Big news from the habitable zone
THE ANNOUNCEMENT by NASA scientists that they have finally found another solar system reminiscent of our own is the fulfillment of an age-old quest in astronomy. Schoolchildren will now learn that instead of one solar system, astronomers now definitively know there are more planetary systems - and some are even in our cosmic backyard.
In a sense, this discovery continues the revolution started by Copernicus and Kepler and Galileo, in which Earth (and then the sun) lost its special place as the center of the universe.
At a recent press conference, NASA astronomers said they had found five planets around 55 Cancri, a sunlike star which lies 41 light-years away in the constellation of Cancer. One of the planets lies in the so-called habitable zone - an area around the star where temperatures are conducive to life - and astronomers speculated that it may have moons with earthlike attributes.
The latest discovery bolsters a far older view that the processes that gave rise to the planets in our solar system are not unique, and there are planets elsewhere. The philosopher Epicurus wrote in the fourth century BC that "there are infinite worlds both like and unlike this world of ours."
But it was the philosopher Immanuel Kant who made the strongest case for exoplanets, as these planets have come to be known. In his theory of the heavens in 1755 - a time when only six planets were known - Kant advanced the idea that there were planets in the solar system beyond Saturn and that planets were not confined to our solar system.
"Our planetary system has the sun as its central body, and the fixed stars which we see are, in all probability, centers of similar systems," he wrote.
The first of Kant's predictions was proven true during his lifetime. In 1781, William Herschel, a German musician and astronomer who had immigrated to England, stumbled upon a faint, hazy object that moved in a planetlike orbit beyond Saturn. It was named Uranus. This caused a lot of excitement about planet hunting, and amateur astronomers trained their telescopes onto the night sky in the hope of finding more.
But while cataloging the planets in our own solar system was difficult, searching for planets beyond our solar system was even more complex. The separation between stars is immense and measured not in miles but in light-years (A light-year is almost 6 trillion miles.) As a consequence, even planets around nearby stars, a few light-years away, are incredibly faint. Planets produce no light of their own but reflect their parent suns. The voyage across interstellar space so diminishes the reflected light that it becomes almost impossible to take images of such planets.
It was only in 1995 that the first planet beyond the solar system was observed. As our telescopes have gotten better, more of these extremely faint exoplanets have come into view.
In some cases - as with the planets around 55 Cancri - the discovery was made by an indirect means, from the planet's gravitational perturbation to the movement of the parent star.
As of today, astronomers have counted roughly 260 exoplanets around nearby stars; most of these are relatively large, about the size of Jupiter.
There is a lot of excitement surrounding the 55 Cancri discovery because astronomers realize that one of the planets lies in the so-called habitable zone - an area around the star where temperatures are such that liquid water can exist. Scientists maintain that the presence of water increases the possibility of finding life. This particular planet is almost the size of Saturn, so probably too large to sustain life as we know it. But it may possess large moons that are earthlike.
Of course, the question that follows is whether there are many planets like the Earth out there? After all, the sun is an ordinary star, and there are millions of similar stars in our galaxy. If there are Jupiter and Saturnlike planets around sunlike stars, is it not conceivable that they also have earthlike planets orbiting them?
Searching for direct evidence of earthlike planets - Earth is much smaller than Saturn or Jupiter - will take a new generation of powerful telescopes. But surely we will succeed. And then, of course, we will have to deal with the biggest question of all: Is there other life out there? Will we ever learn the answer?
Saswato R. Das writes about astronomy and astrophysics. This column first appeared in the International Herald Tribune.
