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.
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