Of the 5,000 exoplanets discovered since 1995, the GJ 367b, found 31 light-years away, is certainly unique. First, according to its size: 9,000 km in diameter (compared to Earth’s 12,700), it barely surpasses Mars, making it one of the smallest exoplanets known to date. It is, of course, the smallest mass: only 55% of the Earth’s mass. On the other hand, its density seems to be higher than that of Earth. So it will have a large iron core representing about 4/5 of its distance … To complete the image, GJ 367b experiences a surface temperature of 1300 ° C. The red dwarf, which is twice the size of our Sun, orbits only a million kilometers from its star. This result was published in the journal Science.

Measure 3km / h at a speed of 300,000 billion km from Earth

This discovery was made possible thanks to the TESS satellite and the Harps instrument. Launched in 2018, TES (Transiting Exoplanet Survey Satellite) is dedicated to the search for exoplanets using the transit method. It measures the small eclipse caused by a star as it passes between the star and the earth. In the case of GJ 367 b, the brightness of the star decreases by only 0.03%. Harps (High Accuracy Radial Velocity Planet Search) is a spectrograph mounted on the 3.6 m telescope of the La Silla Observatory (ESO) in Chile. It is sensitive to the motion of the star due to the gravitational pull of the exoplanet. Here, too, the effect is minimal: the star travels at a speed of only 3 kilometers per hour. At a low speed of 300,000 billion kilometers from Earth!

The GJ 367b lasts eight hours a year

“Such measurements, within the capacity of the equipment, are possible thanks to the very short rotation period of the GJ 367b., Explains Science and the future Xavier Bonfils from the Institute of Planetology et al Astrophysic de Grenoble who participated in the study. In just 8 hours, the planet orbits its star. TESS discovered it from space, and after 27 days of continuous observation, was able to observe many parts of the planet in front of its star, which made it possible to increase the scale and improve accuracy. Something similar to the harps that were later pointed to the Exoplanet. Using the information they had, the researchers were able to operate their computer models to visualize what the asteroid looked like. In addition to her large heart, she has an ice sheet, a thin silicate coating, and perhaps a nice atmosphere of hydrogen and helium.

The specific goal of the future space telescope

All of this would make her look like Mercury, who is relatively close to her star and has an iron-disproportionate heart. Did the two planets have the same fate, each in their own solar system? “Suppose some mechanisms play a role, Think Xavier Bonfiles. For example, the proximity of a star and heat inevitably creates a strong erosion at the surface and increases the relative proportion of the iron core. But the most interesting thing is that a planet with a density comparable to that of Mercury has been discovered. So far, Telluric exoplanets have been similar in density to Earth, Venus or Mars. GJ 367b shows that such planets are as diverse as gas giants. However, all these results should be considered with caution, because the margin of error is important in such subtle measurements. But the robotic portrait of the asteroid will need to be updated in the coming months.GJ 367b will be one of the main targets Telescope Spatial James Webb [qui doit être lancé le 22 décembre 2021], Xavier Bonfiles emphasizes. Its brilliance makes it an ideal target. We need to be able to search for an atmosphere there and analyze it and, if necessary, find out if its silicate crust will evaporate under the sun.