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Life on other planets

Seth Shostak— © Guardian Newspapers Limited 2004

LAST WEEK, astronomers announced that they had made a giant breakthrough by finding something small. They turned up a planet that happens to be the most diminutive ever discovered beyond our own solar system, at least around a common type of star. The object is roughly twice the Earth's girth which, as planets go, is decidedly petite.

Although this runty world is, in the grand matrix of the cosmos, probably nothing special, it is possible that it heralds wonderful things. This remote rock could be akin to the flotsam and jetsam Columbus saw just before the New World hove into view. It might be the harbinger of a major discovery.

What do we know about this fresh find? First off, it belongs to a red dwarf star with the pedestrian name Gliese 876, a sun that is only about one third as hefty as our own. The planet speeds around this pint-size star every two days in an orbit that is barely larger than a loose-fitting belt — only one tenth the size of Mercury's path around the Sun. Despite the fact that red dwarfs are relatively cool, the small orbit means that temperatures on this world are extra toasty: several hundred degrees. In other words, this planet is unlikely to have any inhabitants; it is just too hot.

Okay, then why get excited by the rooting up of a small world around a small star? The reason derives from our changing view of these bantam suns, and their suitability for hosting planets where life could arise.

A few years ago, red dwarfs were thought to be the stellar hoarstones of barren real estate. Inevitably barren. Everyone knew their dull glow could only warm planets that happened to orbit close by. However, such star-hugging worlds would quickly be locked into a kind of static embrace, with one hemisphere always staring at the star, much in the way one hemisphere of the moon always faces the Earth. It was assumed that these worlds would become savagely hot on one side, and desperately cold on the other, destroying their atmosphere together with any chances for life. Researchers hunting for radio signals from alien societies did not bother to aim their antennae in the direction of red dwarfs.

Times change. More recent, and more realistic, analysis of what would happen to a planet that closely circles a red dwarf has shown that its atmosphere would quickly spread the heat. Greg Laughlin, an astronomer at the University of California at Santa Cruz who was part of the Gliese 876 discovery team, notes that "Even Venus, a local planet that only slowly turns on its axis, has pretty much the same temperatures all the way around." That is because Venus has an atmosphere.

So the paradigm has shifted, as the geeks like to say, and places we once thought were flat-out uninhabitable are now possible abodes for life. Red dwarfs are no longer regarded as hopeless cosmic desert.

This is revolutionary because red dwarfs are the most plentiful stars in the universe. For every Sun-like star, there are eight red dwarfs. If red dwarfs commonly have planets — a possibility given impetus by the discovery of the world around Gliese 876 — and if some of these worlds orbit at slightly greater, and therefore cooler, distances from their home star, then habitable worlds could be as abundant as summer mosquitoes. Most of the life in the universe could bask in the ruddy light of red dwarfs.

Startling possibility

This leads to a startling possibility, because red dwarfs have a characteristic that distinguishes them from stars such as the sun: longevity. Small stars, like small dogs, live longer. Our own sun has been boiling away for nearly five billion years; it has another five billion to go before it starts to shudder and die. But a red dwarf would offer much more time for development, 100 billion years or more, because these dim bulbs are parsimonious with their fuel.

If life, and occasionally intelligent life, exists elsewhere, then the most ancient civilisations are surely encamped around the oldest stars; and the oldest that still shine are red dwarfs. Of course, 14 billion years after the big bang, even the most aged of red dwarfs are still teenagers. But if some have planets on which biology bloomed early, that life has a history that is two or three times as long as the span between the Earth's earliest micro-organisms and the ascent of man.

Such an ancient society, with far more time to exploit science, might easily be able to betray its existence. No, we have not found evidence for such civilisations yet, but if we do, it is conceivable that they developed on a world of which the rock found around Gliese 876 is merely a first example. That overheated planet might be the first signpost of myriad worlds where life could, and possibly does, flourish.

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