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Discovery of a lava and iron planet

During its last update, carried out on Wednesday 1is In December, the Internet site Exoplanet.eu, better known by its name Encyclopedia of extrasolar planets, had 4,878 entries. We will have to add one 4 879e with the discovery, announced in Science Thursday, December 2, by an international team, from a curious extrasolar planet. A planet of lava and iron.

The star in question is one of the findings of TESS, a space telescope launched by NASA in 2018 to monitor small nearby stars, in the hope that possible terrestrial-type exoplanets pass in front of them and temporarily – and very slightly – decrease their light flow. A detection mode called “transit technique”. The newcomer is called GJ-367b, because it revolves around GJ-367, a star located 30.7 light years from us – which is to say in the suburbs of the Solar System, if we consider that our galaxy , the Milky Way, stretches some 100,000 light years. Half smaller and less massive than the Sun, GJ-367 belongs to the category of red dwarfs, which are the lightest, coolest and least luminous stars, but also the most abundant in the galaxy.

As one of the study’s co-authors, Xavier Delfosse, from the Grenoble Institute of Planetology and Astrophysics (CNRS-Université Grenoble-Alpes) explains, when an exoplanet is discovered, “The objective is to obtain both its mass and its radius, to determine its density and thus have information on its composition”. Density constitutes, in fact, a primordial factor from which we deduce what the planets are made of: if we rely on the stars of the Solar System, “A rocky planet like Earth has a density of 5.5 grams per cubic centimeter, continues Xavier Delfosse. For Neptune-type planets, which have an ice core surrounded by an envelope of gas, the density is between 1.5 g / cm3 and 2 g / cm3 while gas giants like Jupiter or Saturn, with a large atmosphere of hydrogen and helium, have a density of around 1 g / cm3. »

The surface temperature is around 1,500 ° C

The transit technique gave the radius of GJ-367b: 70% of the radius of the Earth. As for the mass, it was obtained thanks to the other method of detecting exoplanets, known as “radial speeds”, which analyzes the tiny oscillations that the mass of the planet subjects to the position of its star. The researchers were thus able to calculate the density of GJ-367b and were not disappointed with the result: 8.1 g / cm3, that is to say a figure higher than that of the planets of the solar system and a figure close to the density of iron.

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