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REMEMBER the amazing good luck that Captain Kirk and his crew seemed to have
whenever they beamed down to a planet? The gravity was always normal, the
temperature comfortable and the air breathable. And, more often than not, they
were greeted by surprisingly human-looking life forms. Was this just wishful
thinking, or do Earth-like planets, the kind on which 鈥渓ife as we know it鈥� could
flourish, really litter the Galaxy?

A few years ago, there would have been little hope of answering that
question, but within a few years we should know. With an array of space
missions, scientists are preparing to explore the region known as the habitable
zone鈥攖he comfortable region around a star in which an Earth-like planet is
most likely to be found. We may soon see the fingerprint of such a distant
planet passing in front of its sun, briefly dimming its yellow-white glow.

We already have reasons to believe that planets are the norm. In most
theories of star formation, a cloud of gas and dust collapses into an accretion
disc. Most of the central material forms a star, and planets form farther out.
We have seen accretion discs around young stars, so the theories seem to be
right.

And in the past few years, astronomers have detected giant, Jupiter-sized
planets in more than a dozen star systems beyond our own. These giants are
unlikely to be good environments for life like our own: they are probably like
Jupiter, with thick atmospheres of noxious gases, including methane and ammonia,
above a superhot, planet-deep sea of liquid hydrogen.

Even so, their existence is encouraging. 鈥淚f you see giant planets around a
few per cent of nearby stars, despite the extreme limitations of our current
ability to detect them, it tells you that their formation really is a very
common process,鈥� says Scott Tremaine of Princeton University. 鈥淎nd if so, it鈥檚
hard to believe that you wouldn鈥檛 also commonly form terrestrial planets.鈥� That
is, planets like the small, rocky worlds of our inner Solar System.

Of course, being small and rocky isn鈥檛 enough to make a planet ideal for
life. Venus and Mars are superficially similar to Earth, but Venus is far too
hot and its atmosphere is thick and corrosive, whereas Mars is probably too cold
and has only a tenuous atmosphere that is quite unbreathable. Only Earth
occupies the comfortable middle ground, satisfying the Goldilocks criterion: not
too hot, not too cold, just right.

For a start, to hold an atmosphere that鈥檚 not too thick or too thin a planet
probably needs to be between a tenth and ten times the mass of Earth.
Most astronomers believe it should also orbit within the habitable
zone鈥攖he region in which a planet can sustain liquid water on its surface
(see 鈥淚n the comfort zone鈥�). The size and location of the zone varies with
the brightness of the host star: for our Sun, the habitable zone begins just
outside the orbit of Venus and ends just before the orbit of Mars
(see Diagram).FIG-22044801.jpg

A few of the extrasolar giants we have detected are in or close to their
star鈥檚 habitable zone, notably the planets around 16 Cygni B and 47 Ursa
Majoris. In 1996, Darren Williams, James Kasting and Richard Wade of
Pennsylvania State University realised that even if the giants are inhospitable,
their moons might support life鈥攊f they are big enough to hold a thick
atmosphere. But these moons would be desperately difficult to detect.

A more reasonable challenge is to find Earth-like planets in the habitable
zones of other stars. The planets discovered so far have been detected
indirectly, by watching their parent stars wobble slightly as the planets swing
around them. Unfortunately, something like the Earth has a thousandth of the
mass of these giants, and the wobble would be too small to see.

Direct imaging is also unrealistic. Even the Jupiter-sized giants have yet to
be seen in a telescope. If there is another Earth, even around one of our
nearest neighbour stars, its feeble reflected light would be utterly swamped by
the glare of the host star, perhaps 10 billion times as powerful.

And yet there is a startlingly straightforward way to detect such planets.
Consider what an alien astronomer far from Earth would see if it aimed its
telescope toward us. It would probably see the faint point of light that was our
Sun, and nothing more. But suppose the astronomer happened to be in exactly the
same plane as the Earth鈥檚 orbit. Once a year, the Earth would pass in front of
the Sun, dimming the Sun鈥檚 light slightly. For 13 hours or so, the Sun would be
dimmer by about a part in 10 000.

