The
origin of life and the habitability of worlds other than Earth are two of the
biggest mysteries facing science today. Much research has been dedicated to
these topics, but there is still a lack of definite answers.
Jan
Hendrik Bredehöft from the
UK's Open University has been considering habitability on other worlds. "I'm
one of those guys who takes a piece of meteorite, grinds it up and finds out
what the organic chemistry is in there," said Bredehöft.
Based
on these types of studies, he has come to believe that habitable worlds can be
split into four categories, each with varying likelihoods of being home to
extraterrestrial organisms. This has great potential for assisting the search
for life in the universe, particularly as technology is now progressing to the
stage where direct
imaging of extrasolar planets is possible. Bredehöft presented his ideas at Europlanet's
latest Planetary Science Congress.
His
four groups of habitable worlds are: Earth-like, Mars-Like, Europa-like
and water-worlds.
Taking
each of these in turn, he considered their potential for hosting complex life.
Earth-like words are the first class, and a kind of "control" since we already
know such worlds are capable of sustaining complex life. Earth-like worlds
feature an appropriate atmosphere, liquid water, moderate temperature ranges,
and stable climates.
The
second class of planets are those that were once much like Earth, such as Mars
and Venus. "For some reason these planets left the classical habitable zone," said Bredehöft.
"Mars became too dry, there's very little water left, at least not liquid
water. Venus became just so enormously hot due to the greenhouse effect."
Still,
Bredehöft believes there is some chance for life to
exist on this type of world. He reasons that organisms could have developed
when the planet was more hospitable, and this life could maintain a grip even
through the hard times. "Once life has established itself it is really hard to
kill off," said Bredehöft. "There
have been absolutely devastating
events in Earth's history that might have wiped out all kinds of life, but
usually these served to further enhance biodiversity, rather than destroy it."
A
chilly existence
Bodies that possess liquid water, but under an ice
layer rather than on the surface, make up the third class of worlds.
Jupiter's
moon Europa is a classic example from our own
cosmic neighbourhood. Could there be life in places like this? Bredehöft's ideas here are particularly pertinent as often
these worlds do not fit neatly into the conventional view of habitable zones. Europa, for example, lies beyond the solar system's
temperature zone where water can remain as a liquid on a planet's surface.
However,
there is still potential for life.
The
traditional view of habitable zones thinks of a local star as being the prime
energy source. But on icy worlds like Europa, other
factors come into play, such as the gravitational pull of another planet.
Worlds with liquid water hidden beneath icy layers could potentially be
inhabited by simple organisms despite being far from the conventional habitable
zone, so long as energy is provided in some other way.
Water-worlds
The
fourth kind of habitable planets are made almost entirely of water. These
hypothetical worlds would be Mercury to Earth-sized and would feature extensive
oceans. Unlike oceans on Earth, the water on these types of planets would
not be in contact with silicates or other rocks.
"These planets can either be completely made of water with high pressure
ice at the core, or they can have bodies of liquid water that are separated
from a silicate core by a thick layer of high pressure ice," said Bredehöft.
One
theory for life's origin on Earth says organic material collected in shallow
pools and then became concentrated by clinging to the surface of rocks.
Eventually, this early life spread into the wider ocean. Another theory for
life's origin is that the necessary chemistry occurred at hydrothermal volcanic
vents. On water worlds, however, these scenarios are impossible.
Therefore, Bredehöft thinks life is not likely to
originate on such planets.
"The
amount of water on such a planet would be so huge, you
would need unbelievable amounts of carbon components concentrated together for
a chance of life. It's far too diluted," said Bredehöft.
Considered
opinions
After
considering all the facts, Bredehöft said the best bet
to find extraterrestrial ecosystems is to hunt for Earth-like planets, after
all. However, he doesn't think Earth-like worlds will necessarily have advanced
life.
"We
don't know whether the level of complexity or the size of organisms living on
Earth is essentially a logical outcome of evolution or whether it is just some
fluke experienced here," said
Bredehöft. "Is having
talking intelligent beings on the surface of the planet the pinnacle of
evolution? We just assume so because we like to see ourselves as something
special."
With
the rapid pace of development in planet-hunting technology, it is only a matter
of time until we learn much more about exotic extrasolar
planets and moons, and are able to glean vital information about their
properties. Until then though, scientists like Bredehöft
will continue to theorise about discoveries.
So
in Bredehöft's carefully considered opinion, what
kind of organisms are we most likely to find? "Probably something slimy," he said.
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