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Location: UFOUpDatesList.Com > 2004 > Feb > Feb 12

Water, Water Everywhere

From: Frank Warren <frank-warren.nul>
Date: Thu, 12 Feb 2004 07:35:11 -0800
Fwd Date: Thu, 12 Feb 2004 14:26:04 -0500
Subject: Water, Water Everywhere

Source: The Independent


11 February 2004

Water, Water Everywhere

Conventional wisdom is that extraterrestrial life is most likely
to be found on Earth-like worlds. But astronomers now believe
that giant landless planets with oceans hundreds of kilometres
deep could be teeming with organisms.

By Marcus Chown

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.

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.

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.

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.

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

"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."

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.

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.

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.

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.

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.

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.

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.

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.

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.

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."

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.

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.

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.

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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