De nombreuses planètes potentiellement habitables d'une taille proche de la Terre sont en orbite autour d'étoiles dites "naines rouges", plus petites et moins chaudes que le soleil et très fréquentes dans notre galaxie, la Voie Lactée, selon des chercheurs.

Dans une recherche publiée mercredi, qui se base sur un catalogue d'exoplanètes découvertes par le télescope américain Kepler, les astronomes du centre d'astrophysique de l'université d'Harvard (CfA) estiment que 6% des "naines rouges" ont des planètes d'une taille similaire à la Terre et potentiellement habitables.

Dans la mesure où ces "naines rouges" sont les étoiles les plus fréquentes dans notre galaxie, au nombre de 75 milliards, on pourrait compter quelque 4,5 milliards de planètes potentiellement habitables. L'exoplanète soeur de la Terre la plus proche se trouverait à seulement 13 années-lumière (une année lumière correspond à 9.460 milliards de kilomètres), selon ces scientifiques.

"Nous pensions que nous devrions explorer de vastes distances pour trouver une planète comme la Terre, mais maintenant nous réalisons qu'une autre planète comme la Terre est probablement dans notre voisinage cosmique, attendant d'être découverte", relève Courtney Dressing, une astronome de l'université de Harvard, et principal auteur.

Malgré le fait que les "naines rouges" soient plus petites et moins chaudes que les autres étoiles comme notre soleil, elles offrent des conditions propices aux planètes comme la Terre, explique-t-elle.

Une "naine rouge" moyenne atteint seulement le tiers de la taille du soleil et est un millier de fois moins brillante. Aucune de ces étoiles n'est visible de la Terre à l'oeil nu.

Ces chercheurs ont identifié 95 planètes en orbite autour de "naines rouges" qui pourraient être habitables. Mais seulement trois, ou 6% d'entre elles, ont des températures et une taille proches de la Terre.

"Ce taux laisse penser qu'il sera nettement plus facile de rechercher la vie au-delà de notre système solaire que ce que nous pensions", note David Charbonneau, co-auteur de l'étude.

Pour autant une exoplanète habitable autour d'une "naine rouge" serait un monde totalement différent du nôtre.

Car ces exoplanètes sont proches des étoiles. Mais cela n'empêcherait pas la vie d'exister, avec une atmosphère suffisament épaisse ou des océans profonds pour distribuer la chaleur autour de la planète.

En outre, comme les naines rouges ont une existence plus longue que celles d'étoiles comme le soleil, les exoplanètes autour sont beaucoup plus vieilles et la vie y serait aussi plus ancienne.

Télécharger l'étude (PDF)

Une vidéo (en anglais) sur les naines rouges et les planètes habitables:

Voir aussi: une galerie d'illustrations compilées par nos collègues américains du Huffington Post
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  • NASA's Kepler Mission Discovers Planet

    In this handout illustration made available on December 5, 2011 by NASA, the Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star is digitally illustrated. For the first time NASA's Kepler mission has confirmed a planet to orbit in a star's habitable zone; the region around a star, where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth, making it the smallest yet found to orbit in the middle of the habit. Clouds could exist in this earth's atmosphere, as the artist's interpretive illustration depicts. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images)

  • NASA's Kepler Mission Discovers Planet

    In this handout illustration made available on December 5, 2011 by NASA, a diagram compares our own solar system to Kepler-22, a star system containing the first 'habitable zone' planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth. The diagram displays an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star and is about 2.4 times the size of Earth. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images)

  • Extrasolar Planet HD 209458 b, Osiris

    Artist's conception released by NASA of extrasolar planet HD 209458 b, also known as Osiris, orbiting its star in the constellation Pegasus, some 150 light years from Earth's solar system. Scientists have used an infrared spectrum -- the first ever obtained for an extrasolar planet -- to analyze Osiris' atmosphere, which is said to contain dust but no water. The planet's surface temperature is more than 700 Celsius (1330 Fahrenheit).'

  • Planet & Its Parent Star

    Picture released 04 October 2006 by the European Space Agency shows an artist's impression of a Jupiter-sized planet passing in front of its parent star. Such events are called transits. When the planet transits the star, the star's apparent brightness drops by a few percent for a short period. Through this technique, astronomers can use the Hubble Space Telescope to search for planets across the galaxy by measuring periodic changes in a star's luminosity. The first class of exoplanets found by this technique are the so-called 'hot Jupiters,' which are so close to their stars they complete an orbit within days, or even hours. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM

  • Hot Jupiter

    Picture released 04 October 2006 by the European Space Agency shows an artist's impression of a unique type of exoplanet discovered with the Hubble Space Telescope. This image presents a purely speculative view of what such a 'hot Jupiter' (word dedicated to planets so close to their stars with such short orbital periods) might look like. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM

  • The Goldilocks Planet: Glises 581 G

    Scientist have found a new potentially habitable planet.

  • Imagining Extrasolar Planets

    From the Spitzer Science Center. While astronomers have identified over 500 planets around other stars, they're all too small and distant to fill even a single pixel in our most powerful telescopes. That's why science must rely on art to help us imagine these strange new worlds. From Spitzer Space Telescope. Even without pictures of these exoplanets, astronomers have learned many things that can be illustrated in artwork. For instance, measurements of the temperatures of many "Hot Jupiters," massive worlds orbiting very close to their stars, hint that their atmospheres may be as dark as soot, glowing only from their own heat. While "Hot Jupiters" would be relatively dark in visible light, compared to their stars, their brightness is proportionally much greater in the infrared. Illustrating this dramatic contrast change helps explain why the infrared eye of NASA's Spitzer Space Telescope plays a key role in studying exoplanets. As our understanding evolves, so must the artwork. Astronomers found a blazing hot spot on the exoplanet Upsilon Andromedae b that at first, appeared to face towards its star. More data has revealed that the hottest area is actually strangely rotated almost 90 degrees away, near the day/night terminator. WASP 12b is as hot as the filament in a light bulb, and would be blazing bright to our eyes. Most interestingly, if it proves to have a strongly elliptical orbit, as first thought, calculations show it would be shedding some of its outer atmosphere <b>...</b>