May 17
For the first time, astronomers have discovered a planet far, far away that might be similar to Earth. This distant world, which pirouettes around a dim bulb of a star with the unglamorous name Gliese 581, may possibly sport a landscape that would be vaguely familiar to us — a panorama of liquid oceans and drifting continents. If so, there's the chance that it's a home to life — perhaps even advanced life.
It's different mostly because it's small. Nearly all the earlier discoveries were of massive worlds, lumbering giants comparable to Jupiter or Saturn. Such behemoths are likely to be buried in thick and toxic atmospheres, and seem ill-suited for supporting life.
However, by measuring the motions of bantam stars, such as the red dwarf Gliese 581, it's possible to uncover lighter-weight worlds, since detectability depends on the ratio of stellar to planetary mass. Gliese 581c, as the new find is called, is the smallest yet discovered around a normal star, a mere 50 percent larger across than Earth. This diminutive size suggests (but does not prove) that it's a rocky world, like Venus, Earth or Mars.
In another stroke of luck, it turns out that this planet is likely to be - at just the right temperature. Unlike Earth, it hugs Gliese 581 with a tight grip. It's five times closer to its runty star than Mercury is to our Sun. On the other hand, Gliese 581 is only a few percent as luminous as the Sun. These two factors roughly cancel, and a simple calculation suggests an average temperature similar to the temperate zones of Earth.
A planet this close to its stellar master will most likely be tidally locked, with one side always facing its sun, and the other side perpetually turned toward the cold darkness of space. But computer models have suggested that if such a world has an atmosphere, strong winds will distribute the heat of the sunny hemisphere around the planet. There should be a belt of moderate temperatures somewhere near the twilight ring between light and dark. This idea has received a bit of confirmation from recent infrared measures of another newly discovered planet, a tidally locked world named HD 189733b. This gas giant seems to show a moderating of the temperature difference between its light and dark sides due to high-speed winds.
The bottom line is exciting. Out of the hundreds of planets so far uncovered around other stars, Gliese 581c is the best candidate for habitation. It could conceivably boast such terrestrial amenities as liquid oceans, a benign atmosphere, and plate tectonics to churn metal ore close to the surface, useful for any advanced beings with a penchant for technology.
Gliese 581 is, as astronomical distances go, relatively close: only 20 light-years away. It's one of the few star systems which, if inhabited, might provoke conversation. A simple exchange, along the lines of "how are you?" followed by "fine, and you?" would require a mere four decades. Tedious, but not unthinkable.
However, irrespective of whether the world orbiting Gliese 581 is host to chatty beings or not, its discovery is highly suggestive. In the mid-twentieth century, astronomers debated whether planets were extraordinarily rare or as common as crickets. We now know the latter is true, and the number of planets in our own galaxy could easily tally in the hundreds of billions.
go
Edited by owlscrying, 18 May 2007 - 10:12 AM.