Les astronomes vont entraîner les spectrographes de haute précision de Webb sur deux exoplanètes rocheuses intrigantes.
Imaginez que la Terre soit beaucoup, beaucoup plus proche du Soleil. Si proche qu’une année entière ne durerait que quelques heures. Si proche que la gravité a enfermé un hémisphère dans la lumière permanente du jour et l’autre dans l’obscurité éternelle. Si près que les océans se dissolvent, les roches commencent à fondre, et les nuages font pleuvoir de la lave.
Bien que rien de tel n’existe dans notre propre système solaire, les planètes de ce type – rocheuses, de la taille de la Terre, extrêmement chaudes et proches de leur étoile – ne sont pas rares dans le Milky Way galaxy.
What are the surfaces and atmospheres of these planets really like? NASA’s James Webb Space Telescope is about to provide some answers.
Geology from 50 Light-Years: Webb Gets Ready to Study Rocky Worlds
With its mirror segments beautifully aligned and its scientific instruments undergoing calibration, NASA’s James Webb Space Telescope (Webb) is just weeks away from full operation. Soon after the first observations are revealed this summer, Webb’s in-depth science will begin.
Included in the investigations planned for the first year are studies of two hot exoplanets classified as “super-Earths” for their size and rocky composition: the lava-covered 55 Cancri e and the airless LHS 3844 b. Scientists will train Webb’s high-precision spectrographs on these planets with a view to understanding the geologic diversity of planets across the galaxy, as well as the evolution of rocky planets like Earth.
Super-Hot Super-Earth 55 Cancri e
55 Cancri e orbits less than 1.5 million miles from its Sun-like star (one twenty-fifth of the distance between Mercury and the Sun), completing one circuit in less than 18 hours. With surface temperatures far above the melting point of typical rock-forming minerals, the day side of the planet is thought to be covered in oceans of lava.
Planets that orbit this close to their star are assumed to be tidally locked, with one side facing the star at all times. As a result, the hottest spot on the planet should be the one that faces the star most directly, and the amount of heat coming from the day side should not change much over time.
But this doesn’t seem to be the case. Observations of 55 Cancri e from NASA’s Spitzer Space Telescope suggest that the hottest region is offset from the part that faces the star most directly, while the total amount of heat detected from the day side does vary.
Does 55 Cancri e Have a Thick Atmosphere?
One explanation for these observations is that the planet has a dynamic atmosphere that moves heat around. “55 Cancri e could have a thick atmosphere dominated by oxygen or nitrogen,” explained Renyu Hu of NASA’s Jet Propulsion Laboratory in Southern California, who leads a team that will use Webb’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) to capture the thermal emission spectrum of the day side of the planet. “If it has an atmosphere, [Webb] a la sensibilité et la gamme de longueurs d’onde nécessaires pour la détecter et déterminer de quoi elle est faite”, a ajouté M. Hu.
Ou est-ce qu’il pleut de la lave le soir sur 55 Cancrie ?
Une autre possibilité intrigante, cependant, est que 55 Cancri e n’est pas verrouillée de façon tidale. Au lieu de cela, elle pourrait être comme Mercure, tournant trois fois pour deux orbites (ce que l’on appelle une résonance 3:2). En conséquence, la planète aurait un cycle jour-nuit.
“Cela pourrait expliquer pourquoi la partie la plus chaude de la planète est décalée”, a expliqué Alexis Brandeker, un chercheur de l’Université de Stockholm qui dirige une autre équipe étudiant la planète. “Tout comme sur Terre, il faudrait du temps pour que la surface se réchauffe. Le moment le plus chaud de la journée serait dans l’après-midi, et non pas tout de suite à midi.”
L’équipe de Brandeker prévoit de tester cette hypothèse en utilisant NIRCam pour mesurer la chaleur émise par le côté éclairé de 55 Cancri e au cours de quatre orbites différentes. Si la planète a une résonance de 3:2, ils observeront chaque hémisphère deux fois et devraient être en mesure de détecter toute différence entre les hémisphères.
Dans ce scénario, la surface se réchaufferait, fondrait, voire se vaporiserait pendant la journée, formant une atmosphère très fine que Webb pourrait détecter. Le soir, la vapeur se refroidit et se condense pour former des gouttelettes de lave qui retombent en pluie sur le satellite.surface, redevenant solide à la nuit tombée.
Une super-Terre un peu plus froide LHS 3844 b
Alors que 55 Cancri e permet de découvrir la géologie exotique d’un monde couvert de lave, LHS 3844 b offre une occasion unique d’analyser la roche solide d’une exoplanet surface.
Like 55 Cancri e, LHS 3844 b orbits extremely close to its star, completing one revolution in 11 hours. However, because its star is relatively small and cool, the planet is not hot enough for the surface to be molten. Additionally, Spitzer observations indicate that the planet is very unlikely to have a substantial atmosphere.
What Is the Surface of LHS 3844 b Made of?
While we won’t be able to image the surface of LHS 3844 b directly with Webb, the lack of an obscuring atmosphere makes it possible to study the surface with spectroscopy.
“It turns out that different types of rock have different spectra,” explained Laura Kreidberg at the Max Planck Institute for Astronomy. “You can see with your eyes that granite is lighter in color than basalt. There are similar differences in the infrared light that rocks give off.”
Kreidberg’s team will use MIRI to capture the thermal emission spectrum of the day side of LHS 3844 b, and then compare it to spectra of known rocks, like basalt and granite, to determine its composition. If the planet is volcanically active, the spectrum could also reveal the presence of trace amounts of volcanic gases.
The importance of these observations goes far beyond just two of the more than 5,000 confirmed exoplanets in the galaxy. “They will give us fantastic new perspectives on Earth-like planets in general, helping us learn what the early Earth might have been like when it was hot like these planets are today,” said Kreidberg.
These observations of 55 Cancri e and LHS 3844 b will be conducted as part of Webb’s Cycle 1 General Observers program. General Observers programs were competitively selected using a dual-anonymous review system, the same system used to allocate time on Hubble.
The James Webb Space Telescope is the world’s premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.