An international team of astronomers has recently spotted an exoplanet around a sun-like star, much deeper than any ever-discovered rock planet. Analyzes by researchers led by Alexandre Santerne of the French University of Aix-Marseille suggest that the exoplanet may be similar to the Mercury of our solar system. How this 340-light-distant world was formed, however, remains is a mystery.
The exoplanet named K2-229b was discovered in 2016 by the Space Telescope Kepler. The measurements revealed a total of three planets around a star of spectral class K in the constellation Virgo. While the two outer worlds are not extraordinary finds, the innermost exoplanet proved to be unusually exciting: with a diameter only 20 percent bigger than that of the Earth, it is about 2.59 times as massive, making it as dense as Mercury, the astronomers report in the journal Nature Astronomy.
Existing research has shown that 70 percent of our innermost, and at the same time smallest, planet consists of a metallic core, over which a comparatively thin shell of silicate rock lies. That sets it apart from all other rock planets in the solar system. Mars, Earth and Venus have a reverse composition: The metallic core accounts for about 30 percent, the rest is rock. Possible scenarios of how the planet K2-229b is so constructed, and thus contradicts the previously valid theory for the formation of planets, is unclear.
The team around Santerne, however, has a number of ideas on how this exoplanet could have formed. Perhaps K2-229b was significantly larger shortly after its formation and was rendered skinnier by a cosmic disaster, such as the collision with another object, which stripped off part of its outer shell. A role could also have also been played by the close proximity of the planet to the mother star. Indeed, K2-229b orbits at a distance of 0.012 astronomical units around the star; that is about one hundredth of the distance between the earth and the sun. Stellar magnetic field eruptions could also have taken away parts of the exoplanet’s shell.