杏吧原创

NIGHT in night out, the world鈥檚 television and movie screens radiate the
sights and sounds of extraterrestrial life. From Star Wars and Star
Trek to The X-Files, the message is clear: the Universe is
filled with alien life forms of all shapes and sizes, every conceivable level of
intelligence, and covering the gamut of attitudes from friendly benevolence to
outright hostility. The idea that planets with animal life are commonplace has
firmly taken root.

We don鈥檛 just have science fiction writers to thank for this, but two
celebrated astronomers. In 1973, Frank Drake and Carl Sagan gave us the Drake
equation, a method of working out the potential number of advanced civilisations
in our Galaxy. Using the best information available at the time, Sagan estimated
that there may be a million civilisations in the Milky Way alone. And since our
Galaxy is only one of hundreds of billions of galaxies in the Universe, there
could be billions of planets with intelligent, alien species.

It鈥檚 a breathtaking concept. But is it credible? How probable is it that
single-celled life on other planets could have evolved into more complex
varieties, culminating in the development of animals and a sophisticated
culture.

In our book Rare Earth: Why Complex Life is Uncommon in the Universe,
Don Brownlee and I argue that even the simplest forms of animal life are
exceedingly rare in our Galaxy, and in the Universe beyond. That鈥檚 not to say
all life is rare; but we do take issue with the notion that just because
microbial life is common in the Universe, you鈥檒l automatically get animals and
higher plants, let alone intelligent beings.

But let鈥檚 start from the beginning. Why do so many people believe animal life
could be common elsewhere? One important reason is the discovery that microbes
can survive, and even thrive, in the searing temperatures and crushing pressures
of the deep sea and beneath the surface of the Earth. Similar conditions exist
on other planets, and people take this to mean that if primitive life can
survive on Earth, why not elsewhere in the Universe?

Then there鈥檚 the fossil record. This tells us that life on Earth began soon
after the end of the period when planets were being bombarded about 3.8 billion
years ago. The fact that life appeared so early in Earth鈥檚 history suggests that
it forms quite easily. If it can happen here, it鈥檚 reasonable to ask why
shouldn鈥檛 it have been happened countless times elsewhere?

We think, however, that this line of reasoning is flawed. Just because life
evolves does not mean that animal life will then eventually appear. The chances
of animal life springing up are probably more remote than many scientists
believe. Why? Because Earth is actually rather a special place, and there are
unlikely to be many planets on which conditions are appropriate for simple life
forms to evolve into complex organisms such as plants and animals.

What are those conditions?

One is stability. Life needs a relatively stable environment like that on
Earth, which has a stable orbit and receives a stable amount of energy from the
Sun. Most planets we know of outside the Solar System have irregular or unstable
orbits. And the energy from stars in other solar systems and galaxies tends not
to be constant鈥攃ertainly not for such a long time. Had our Sun鈥檚 energy
output undergone rapid swings during life鈥檚 long development, there would be
little chance of animal life on this planet.

Also, astronomers have calculated that we probably aren鈥檛 hit by comets and
asteroids as often as other planets. Such collisions cause mass extinctions,
even wipe out life altogether. We don鈥檛 know why collisions with Earth are rare,
but there is evidence that we have Jupiter to thank. The giant planet shields us
from such catastrophes by acting as a comet and asteroid catcher, a gravity-sink
that sweeps the Solar System clean of cosmic garbage that might otherwise have
collided with Earth.

But not all Jupiter-sized planets have the stable orbits needed to do this
job. A lot of solar systems contain 鈥渂ad鈥� Jupiters that are either orbiting
close to their stars, or have highly eccentric or unstable orbits. In the
star-packed interiors of other galaxies, the frequency of supernova explosions
and close encounters with neighbouring stars could occur just too frequently to
bring about the long-term stability we think is needed for the development of
animal life.

Another happy accident is in the nature of Earth itself. Our planet provides
just the right mix of metals for life to evolve and thrive. It is also the only
planet in our Solar System with a moon of an appreciable size compared to the
planet it orbits. This keeps our tilt stable, which keeps climate relatively
constant. And Earth is the only planet in the Solar System that has tectonic
plates, which lead to continental drift, the most crucial element in maintaining
a long-term climate that allows liquid water to exist. Plate tectonics helps
limestone to form, which scrubs greenhouse gases from our atmosphere.

Water is critical to life. And Earth happens to be the right distance from
the Sun to enable liquid water to have existed on its surface for more than 4
billion years. A little farther from the Sun and we鈥檇 have frozen; any closer
and most of the water would have boiled away, as happened on Venus.

Even Earth鈥檚 position in our Galaxy is important. If we were closer to the
galactic centre, animal life would be at risk from mass extinctions. But if we
were further away, there wouldn鈥檛 be enough of the heavy elements needed to
build rocky planets, and to fuel the radioactivity that keeps plate tectonics
rolling.

Ever since Copernicus removed our planet from the centre of the Universe and
placed it in orbit around the Sun, its status has steadily been downgraded. We
have gone from the centre of the Universe to a small, insignificant planet
orbiting a small, undistinguished star in an obscure fringe of the Milky Way.
This is the 鈥減rinciple of mediocrity鈥�, the idea that Earth is one of a very
common class of planets, and it鈥檚 time we got rid of it. Mediocre and
commonplace scarcely does justice to the special planet we call home.