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New Planet Could Be Earthlike, Scientists Say

By DENNIS OVERBYE  /  April 25, 2007

The most enticing property yet found outside our solar system is about
20 light-years away in the constellation Libra, a team of European
astronomers said yesterday.

The astronomers have discovered a planet five times as massive as the
Earth orbiting a dim red star known as Gliese 581.

It is the smallest of the 200 or so planets that are known to exist
outside of our solar system, the extrasolar or exo-planets. It orbits
its home star within the so-called habitable zone where surface water,
the staff of life, could exist if other conditions are right, said
Stephane Udry of the Geneva Observatory.

“We are at the right place for that,” said Dr. Udry, the lead author
of a paper describing the discovery that has been submitted to the
journal Astronomy & Astrophysics.

But he and other astronomers cautioned that it was far too soon to
conclude that liquid water was there without more observations. Sara
Seager, a planet expert at the Massachusetts Institute of Technology,
said, “For example, if the planet had an atmosphere more massive than
Venus’s, then the surface would likely be too hot for liquid water.”

Nevertheless, the discovery in the Gliese 581 system, where a Neptune-
size planet was discovered two years ago and another planet of eight
Earth masses is now suspected, catapults that system to the top of the
list for future generations of space missions.

“On the treasure map of the universe, one would be tempted to mark
this planet with an X,” said Xavier Delfosse, a member of the team
from Grenoble University in France, according to a news release from
the European Southern Observatory, a multinational collaboration based
in Garching, Germany.

Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics,
who studies the structure and formation of planets, said: “It’s 20
light-years. We can go there.”

The new planet was discovered by the wobble it causes in its home
star’s motion as it orbits, using the method by which most of the
known exo-planets have been discovered. Dr. Udry’s team used an
advanced spectrograph on a 141-inch-diameter telescope at the European
observatory in La Silla, Chile.

The planet, Gliese 581c, circles the star every 13 days at a distance
of about seven million miles. According to models of planet formation
developed by Dr. Sasselov and his colleagues, such a planet should be
about half again as large as the Earth and composed of rock and water,
what the astronomers now call a “super Earth.”

The most exciting part of the find, Dr. Sasselov said, is that it
“basically tells you these kinds of planets are very common.” Because
they could stay geologically active for billions of years, he said he
suspected that such planets could be even more congenial for life than
Earth. Although the new planet is much closer to its star than Earth
is to the Sun, the red dwarf Gliese 581 is only about a hundredth as
luminous as the Sun. So seven million miles is a comfortable huddling

How hot the planet gets, Dr. Udry said, depends on how much light the
planet reflects, its albedo. Using the Earth and Venus as two extreme
examples, he estimated that temperatures on the surface of the planet
should be in the range of 0 degrees to 40 degrees centigrade.

“It’s just right in the good range,” Dr. Udry said. “Of course, we
don’t know anything about its albedo.”

One problem is that the wobble technique only gives masses of planets.
To measure their actual size and thus find their densities,
astronomers have to catch the planets in the act of passing in front
of or behind their stars. Such transits can also reveal if the planets
have atmospheres and what they are made of.

Dr. Udry said he and Dr. Sasselov would be observing the Gliese system
with a Canadian space telescope named MOST to see if there are any
dips in starlight caused by the new planet. Failing that, they said,
the best chance for more information about the system lies with the
Terrestrial Planet Finder, a NASA mission, and the Darwin missions of
the European Space Agency, which are designed to study Earthlike
planets, but have been delayed by political, technical and financial

“We are starting to count the first targets,” Dr. Udry said.

For plants on alien worlds, it isn’t easy being green
11 April 2007  /  Jeff Hecht

The greenery on other planets may not be green. Astrobiologists say
plants on Earth-sized planets orbiting stars somewhat brighter than
the Sun may look yellow or orange, while those on planets orbiting
stars much fainter than the Sun might look black.

Vegetation colour matters to astrobiologists because they want to know
what to look for as a sign of life on planets outside the solar
system. Terrestrial photosynthesis depends mostly on red light, the
most abundant wavelength reaching the Earth’s surface, and blue light,
the most energetic. Plants also absorb green light, but not as
strongly, so leaves look green to the eye.

