Astrophysics, new planet discovered that is similar to Mercury and as big as Earth
The size of Earth with a Mercury-like composition. This would be the unusual nature of exoplanet K2-229b, about 340 light years away from Earth.
The discovery was made by a team of researchers led by Alexandre Santerne, of the Laboratory of Astrophysics in Marseille, which included Francesca Faedi and Aldo Bonomo, of the Italian National Institute for Astrophysics-INAF. The scientists characterized the exoplanet using data from NASA’s Kepler-2 space telescope observations and from the HARPS spectrograph installed on the telescope of the European Southern Observatory-ESO at La Silla, in Chile.
Similar to Earth in size, K2-229b has a mass 2.6 times that of our planet. According to the researchers, this difference depends on the planet’s huge iron core, which would make up about 70% of the planet’s mass.
In this respect, the exoplanet is similar to ‘our’ Mercury, with a mass fraction of about 70% metallic core and 30% silicate mantle, unlike Venus, Mars, Earth and other extrasolar terrestrial planets that have a mass fraction of about 70% silicate mantle and about 30% metallic core.
“K2-229b is the first planet to be discovered that is the size of Earth but has an internal composition similar to Mercury”, explained Bonomo, co-author of the article that describes the discovery, published in Nature Astronomy.
“It is also the first time that a significant difference has been highlighted between the chemical composition of the planet and that of its star, and from the latter we would have expected a different internal structure of the planet, similar to Earth. At the moment, we cannot understand very well what process led K2-229b to have a Mercury-like composition, although several hypotheses have been proposed to explain it”.
K2-229b is the innermost of the three planets in the K2-229 planetary system, the existence of which was revealed by the Kepler space telescope in 2016. By studying the measurements taken by the HARPS spectrograph, the researchers could determine the mass of K2-229b, and therefore its structure, consisting of a huge iron core and a relatively thin silicate mantle.
But how was this particular structure formed? There are several hypotheses. The most likely hypothesis is that, just like what happened some billions of years ago with Mercury, the planet’s outer layers were torn away after a huge impact. Or, alternatively, that the planet formed in an inner zone of the protoplanetary disk with low silicate concentration, or else, that its silicate mantle evaporated due to the strong radiation from its, very close, star.
Upcoming missions are expected to provide more clues to better understand the formation of Mercury-like exoplanets.
“With ESA’s PLATO mission , planned for launch in 2024, we will finally have the opportunity to study in detail planets like K2-229b orbiting bright stars of different mass and chemical composition”, Francesca Faedi said.
Promoted by the European Space Agency (ESA), the PLATO mission is funded by Italy through the Italian Space Agency (ASI).
“Thanks to these observations, we will better understand the planetary formation and migration mechanisms and place our Solar System in a wider global context”, continued the INAF researcher who is involved, together with Bonomo, in the scientific preparation of the PLATO mission, devoted to the search for exoplanets.