Astronomers are planning just such an experiment, looking for these transits
in nearby stars. Because the Earth鈥檚 atmosphere makes stars twinkle, such a
sensitive measurement is all but impossible from the ground, so astronomers and
space scientists at NASA鈥檚 Ames Research Center in Moffett Field, California,
have proposed a space-based mission known as Kepler. Perhaps as early as 2003,
they plan to put a telescope with a 1-metre aperture into orbit about the Sun,
trailing the Earth by a few million kilometres.

Of course, there鈥檚 no reason to expect our viewpoint to lie in the plane of
any particular planetary system: the odds of such a chance alignment are about 1
in 200. So Kepler will spend four years continuously monitoring the brightness
of 100 000 stars in the hope of seeing at least some planetary systems
edge-on.

鈥淭he mission is expected to find about 500 Earth-like planets,鈥� says
principal investigator William Borucki at Ames. Built into that estimate,
however, is the assumption that most stars do indeed have planetary systems.
While many astronomers believe that, it will take experiments like Kepler to
settle the matter. 鈥淚f you did find 500, you鈥檇 say that most stars do have such
planets,鈥� Borucki says.

By measuring the degree to which the host star dims during a transit,
astronomers can estimate the size of the planet and therefore its mass. Its
orbital period is simply the time between successive transits. Meanwhile,
spectroscopic studies tell the temperature, and therefore the brightness and
mass, of the host star.

With all those numbers in hand, astronomers can work out whether a given
planet lies in the habitable zone of its host star鈥攎aking it a likely
target in the search for water and, ultimately, life.

The possibility of life, of course, is what makes the search for Earth-like
planets so exciting. And yet, because no life has been discovered beyond our
home planet, we have no way to tell whether life needs Earth-like conditions.
That鈥檚 an issue that researchers involved with the search for extraterrestrial
intelligence (SETI) have been wrestling with for some time.

鈥淚t鈥檚 very conservative, of course, to assume that they are like us,鈥� says
Seth Shostak, a radio astronomer at the SETI Institute in Mountain View,
California. Nevertheless, Shostak and his colleagues will have a keen eye on the
results from Kepler and similar missions. 鈥淲e would be buoyed substantially by
knowing that Earth-like planets were common,鈥� he says. Any obviously Earth-like
planets to be identified would, of course, be added to the SETI search lists,
and such a discovery may make it easier for SETI projects鈥攖oday still seen
by many as 鈥渇ringe science鈥濃�攖o acquire funding.

The Kepler mission will probably be the first to detect Earth-like planets
beyond our Solar System, but several missions with similar goals are also in the
planning stages. The first of them, the Keck Interferometer, should be able
detect objects the size of Uranus (four times the diameter of the Earth) up to
about 80 light years away. Starting next year, the two Keck telescopes on Mauna
Kea in Hawaii will be used as a two-element interferometer with a baseline of 85
metres鈥攎imicking the resolution of a telescope with a diameter of 85
metres. Its optics will make the light of the host star cancel itself out by
destructive interference, so astronomers can examine any companion planets.

The Space Interferometry Mission (SIM), scheduled for launch by NASA in 2005,
will use a set of space-based telescopes to perform a similar task: they will
probe nearby stars for planetary companions, detecting objects down to a few
times the size of the Earth.

More sophisticated still is the Terrestrial Planet Finder (TPF), proposed by
scientists at the Jet Propulsion Laboratory in Pasadena, California. TPF, which
could be launched as soon as 2010 if the project goes ahead, will be a
space-based array of four telescopes, with an overall resolution 100 times
better than the Hubble Space Telescope. It would allow astronomers to image
individual terrestrial planets, detect planetary systems as much as 50 light
years away, and do a full survey of stars within this region. And most
importantly, it will use a high-resolution spectrometer to analyse the chemical
composition of the atmosphere of any planet it finds. In particular, TPF will
try to sniff out ozone and methane, reactive chemicals that would hint at the
presence of life.

Meanwhile, the European Space Agency is planning a similar mission, known as
Darwin. This six-telescope array would again use interferometry to image
planetary systems and study their spectra. TPF and Darwin may be combined into a
single mission to save money.

