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Publié par trichard à 11:53:34 dans BIOSOPHIE | Commentaires (0) | Permaliens
http://www.nature.com/news/2007/070423/full/446956b.html / Published online: 25 April 2007; | doi:10.1038/446956b
Kerri Smith
Analysis of two damaged brains, preserved in a museum since the nineteenth century, could force neuroscientists to rethink the area where language resides in the brain.
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Publié par trichard à 12:28:24 dans PHYSIOLOGIE | Commentaires (0) | Permaliens
http://www.nature.com/news/2007/070423/full/070423-5.html / Published online: 25 April 2007; | doi:10.1038/news070423-5
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Astronomers
have found an Earth-like planet circling a dim red star not far, in
galactic terms, from our Solar System. The planet, just five times the
mass of our own, might be the best hope yet of a world that can support
life.
The
extrasolar planet, or exoplanet, is orbiting one of our closest stellar
neighbours, the red dwarf star Gliese 581, just 20.5 light years away.
Stéphane Udry of the Geneva Observatory in Switzerland and his
colleagues spotted the planet by detecting wobbles in the parent star,
caused by the orbiting planet's gravity.
The
planet is much closer to its star than we are to the Sun - orbiting at
one-fourteenth of the Earth-Sun distance. But because Gliese 581 is a
red dwarf, which emits less light and heat than the Sun, the planet is
in the so-called 'habitable zone' for its star. The researchers'
calculations suggest that the planet's average temperature is between 0
and 40 °C - perfect for liquid water, and perhaps even life, to exist.
But
this is a very crude temperature estimate, says Udry's colleague Michel
Mayor, principal investigator for HARPS (High Accuracy Radial velocity
Planetary Searcher), the instrument that made the observations in La
Silla, Chile. To get a better idea, more information about the nature
of the planet would be needed - for example, whether it has an
atmosphere. "For the time being, it is difficult to know more," he says.
In with a chance
The
new planet is the closest in mass to Earth ever discovered outside our
Solar System -the previous nearest match was roughly 5.5 times the mass
of Earth and in a much more distant orbit from its star. The technique
used by Udry's team can only put a lower limit on the planet's likely
mass, and its size can therefore only be guessed at: if the planet is
rocky and Earth-like, its radius should be around 1.5 that of Earth. If
the planet is ocean-like, it will be slightly bigger. The researchers
have submitted their results to the journal Astronomy & Astrophysics.
Information
about the planet's composition can only be gleaned if the planet is
passing in front of, or transiting, its star, and the chances of seeing
that happen with any one planet is about 2%, says Mayor. But this
doesn't mean that they will stop looking. "We have good reason to
believe that this kind of planet exists around other stars," he says.
And if there are a lot of planets whizzing around their stars, at some
point a transiting planet will be seen.
The latest discovery follows news two years ago of two
other planets orbiting Gliese 581, one roughly eight times the Earth's
mass, and the other around 15 times Earth mass.
If
Udry's models are correct, the new planet would be a so-called
'super-Earth' - a very exciting prospect, says exoplanet expert David
Charbonneau at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts. "If the planet is a rocky super-Earth, then
perhaps it has a surface with liquid water and life," he suggests.
There
is another, less exciting option, however, which would make the planet
slightly less homely, Charbonneau adds: "If instead the planet is a
'sub-Neptune', then it would have a large gas envelope that buries the
surface below, making it inhospitable for life."
References : Udry U., et al. Astron. Astrophys. (submitted) .
La première planète potentiellement habitable
Une planète "habitable" à vingt années-lumière de la Terre

Vue d'artiste du système planétaire autour de la naine rouge Gliese 581.
Publié par trichard à 22:54:57 dans PLANETOLOGIE | Commentaires (0) | Permaliens
Mais Francis Hueber, un chercheur au Muséum national d'histoire naturelle de Washington, a émis dès 2001 une thèse selon laquelle il s'agirait bien de champignons. Lui-même et son équipe se sont appuyés sur une étude isotopique de Prototaxites de diverses provenances, notamment le Canada, l'Australie et l'Arabie Saoudite. Ils ont pour cela examiné et mis en rapport les ratios des isotopes 12 et 13 du carbone. Ceux-ci sont répartis régulièrement dans les plantes, qui produisent cet élément à partir du CO2 présent dans l'air, et varient peu d'une espèce à l'autre. Par contre, chez les champignons, qui ne réalisent pas la photosynthèse, le carbone provient de la nourriture, comme pour les animaux et présentent des ratios variables.
Selon Francis Hueber, les variations des ratios observés entre les valeurs isotopiques du carbone sont trop différentes pour qu'il puisse s'agir de plantes. Une dernière observation manque encore cependant pour valider définitivement la thèse des chercheurs: la découverte de spores fossiles, dont l'existence est liée à toutes les espèces de champignons et qui en constituent le seul moyen de reproduction.

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Sections fossiles de Prototaxites (Crédits : Muséum national d'histoire naturelle de Washington)
Publié par trichard à 22:41:41 dans PHYLOGENIE | Commentaires (0) | Permaliens
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Clouds
are bigger than they look, according to new measurements by atmospheric
scientists in Israel and the United States. They say that clouds are
surrounded by a 'twilight zone' of diffuse particles, invisible to the
naked eye, extending for tens of kilometres around the cloud's visible
portion.
These
vast, sparse haloes of droplets may have been overlooked in atmospheric
studies, the researchers say. And they think that this could have
skewed attempts to understand how clouds influence climate.
Clouds
are one of the biggest sources of uncertainty in efforts to measure and
predict global warming. They have two opposite effects: increasing
warming by absorbing heat radiated from the planet's surface (which is
why cloudy nights are warmer), while offsetting this by reflecting
sunlight back into space from cloud tops.
Most
atmospheric scientists now think that clouds have an overall global
cooling effect. Measurements of warming trends therefore have to take
into account whether the skies are cloudy or not, and model forecasts
of future warming may hinge on whether they predict more or less
cloudiness...
References : Koren I., et al. Geophys. Res. Lett., 34. L08805 (2007).
Publié par trichard à 22:26:40 dans PLANETOLOGIE | Commentaires (0) | Permaliens