Jayawardhana - Strange new worlds: the search for alien planets and life beyond our solar system

For my parents

Empty space is like a kingdom, and earth and sky are no more than a single individual person in that kingdom. Upon one tree are many fruits, and in one kingdom there are many people. How unreasonable it would be to suppose that, besides the earth and the sky that we can see, there are no other skies and no other earths.

Teng Mu, Chinese scholar

Sung Dynasty (9601279)

We are living in an extraordinary age of discovery. After millennia of musings and a century of false claims, astronomers have finally found definitive evidence of planets around stars other than the Sun. A mere twenty years ago, we knew of only one planetary system for sureours. Today we know of hundreds of others. Whats more, thanks to a suite of remarkable new instruments, we have peered into planetary birth sites and captured the first pictures of newborns. We have taken the temperature of extra solar giant planets and espied water in their atmospheres. Numerous super-Earths have been found already, and a true Earth twin might be revealed soon. It is still the early days of planet searchesthe bronze age as one astronomer put itbut the discoveries have already surprised us and challenged our preconceptions many times over. Whats at stake is a true measure of our own place in the cosmos.

At the crux of the astronomers pursuit is one basic question: Is our solar systemwith its mostly circular orbits, giant planets in the outer realms, and at least one warm, wet, rocky world teeming with lifethe exception or the norm? It is an important question for every one of us, not just for scientists. Astronomers expect to find alien Earths by the dozens within the next few years, and to take their spectra to look for telltale signs of life perhaps before this decade is out. If they succeed, the ramifications for all areas of human thought and endeavorfrom religion and philosophy to art and biologyare profound, if not revolutionary. Just the fact that we are potentially on the verge of so momentous a discovery is in itself remarkable.

Human beings have speculated about other worlds and extraterrestrial life for millennia, if not longer. Some ancient civilizations considered the heavens to be the abode of gods. Others believed that souls would migrate to the Sun, the Moon, and the stars after death. By the fifth century BC, a number of Greek philosophers considered the likelihood of multiple worlds and proposed that heavenly bodies are made of the same material as the Earth. Those ideas were central to their doctrine of atomism, the idea that the entire natural world was made up of small, indivisible particles. Metrodorus of Chios, a student of Democritus, is said to have written: A single ear of corn in a large field is as strange as a single world in infinite space. In the year 467 BC, a bright fireball appeared in the skies of Asia Minor, and fragments of it fell near the present-day town of Gallipoli. The event affected the thinking of many, including the young philosopher Anaxagoras of Clazomenae who wrote: The Sun, the Moon and all the stars are stones on fire. The Moon is an incandescent solid having in it plains, mountains and ravines. The light which the Moon has is not its own but comes from the Sun. (He also said that the purpose of life is to investigate the Sun, Moon and heaven.) The Roman poet Lucretius believed in other worlds in other parts of the universe, with races of different men and different animals.

Other prominent Greek philosophers, most notably Plato and Aristotle, espoused the opposing viewthat the Earth is unique. The Earth-centric model of the cosmos, based on the teachings of Aristotle and Ptolemy, gained prominence over time and dominated the European worldview until the late Middle Ages. Conveniently, the privileged position claimed for our planet and humankind suited the church teachings. There was little discussion of extraterrestrial life, with a few exceptions. The tide started to turn with the publication of Nicolas Copernicuss influential volume On the Revolutions of Celestial Bodies just before his death in 1543. He posited that the Sun occupied the center of the universe, thus displacing the Earth from its unique niche.

But the true revolution occurred with the invention of the telescope at the beginning of the next century. Galileos 1610 discovery of four moons circling Jupiter proved the existence of heavenly bodies that did not orbit the Earth. He also showed that Venus exhibited a full set of phases, just like the Moon, as predicted by Copernicuss Sun-centered model. Perhaps even more dramatic was the revelation from Galileos telescopic observations that the Moon was quite similar to the Earth in many ways. His beautiful sketches of the lunar landscape show mountains and valleys. Here was another world in its own right, with familiar topography.

I remember the first time the concept of another world entered my mind when I was a child. It was during a walk with my father in our garden in Sri Lanka, where I grew up. He pointed to the Moon and told me that people had walked on it. I was astonished: the idea that one could walk on something in the sky boggled my mind. Suddenly that bright light in the sky became a place that one could visit. To be sure, it was the possibility of adventure, rather than the great philosophical implications of there being other worlds, that impressed me. Looking back, that moment has had a defining impact on the path I have taken in life. Like many kids, I dreamt of becoming an astronaut. That desire fostered my interest in science and eventually led me to a career in astrophysics.

The first time I heard about planets being detected around other stars was in the summer of 1991, while I was an intern at The Economist in London. The science editor, Oliver Morton, mentioned that astronomers were about to announce a planet orbiting a stellar cinder called a pulsar. I didnt quite grasp the significanceand was a bit annoyed that the planet story bumped from that weeks issue an article I had written! Six months later, that particular claim was retracted, but a different pulsar with planets was found by then. A few years later, I interviewed several astronomers searching for Jupiter-like planets around normal stars for a news item in Science magazine. Despite fifteen years of searching, they had not found any as of 1994, so some wondered whether Jupiters might be rare.

Early ideas about the origin of the solar system implied that planets are a natural outcome of the Suns birththus they should be common around other stars too. In 1155, the Prussian philosopher Immanuel Kant proposed that planets coalesced out of a diffuse cloud of particles surrounding the young Sun. His model attempted to explain the order of the planets: the inner ones were denser because heavier particles gathered near the Sun while the outer planets grew bigger because they could collect material over a larger volume. Unfortunately, soon after his book was printed, Kants publisher went bankrupt, and not even King Frederick the Great, to whom it was dedicated, got to see Kants ambitiously titled book