared in The Astrophysical Journal.
The first thing they looked for in each system was whether a small terrestrial planet could exist in a stable orbit.
They found that for a smaller planet to be habitable, it must be far enough from its larger cousins and within the `habitable zone', the region around the star, which can support liquid water at all times.
After all, it was in the primordial ooze of earth's oceans that life forms first appeared.
Based on this premise, around a quarter of the systems contained regions where life-friendly planets could, in principle, exist. In 2005, the first evidence of the existence of the organic building blocks of life, in an earth-like orbit around a young sun like star was found by NASA's Spitzer Space Telescope, reported New Scientist. One of the stars observed in a nearby stellar nursery showed signs of existence of the organic molecules — acetylene and hydrogen cyanide.
These gases when combined with water can form several different amino acids needed to form proteins as well as one of the four chemical letters, or bases in DNA called adenine. And in 2005, Washington Post reported the discovery of a large `rocky' earth-like planet using a new technology, which measures the wobble in the star of the planet to deduce the planet's presence.
The then newly discovered planet was an oven-like world about 7-1/2 times the size of earth. The smallest earth-like planet was found in the year 2006 by an international team of astronomers and was reported in the journal Nature.
Mass and distance
With a mass five times the size of earth it can be found 25,000 light-years away in the Milky Way, orbiting a red dwarf star. Professor Michael Bode from Liverpool John Moores University, a principal investigator for the RoboNet project, which collaborated on this research, told BBC News: "This is the most earth-like planet we have discovered to date, in terms of its mass and the distance from its parent star."
However it was found that the planet's cold temperatures make the chance of finding life very unlikely.
So, will the recent discovery of a very likely candidate end the quest for the Holy Grail of astronomy or will it prove to be another damp squib?
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