Over the past 25 years, astronomers have discovered a wide variety of exoplanets, made up of rocks, ice and gas, through the construction of astronomical instruments designed specifically for planetary research.
Also, using a combination of different observation techniques, they were able to determine a large number of masses, sizes and therefore densities of the planets, which helps them estimate their internal composition and increases the number of planets that have been discovered outside the Solar System.
However, studying the atmospheres of rocky planets, which would make it possible to fully characterize these Earth-like exoplanets, is extremely difficult with the instruments currently available. For this reason, atmospheric models of rocky planets have not been tested.
It is therefore interesting that the astronomers of CARMENES (Calar Alto high-Resolution search for M nains with Exoearths with Near-infrared and Optical Scale Spectrographs), a consortium in which the Instituto de Astrofisica de Canarias (IAC) is a partner, have recently a published a study, conducted by Trifon Trifonov, astronomer at the Max Planck Institute for Astronomy in Heidelberg (Germany), on the discovery of a hot super-Earth orbiting a red dwarf star near Gliese 486, just 26 light years from the Sun.
To do this, the scientists used the combined techniques of transit photometry and radial velocity spectroscopy, and used, among other things, observations with the MuSCAT2 instrument (Multicolored simultaneous camera for the study of exoplanet atmospheres in transit) on the 1.52 m Carlos Sánchez telescope at the Teide Observatory. . The results of this study were published in the journal Science.
The planet they discovered, named Gliese 486b, has a mass 2.8 times that of Earth and is only 30% larger. “By calculating its average density from the measurements of its mass and its radius, we deduce that its composition is similar to that of Venus or the Earth, which contain metallic nuclei”, explains Enric Pallé, researcher at the IAC and co-author of the article.
Gliese 486b revolves around its host star on a circular path every 1.5 days, at a distance of 2.5 million kilometers. Despite being so close to its star, the planet has probably retained some of its original atmosphere (the star is much cooler than our Sun) so it is a good candidate to observe in more detail. with the next generation of space and floor. telescopes.
For Trifonov, “the fact that this planet is so close to the sun is exciting because it will be possible to study it in more detail using powerful telescopes such as the prominent James Webb Space Telescope and the ELT (Extremely Large Telescope) under construction. “
The Gliese 486b takes the same time to rotate on its axis as it does to orbit its host star, so that it always has the same side facing the star. Although Gliese 486 is much weaker and cooler than the Sun, the radiation is so intense that the planet’s surface heats up to at least 700K (around 430 degrees C). For this reason, the surface of Gliese 486b probably looks more like the surface of Venus than that of Earth, with a hot, dry landscape, with burning lava rivers. However, unlike Venus, Gliese 486b can have a thin atmosphere.
Calculations made with existing planetary atmospheric models may be consistent with hot surface and thin atmosphere scenarios because stellar irradiation tends to evaporate the atmosphere, while the planet’s gravity tends to hold it back. . Determining the balance between the two contributions is difficult today.
“The discovery of Gliese 486b was a fluke. If it had been a hundred degrees warmer, its entire surface would be lava, and its atmosphere would be vaporized rock, ”explains José Antonio Caballero, researcher at the Center for Astrobiology (CAB, CSIC-INTA) and co -author of the article. “On the other hand, if Gliese 486b had been about a hundred degrees cooler, it would not have been suitable for follow-up observations.”
Future observations planned by the CARMENES team will attempt to determine its orbital tilt, which allows Gliese 486b to cross the line of sight between us and the star’s surface, obscuring some of its light and producing what the we call transits. .
They will also perform spectroscopic measurements, using emission spectroscopy, when areas of the hemisphere illuminated by the star are visible as phases of the planet (analogous to the phases of our Moon), during orbits de Gliese 486b, before it disappears behind the star. The observed spectrum will contain information about the conditions of the warm illuminated surface of the planet.
“We can’t wait for the new telescopes to become available,” Trifonov admits. “The results that we will be able to obtain with them will help us to better understand the atmospheres of rocky planets, their extent, their very high density, their composition and their influence in the distribution of energy around the planets.
The CARMENES project, whose consortium is made up of 11 research institutes in Spain and Germany, aims to monitor a set of 350 red dwarf stars to search for planets like Earth, using a spectrograph on the 3.5 m telescope from the Calar Alto observatory (Spain). The present study also used spectroscopic measurements to derive the mass of Gliese 486b. Observations were made with the MAROON-X instrument on Gemini North (8.1m) in the United States, and archival data was taken from the 10m Keck Telescope (USA) and Telescope 3, 6 m from ESO, (Chile).
The photometric observations come from the TESS (Transiting Exoplanet Survey Satellite) space observatory of NASA, (USA), whose data were basic to obtain the radius of the planet, from the MuSCAT2 instrument on the Carlos Sánchez telescope of 1 , 52 m at the Teide observatory (Spain) and the LCOGT (Las Cumbres Observational Global Telescope) in Chile, among others.
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