Planets are like insurance premiums: smaller is usually
better. But better worlds are in short supply.
As I write this, a total of 347 exoplanets — worlds
orbiting other stars — have been reported in the scientific literature. This
roster is expanding at roughly one planet a week, and it's a decidedly beefy
crowd. Many exoplanets are comparable in mass
to Jupiter, and the biggest is nearly eight thousand times as hefty as
Earth. Such bulky orbs are likely to be wrapped in thick, malodorous
atmospheres: hardly the type of place that ET would be pleased to call home.
But what about those smaller, better worlds, at the
bottom of the heap? Are there any lightweights among the exoplanet crowd?
Well, there's Gliese
581e, which weighs in at a mere 1.9 times the tonnage of Earth, and is the
least massive exoplanet known to circle an ordinary star. If the density of
this planet is similar to that of terra firma, then its diameter is a mere 24%
greater than the world under your feet.
Last month's discovery of Gliese 581e was a big deal.
News stories lead with the titillating tidbit that, based on its size, this
planet could be a sibling of Earth, hinting that it might be suitable for
life. Buried about five paragraphs into the text was the less cheering fact
that Gliese 581e orbits only 3 million miles from its home star. Admittedly,
that sun is far fainter than our own, but given this planet's proximity Gliese
581e will be baked in starlight as strong as that which shines on Venus. It's
not Shangri La.
Gliese 581e is no cousin of Earth. But there's a race
underway to find such
cousins, and a winner may cross the finish line in the next thousand days.
If that happens, kudos should go to new instruments — for
example, HARPS. That sounds like a music club for angels, but the High
Accuracy Radial velocity Planet Searcher is in fact a spectrograph mounted on
the 3.6 m telescope at the European Southern Observatory in Chile. HARPS is
able to sense stellar wobbles with stunning accuracy, and is the device that
uncovered Gliese 581e. Back in 1995, when astronomers grabbed headlines with
the discovery of a planet around the star 51 Pegasi, they were able to measure
a star's back-and-forth shaking to within 5 meters per second, or roughly the
speed of a bicycle. Today, HARPS can easily perceive shakes as small as 1 meter
per second, and is approaching an accuracy of one-third that speed.
Eventually, it might reach 10 cm per second, or the pace of an ant. For
comparison, the Sun's wobble due to Earth is about 3 cm per second.
While wobble-watching experiments are already out of the
gate and down the track, the favorite in the exoplanet horse race is NASA's Kepler
mission, which just began its four-year reconnaissance for small worlds.
Kepler's designers expect the telescope to eventually find dozens of Earth's
doppelgangers — planets that are not merely the same size as our own, but
in orbits that would grace them with salubrious temperatures: in other words,
worlds that could boast of thick atmospheres and watery oceans.
The bottom line is that the discovery of a true analog to
Earth is so close; researchers already feel its hot breath. But if such a
planet is found, will the public care?
Gauging from the enthusiasm that greeted the discovery of
Gliese 581e, the answer is emphatically yes. My in-box will be flooded with
emails urging that our SETI experiments target the new world, and of course
we'll do that. But keep this in mind: a single Earth-like planet (or even
several dozen) is like a single kiss – it's not enough. Terra firma has been
around for 4.6 billion years, but life clever enough to transmit signals into
space has been walking Earth's surface for less than a century. If you're
sanguine enough to believe that we'll continue to be technologically proficient
for another 10 thousand years, then the fraction of our planet's lifetime
during which someone was "on the air" on Earth will have been no more than two
parts in a million.
If this estimate is even roughly typical of other worlds,
then we'll need to aim our radio antennas in the directions of 500 thousand
Earth-like planets to have a decent chance of hearing anyone. That may sound
daunting, but new instruments — such as the Allen Telescope Array — can pull
that off in two decades' time, if Earth-like worlds are common.
The data that will tell us whether that latter assumption
is true — and thereby answer a question as old as upright hominids — are
rushing our way.
Shostak is author of "Confessions of an Alien
Hunter"
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