- Picture a giant, blue-white world with a planet-wide
ocean hundreds of times bigger than the Earth's and 10 times as deep. According
to planetary physicists in France and America, such "ocean planets"
could be common - and possibly the best places in our galaxy to find life.
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- Until recently, nobody suspected the existence of giant
water-worlds, but in the past decade astronomers have discovered more than
100 planets orbiting nearby stars. These "extrasolar" planetary
systems have changed our ideas of the kind of planets that are possible.
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- In particular, some of the extrasolar planetary systems
contain "hot jupiters"; gas-giant planets similar to our own
Jupiter, but orbiting perilously close to the fires of their parent star.
In the case of the nearby star 51 Pegasi, for instance, a Jupiter-like
planet orbits eight times closer to its parent star than the distance at
which Mercury - the innermost planet in the solar system - orbits the Sun.
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- The puzzle is that, if such planets are born so close
to their stars, the ferocious heat should long ago have caused them to
evaporate. Planetary physicists have been forced to conclude, therefore,
that the planets were born much farther from their stars and then "migrated"
inward.
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- The picture that has emerged is of a planet being driven
in towards its star during the late stages of planet formation by the gravitational
force between the planet and the remnant of the swirling disc of dust out
of which the planets formed. But, if this could happen to a gas-giant planet
such as Jupiter, why could it not happen to an ice-giant planet like Uranus
or Neptune?
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- "Exactly," says Christophe Sotin of the University
of Nantes in France. If such a planet migrated towards its star, the heat
would cause its huge "mantle" of ice to melt, resulting in a
planet with a gigantic ocean. "Observational techniques are not good
enough to detect such 'ocean planets' yet," Sotin says. "But
we think they could be very common in our galaxy."
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- The possibility of ocean planets is not exactly new;
they were first proposed by David Stevenson of the California Institute
of Technology in Pasadena. However, Sotin and his colleagues are the first
to work out the details and publish them.
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- Sotin learnt about the possibility of ocean planets from
colleagues. He began to figure out what they might look like when he discovered
that his colleagues thought they could have oceans thousands of kilometres
deep. "I realised that was totally preposterous," Sotin says.
He is an expert on ice that forms under high-pressure conditions; expertise
he has used to deduce the internal structure of Jupiter's giant ice-moon,
Callisto. Usually, the melting point of ice goes down at high pressure.
This would mean that deep down in an ice planet, where the ice was being
squeezed by the weight of all the material bearing down from above, it
should be liquid. "This is what led my colleagues to think that all
the ice in an ice planet would melt, making an ocean thousands of kilometres
deep," says Sotin.
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- However, he knew that at ultra-high pressure - the kind
found deep in an ice giant - ice enters a different "phase" known
as "Ice II". Instead of its melting point dropping as the pressure
goes up, it increases, so the ice is prevented from melting. "Consequently,
you only get an ocean about 100 kilometres deep, not thousands of kilometres
deep," Sotin says.
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- Sotin and his colleagues have calculated the depth of
liquid water for ice-giant planets less than 10 times the mass of the Earth.
The depth depends on the surface temperature of the planet, which in turn
depends on the atmosphere, which may contain heat-trapping greenhouse gases
such as carbon dioxide and water vapour. Since the atmospheric composition
of such a planet is not well known, Sotin has calculated the ocean depth
for different surface temperatures.
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- If the surface temperature is 7C, the ocean will be 72
kilometres deep and the seabed temperature and pressure 35C and 11,000
atmospheres respectively. For a higher surface temperature, the ocean will
be deeper, and vice versa. For instance, a surface temperature of 30C leads
to an enormous ocean depth of 133 kilometres.
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- A planet about eight times the mass of the Earth would
have about twice the Earth's diameter and gravity at the surface about
50 per cent higher. An ocean 100 kilometres deep is 10 times the depth
of the deepest ocean trench and 40 times the average depth of the ocean.
With the ocean covering an area six times that of the Earth, we are therefore
talking about a volume 240 times greater. "The Earth's ocean is a
mere puddle by comparison," Sotin says.
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- Others researchers have also suggested the existence
of water worlds, but ones actually born close in to the star, rather than
ones that have migrated there from far away. According to the simulations
of a team led by Sean Raymond of the University of Washington, Seattle,
it is possible to make worlds with more than 100 times the Earth's water.
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- On such ocean planets, there would be no possibility
of any continents poking above the water, as the ocean would be 10 times
deeper than the height of Everest. The climate on such a world would depend
on things such as the orbit of the planet and the orientation of the axis
about which it spins, so in general it is not predictable.
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- But the planet's weather is likely to be extreme. "On
Earth, hurricanes run out of steam when they encounter land but, on an
ocean world, there would be no land to encounter," Sotin says. "There
might be super hurricanes." The winds would pile up enormous ocean
swells, which would never run in to land. "We're talking about the
kind of mountainous waves you get in the Pacific, only more so," Sotin
says.
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- What about the prospects for life? Well, on Earth, we
believe life got started in nutrient-rich waters close to volcanic vents
on the sea floor. The crucial factor, therefore, is how much ice at the
foot of the ocean separates the water from the planet's volcanic interior.
"If it is a thin layer, the prospects for life are good," says
Sotin. "If it's thick, they're not."
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- However, a minority of scientists think that volcanism
isn't necessary to get life started. These are the people, led by Chandra
Wickramasinghe of the University of Wales College, Cardiff, who believe
that life is "seeded" from space, with micro-organisms spread
from star system to star system in cometary dust. If this "interstellar
panspermia" idea is correct, then ocean planets will certainly contain
life. Essential nutrients could rain down from meteorites continually bombarding
the ocean from space.
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- Ocean planets are also exciting to planetary scientists
because they ought to be relatively easy to detect. Currently, our observational
techniques have difficulty detecting even a giant mass such as Jupiter,
and such bodies are not detected directly, only by the effect their gravity
has on their parent star.
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- The race is on to detect an extrasolar planet directly
- by the light it reflects from its sun. For an Earth-like planet - the
Holy Grail of planetary searches - this is immensely difficult. "However,
for an ocean planet, far bigger and with an ocean shining like a giant
silvered mirror, it should be much easier," Sotin says.
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- Currently, several space-based observatories are being
built to detect extrasolar planets directly. They include the European
Space Agency's project Darwin, a flotilla of six spacecraft flying in formation,
each carrying a two-metre infrared telescope; and Nasa's Terrestrial Planet
Finder, an eight-metre visible-light telescope. If ocean planets really
do exist, then the odds are that it will be one of their number, rather
than an Earth-mass planet, that will be discovered first. "If so,"
Sotin says, "We'd better start learning dolphin!"
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- - Marcus Chown is the author of 'The Universe Next Door'
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- © 2004 Independent Digital (UK) Ltd
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- http://news.independent.co.uk/world/science_medical/story.jsp?story=489934
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