Nous venons de trouver 2 des exoplanètes les plus semblables à la Terre, à seulement 12,5 années-lumière

Les astronomes sont convaincus d’avoir trouvé deux nouvelles planètes semblables à la Terre dans notre galaxie, et les deux ressemblent tellement à la nôtre qu’elles font maintenant partie des 19 exoplanètes les plus connues dont l’environnement est potentiellement habitable.

En orbite autour d’une étoile voisine dans la constellation du Bélier, à seulement 12,5 années-lumière de distance, l’une de ces deux planètes pourrait en fait présenter la plus grande similitude avec la Terre que nous ayons découverte jusqu’ici.

“Les deux planètes ressemblent aux planètes intérieures de notre système solaire”explique l’auteur principal Mathias Zechmeister, astrophysicien de l’Université de Göttingen.

“Elles ne sont que légèrement plus lourdes que la Terre et sont situées dans la zone dite habitable, où l’eau peut être présente sous forme liquide.”

Malgré sa proximité, l’étoile de Teegarden n’a été découverte qu’en 2003. Environ dix fois plus légère que notre propre Soleil, l’une des plus petites étoiles que nous connaissons, la vieille naine rouge, âgée d’environ 8 milliards d’années, s’est révélée être un défi pour la recherche.

Selon l’équipe, d’autres systèmes planétaires autour d’étoiles similaires ont toujours été détectés par la méthode du transit, lorsqu’une planète en orbite passe devant une étoile, bloquant la vue de la Terre et provoquant l’obscurcissement de l’objet céleste brillant pendant un court instant.

L’alignement et l’obscurité de Teegarden ne se prêteraient toutefois pas à cette méthode, de sorte que les astronomes ont plutôt utilisé le télescope de nouvelle génération de CARMENES conçu spécialement pour de telles situations. Situé à l’Observatoire de Calar Alto en Espagne, l’instrument a permis aux chercheurs de rechercher tout changement dans la vitesse radiale de la mini-étoile.

Après trois ans d’observation rapprochée, à l’affût des “oscillations” produites par les objets en orbite, plus de 200 mesures indiquent l’existence de deux nouvelles planètes, maintenant appelées Teegarden b et Teegarden c.

Pour s’assurer que les données de vitesse radiale indiquant ces planètes n’étaient pas usurpées par les variations de luminosité de l’étoile, les chercheurs ont complété leurs observations par des données photométriques (mesure de la lumière) recueillies sur l’étoile de Teegarden.

“Ces études démontrent que les signaux des deux planètes ne peuvent pas être dus à l’activité de l’étoile, même si nous n’avons pas pu détecter les transits des deux nouvelles planètes”explique l’astronome Victor Sánchez Béjar de l’Instituto de Astrofísica de Canarias (AIS).

Teegarden b est la planète la plus intérieur ; selon l’équipe internationale, elle a 60 % de chance d’avoir un environnement de surface tempérée, entre 0° et 50°C et probablement plus proche de 28°C. Teegarden c, d’autre part, se trouve plus loin, et a une température de surface plus proche de celle de Mars, à environ -47°C.

(A Mendez/PHL)

Compte tenu de leur masse minimale et de leur exposition au rayonnement solaire, les deux planètes ont créé le Catalogue des Exoplanètes Habitables. En fait, Teegarden b a obtenu l’indice de similarité avec la Terre, parfois noté IST (en anglais : Earth Similarity Index, noté ESI) le plus élevé jamais obtenu.

Bien que cela ne signifie pas nécessairement que l’une ou l’autre planète est habitable, c’est certainement un signe prometteur. M. Zechmeister a dit au Guardian que si ces planètes sont équipées d’atmosphères, elles pourraient très bien être hospitalières à la vie.

“Les planètes Teegarden b et c sont les premières planètes détectées par la méthode des vitesses radiales autour d’une naine rouge si froide”a écrit l’équipe dans un article décrivant cette découverte.

“Les deux planètes ont une masse minimale proche d’une masse terrestre, et étant donné leur composition rocheuse, partiellement en fer ou en eau, on s’attend à ce qu’elles aient des rayons semblables à la Terre.”

(A Mendez/PHL)

Lauren Weiss, astrophysicienne à l’Université d’Hawaï qui n’a pas participé à cette recherche, a déclaré au National Geographic qu’il y avait encore quelques détails techniques à mettre en évidence, mais elle a été impressionnée par la qualité globale des données.

Alors que l’équipe prédit que Teegarden b termine son orbite en 4,9 jours terrestres et c en 11,4 jours, Mme Weiss soutient que leur voyage pourrait aller encore plus vite que cela, ce qui réduirait inévitablement leur habitabilité.

De plus, ajoute-t-elle, nous ne savons pas encore exactement combien de temps il faut à Teegarden pour tourner sur son axe ; étant donné que les astronomes ont utilisé des mesures de vitesse radiale pour obtenir leur découverte, une de ces détections de planète pourrait encore être un artefact de la rotation de l’étoile, mais probablement pas les deux.

En tant que 24e système stellaire le plus proche du nôtre et le quatrième le plus proche avec des planètes potentiellement habitables, Teegardeen est un excellent candidat pour de futures recherches, et son potentiel pour abriter la vie nous a laissé très enthousiastes.

La recherche a été publiée dans Astronomy & Astrophysics.

Source : ScienceAlert – Traduit par Anguille sous roche

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