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Nature 459, 777-778 (11 June 2009) | doi:10.1038/459777a; Published online 10 June 2009

Looking for planets like ours

Michael Brown1

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The hunt for habitable worlds near other stars brings home the realization that our own Solar System might not be as special as we think, says Michael Brown.

BOOK REVIEWEDThe Crowded Universe: The Search for Living Planets

by Alan Boss

Basic Books: 2009. 256 pp. $26, £15.99

Some 250 years ago, the philosopher Immanuel Kant laid out an account of the Universe that sounds remarkably modern. In his Universal Natural History and Theory of the Heavens, he declared that moons go around planets, planets go around stars and stars go around the Milky Way. The Solar System had an understandable origin, and inevitable consequences:

"The planetary structure in which the Sun at the centre makes the spheres found in its system orbit in eternal circles by means of its powerful force of attraction is entirely developed, as we have seen, from the originally distributed basic stuff of all planetary material. All the fixed stars which the eye discovers in the high recesses of the heavens and which appear to display a kind of extravagance are suns and central points of similar systems." In other words, gravity takes stuff and turns it into stars, which are surrounded by swarms of planets, and it has done so everywhere you see a star in the sky.

Looking for planets like ours

ESO

Ground-based telescopes have revealed exoplanets such as the three 'super Earths' in this artist's impression. The Kepler space telescope (below right) could yield even more.

Until recently, this fact could be verified for only one star: the Sun. Then, in 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of the first planet orbiting a star other than the Sun (Nature 378, 355–359; 1995). Today, almost 300 stars are known to have planets around them. It is not quite Kant's "all the fixed stars which the eye discovers", but it's getting close. It has been accepted since the seventeenth century that our Sun is not special, but is one of many stars in the Universe. Now, at the beginning of the twenty-first century, it is clear that our planets are not special either.

Looking for planets like ours

R. MITCHELL-RYALL, T. FARRAR/NASA

Except that some exoplanets are special. It is tempting to describe the many planetary systems that have been discovered so far as weird. Rather than the 'inevitable' orderly arrangement of our own Solar System — with small planets close, large planets far, and everything going around the Sun in near-circular orbits in a common disk — we have instead found that almost anything goes. Planets the size of Jupiter orbit their stars at distances closer than Mercury, other planets have orbits as elliptical as some comets in the Solar System, and others lie farther from their central star than anything in our Solar System. Weird indeed. The only type of planetary system that we have not found, it seems, is one like our own.

But the special position that our home system holds is now in jeopardy. The Crowded Universe tells the story of the development of NASA's Kepler space telescope, which was launched from Earth in March this year to orbit the Sun. Kepler's three-and-a-half-year mission is simple: to find the Earths. Kepler and a similar French-led mission, COROT, are the first ones with a chance to tell us whether planets like ours are as common as Kant hoped or as rare as some astronomers think.

Alan Boss, a planetary scientist at the Carnegie Institution in Washington DC, weaves the story of Kepler with the larger tale of the booming field of exoplanets. As someone whose career in astronomy has spanned the period Boss discusses, I am glad someone was taking notes. It is fun to revisit the days when each new planetary discovery was an exciting event. Multiple teams struggled to outdo the others with firsts. First planet at the distance of the Earth! First transiting planet! First multiple planet system! It is easy to forget that most of the exoplanet field is less than a decade old.

Boss adds the insider story of the definition of planets in our own Solar System, including an account of the inner workings of the International Astronomical Union committee that decided how to classify Pluto, Eris and the other small bodies that we now call dwarf planets. The demotion of Pluto from planet class was unassailably reasonable, but the events leading up to it were some of the more publicly comical occurrences in recent astronomical history. It is a reminder that — for all their command of the physics of the Universe — astronomers, being human, have the capacity for near-infinite folly.

But Pluto is just a distraction for Boss, and rightly so. The meat of The Crowded Universe is the race to find another Earth. It would have been easy for his writing to get caught up in the day-to-day minutiae of meetings and detailed descriptions of research progress. But Boss never forgets that we are privileged to live in a time when a revolution of near-Copernican magnitude is unfolding.

Even if the Kepler and COROT missions do find an abundance of planets, the Kantian revolution will not be complete. The new planets might be exactly the same size as Earth and orbit their stars at the same distance, and although an astronomer might be willing to call such a thing Earth-like, most people will look for more. Does it have liquid water? Does it have a recognizable atmosphere? And, inevitably, could it — does it — support life?

Finding the answers to these questions will take decades. Kepler and COROT are merely steps along the way. In the meantime, we can take solace from Kant: "I am of the opinion that it is not particularly necessary to assert that all planets must be inhabited. However, at the same time it would be absurd to deny this claim with respect to all or even to most of them."

It took nearly 250 years to prove him mostly right the first time. With a little luck and perseverance — and, as Boss shows, a lot of work by astronomers around the world — the final step may just come a little faster.

See http://www.nature.com/astro09 for more on the International Year of Astronomy.

  1. Michael Brown is professor of planetary astronomy in the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
    Email: mbrown@caltech.edu


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