Planet of Promise: Small, Rocky World Could Harbor Life

by Seth Shostak, Senior Astronomer, SETI

Date: 17 May 2007 Time: 10:42 AM ET

One of the several planets within the Gliese 581 star system, called Gliese 581 d, may be one of the most potentially habitable alien worlds known. It is about 8 times the mass of Earth, and located in an orbit just right for liquid water to exist on the surface. Water is a key ingredient for life as we know it. Gliese 581 is a red dwarf star 20.5 light-years from Earth
CREDIT: ESO

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 been a dozen years since the first planet around a star other than the Sun was uncovered. Since then, small teams of astronomers have been flushing out fresh planetary prey at the rate of about one every two weeks. Today, it's easy to have a blase attitude about this continuing drizzle of new worlds. With more than two hundred planets already on the scoreboard, adding yet another sounds redundant.

But this planet is different.

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. Mind you, it's not that nature prefers the creation of such brawny planets; it's only that the wobble technique used to find them strongly favors the heavyweights.

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% 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 - like Baby Bear's porridge - 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.

Mind you, 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.

The conditions for life could be there, but is life itself? As yet, there's no way to know unless the planet has spawned beings that are at least as clever as we are. As part of the SETI Institute's Project Phoenix, we twice aimed large antennas in the direction of Gliese 581, hoping to pick up a signal that would bespeak technology. The first attempt was made in 1995, using the Parkes radio telescope in Australia, and two years later additional observations were undertaken using the 140-foot antenna in Green Bank, West Virginia.

Neither search turned up a signal. But of course, the extraterrestrials might have been off the air when we were listening. Maybe their transmitter power was insufficient for our receivers, or perhaps the bandwidth we covered, from about 1,200 to 3,000 MHz, was the wrong part of the dial. There are many ways not to find an alien broadcast, but the Allen Telescope Array, now being built, will greatly improve our ability to more thoroughly scrutinize this world - and hundreds of thousands of others.

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. This latest discovery is a strong hint that a great number of these could be carpeted in the dirty chemistry we call life. Earth may be unique, but it might not be miraculous.