Extraterrestrial plants will look different because they have evolved
their own pigments based on the colours of light reaching their
surfaces, says Nancy Kiang of the NASA Goddard Institute for Space
Sciences in New York, US.

To determine the best colours for photosynthesis on other planets,
Kiang worked with NASA’s Virtual Planetary Laboratory at Caltech to
determine the light reaching the surfaces of Earth-sized worlds
orbiting their host stars at distances where liquid water – and
therefore life – could exist. The results depended on the star’s
brightness and the planet’s atmosphere.

Autumn colours

Brighter stars emit more blue and ultraviolet light than the Sun. An
oxygen atmosphere would form ozone that blocks the ultraviolet but
transmits more blue light to the ground than on the Earth. In
response, life would evolve a type of photosynthesis that strongly
absorbs blue light, and probably green as well. Kiang says yellow,
orange, and red would likely be reflected, so the foliage would wear
bright autumn colours all year round.

A star slightly dimmer than the Sun would deliver a solar-like
spectrum to the surface of a terrestrial planet, so its foliage would
look much like the Earth’s.

But plants would be different on planets orbiting small M-type stars,
or red dwarfs, which are between 10% and 50% the mass of the Sun. Red
dwarfs, which comprise 85% of the galaxy’s stars, emit strongly at
invisible infrared wavelengths but produce little blue light.

“They’ll definitely be absorbing in the infrared,” unlike terrestrial
plants, Kiang told New Scientist. Because they would benefit by
absorbing visible light, she says they might look black, although she
admits that any colour might be possible. Whatever their colour, the
plants would likely look dark to humans because little visible light
would reach the ground.
Floating and sinking

Photosynthesis might not draw enough energy from infrared light to
produce the oxygen needed to block dangerous ultraviolet light from
the dwarfs.

But if there were at least 9 metres of water on the planet, mats of
algae would be protected from the planet-scalding ultraviolet flares
produced by young red dwarf stars, says Victoria Meadows of Caltech,
principal investigator at the Virtual Planetary Laboratory.

She envisions a bizarre world where microbial mats float near the
surface for efficient photosynthesis when the star is calm, then sink
to a safe depth when a flare hits.

Life could spread further when the stars pass their flare stage, she
told New Scientist: “M stars don’t produce a lot of ultraviolet once
they quiet down, so you don’t need an oxygen layer to shield [life]
from the ultraviolet.”

Journal reference: Astrobiology (vol 7, p 252)

Danger zones mapped for developing planets
18 April 2007 / David Shiga

Stars incubating developing planets would do best to stay at least 1.6
light years away from very massive stellar neighbours. If they venture
any closer than this, they risk having the raw materials needed for
planet formation blown away from them, a new study says.

Previous studies have shown that radiation from very massive stars can
evaporate the planet-forming discs of gas and dust around other nearby
stars. But the exact size of the ‘danger zone’ around massive stars
was not known.

Now, Zoltan Balog of the University of Arizona in Tucson, US, and
colleagues have used NASA’s Spitzer Space Telescope to map the extent
of danger zones around massive stars in the Rosette Nebula, a star-
forming region 5200 light years from Earth.

Previous research has found that about 50% of young stars in star-
forming regions have dusty discs around them. Balog’s team found
similar results in the Rosette Nebula – but only for stars that were
at least 1.6 light years from their nearest massive neighbour.

For those at closer distances, the number retaining dusty discs
dropped to 27%, suggesting that many have had their discs blown away
by the massive stars.

Long stay

“Stars move around all the time, so if one wanders into the danger
zone and stays for too long, it will probably never be able to form
planets,” Balog says.

Watch an animation showing how the disc evaporation might appear over
hundreds of thousands of years.

But stars passing through these danger zones might not be completely
sterilised as far as planet formation is concerned.

If planets can form fast enough, they could coalesce before radiation
has a chance to blow the raw material away. One theory says gas giant
planets like Jupiter could form in less than a million years, which
might be fast enough to avoid the evaporation problem.

Also, one previous study suggests that blowing away some of the gas in
a dusty disc might actually aid the formation of planets, because gas
can make it harder for the dust to clump together and form larger