Some scientists are already speculating about missions beyond TPF. At a
meeting of the American Astronomical Society in Chicago this summer, NASA chief
Daniel Goldin mused that in the next century, 鈥渟cientists will debate the
structure of continents and oceans, weather patterns, climates, storms, and the
nature of seasons on dozens of new worlds鈥�.

Even if our Earth is a lucky quirk of planetary formation and geophysical
history, many scientists believe the odds are good for finding similar planets
out there. Their optimism comes from the sheer numbers: there are, after all,
hundreds of billions of stars in the Galaxy. In a few years鈥� time, the results
from Kepler and subsequent missions will begin to pour in, and this speculation
will give way to numbers, sizes and distances.

But there is another, slightly chilling possibility. Kepler might find
nothing remotely like Earth. 鈥淭hat would tell you that there aren鈥檛 many other
planetary systems like our own in our Galaxy,鈥� says Borucki, 鈥渋f any.鈥� If that
turns out to be the case, our planet will suddenly seem even more special and
fragile鈥攁 single blue-green oasis in a Galaxy where life is the exception
rather than the rule.

* * *

In the comfort zone

WE assume that a planet that can support life is most likely to be found in
the so-called 鈥渉abitable zone鈥�, the region around a star in which a planet could
harbour liquid water on its surface. The size and location of this zone depend
mainly on the star鈥檚 luminosity: the dimmer the star, the tighter an orbiting
planet must hug it in order to be warm enough to support life. The habitable
zone around our Sun contains only the Earth.

Yet this could be an oversimplification. In our own Solar System, we know
that liquid water once flowed on Mars, and, even more intriguingly, may still be
found beneath the icy surface of Jupiter鈥檚 moon, Europa
(see 鈥淲aterworld鈥�).

A planet鈥檚 size is also crucial. 鈥淚f the planet is too small鈥攎uch
smaller than Mars, say鈥攊t can鈥檛 hold onto its atmosphere very well,鈥� says
James Kasting of Pennsylvania State University. 鈥淥n the other hand, if you get a
planet that鈥檚 bigger than about 10 or 15 Earth masses, it gets big enough to
capture a lot of nebular gas as it鈥檚 forming. And we think a planet like that
will end up as a gas giant, rather than as a terrestrial planet.鈥�

But even smaller variations in size and composition could be important. On
Earth we鈥檝e been blessed with the shifting of continental masses known as plate
tectonics, which is critical in releasing carbon dioxide into the atmosphere.
Mars is too small to maintain such activity, so most of the Red Planet鈥檚
CO₂ has become absorbed into the Martian crust, leaving a rarefied atmosphere
just 1 per cent as thick as Earth鈥檚. That thin atmosphere, together with its
remote location, has left Mars bitterly cold, with an average temperature of -20
掳颁.

Venus, meanwhile, is simply too close to the Sun. Evaporation from the
planet鈥檚 surface has created a thick, soupy atmosphere, trapping the Sun鈥檚 heat
and triggering a runaway greenhouse effect. In other circumstances, a powerful
greenhouse effect needn鈥檛 be a bad thing. A planet larger than Earth, in an
orbit larger than Earth鈥檚, could still be habitable if it held onto a thick
atmosphere that warmed the surface to a comfortable level.

Because so many variables are involved, some scientists hedge their bets.
鈥淭he fact that Venus and Mars are so different from Earth gives the lesson that
we should be prepared for surprises,鈥� says Scott Tremaine of Princeton
University, New Jersey. 鈥淭he variety of possible planets in other Solar Systems
is likely to be greater than we anticipate.鈥� But others still put their money on
Earth-sized planets orbiting Sun-like stars in orbits similar to our own. 鈥淲hat
our Solar System tells us is that you have to be the right size and the right
distance from your star,鈥� says Toby Owen of the University of Hawaii. 鈥淚f you
satisfy those criteria, then the chances of being an Earth-like
planet鈥攖hat is, with open bodies of liquid water on the surface鈥攕eem
to be pretty good.鈥