Earth, maybe not unique after all

Saswato R. Das writes about physics and astronomy.

Earlier this month, scientists from NASA's Kepler satellite announced that they had found a rocky planet, with a radius about 40 percent larger than the Earth's, orbiting a sun-like star. The sun and its planet are relatively close by - only about 564 light years (that's 3.3 quadrillion miles)

Dubbed Kepler 10b, the planet orbits its sun furiously - in only 20 hours, compared to our Earth's 365 days, at a distance 1/20th the distance Mercury is from the sun. It's far too hot to sustain life as we know it. But its discovery has prompted great excitement among astronomers, because it is the smallest extrasolar planet we have found.

Astronomers announced the discovery of the first extrasolar planet in 1996. In the intervening 15 years, about 500 have been found. Most of them have been large - Jupiter-sized. But now our technology is getting better, and the smaller ones are being discovered.

Finding planets around other stars is no easy matter. Planets emit no light of their own; they shine by reflecting the light of their parent stars. Given the immense interstellar distances involved, even the nearest stars appear to us only as pinpoints, and it's an immense technological challenge differentiating a planet from its parent star, which is thousands of times brighter.

The first extrasolar planets were, in fact, found by an indirect method - their gravitational signature. A planet's presence affects the motion of a star, and it was this rhythmic gravitational wobble in a star's orbit that was detected in the case of the first extrasolar planets. This method, however, tends to pick out larger planets close to their star, because it relies on a planet's mass - the more massive the planet, the larger the gravitational wobble.

There are other ways to detect smaller planets, and NASA launched the Kepler satellite for that express purpose. Kepler's mission is to find Earth-size planets around other stars, and it does so by searching for planetary transits - a brief dimming of light that's caused when a planet passes in front of its sun. Only 40 percent larger in size, Kepler 10b is about 4 times the mass of the Earth.

The latest discovery supports a far older view that the processes that gave rise to the planets in our solar system are not unique, and there are Earth-sized planets elsewhere in the universe.

The philosopher Epicurus wrote in the 4th century BC that "there are infinite worlds both like and unlike this world of ours." In the 13th century, Albertus Magnus asked, "Do there exist many worlds, or is there but a single world?" The heretic Giordano Bruno, who was burned at the stake in 1600, held that "innumerable suns exist, and innumerable Earths revolve about these suns." And in the 17th century, Sir Christopher Wren observed that "a time will come when men . . . should see planets like our Earth."

But it was the philosopher Immanuel Kant who made the strongest case for extrasolar planets. In 1755, at a time when only six planets were known, Kant advanced the idea 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.

What astronomers are hoping the Kepler satellite will find are Earth-mass planets that lie 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.

Of course, the question that follows is: Are planets like the Earth common? After all, the sun is an ordinary star, and there are millions of similar stars in our galaxy. If there are Jupiter-like planets around sun-like stars, and even ones close in mass and size to the Earth, is it not conceivable that they also have planets with a similar terrestrial environment to the Earth's orbiting them?

When the light from Kepler10b started on its way, about 564 years ago, the human view of the universe was very different: Ptolemy's geocentric model of the cosmos, blessed by the Church, held sway. In 1543, the astronomer Nicolaus Copernicus published his revolutionary book, "De revolutionibus orbium coelestium" ("On the Revolutions of the Celestial Spheres"), which argued for a sun-centered cosmos.

But the real revolution in astronomy happened a few decades later, with the work of Galileo Galilei and Johannes Kepler - in whose honor the NASA spacecraft is named. Galileo's observations through his telescope and Kepler's laws of planetary motion in the early 1600s sounded the death knell for the Ptolemaic model, and heralded the birth of modern astronomy. Today, we know that the sun is a garden-variety star in a not-so-special location within the Milky Way galaxy, which contains some 400 billion stars. And there are billions of galaxies, with billions of stars.

So if there are many sun-like stars with solar systems like our own, there may be many, many planets, and that includes many Earth-like ones.

Even two decades ago, before the first extrasolar planets were announced, many thought the Earth was special. We are seeing now that planets may be as common as galaxies. It was only our technology that wasn't good enough to find them. We first found the large Jupiter-mass extrasolar planets. Now we are finding Earth-sized ones. Soon we will almost certainly find Earth-like planets in the habitable zone.

And then the biggest premise of all - that life is unique - will be tested. Will we find other life in the universe? Will we be able to recognize it? Time and technology will bring the answer.