The exoplanet K2-141b has a year that lasts only a few hours

Rendering of the possible surface of K2-141b (Image courtesy Marco Galliani/INAF)
Rendering of the possible surface of K2-141b (Image courtesy Marco Galliani/INAF)

Two articles, one to be published in “The Astronomical Journal” and one to be published in the journal “Astronomy and Astrophysics”, describe the study of the exoplanet K2-141b, a super-Earth very close to its star, so much that its year lasts only 6.7 hours. A team of researchers led by Luca Malavolta of the University of Padua and the Italian National Institute of Astrophysics used the National Galileo Telescope and its HARPS-N spectrograph to study it.

Planets with orbital periods of less than one Earth day were grouped into a specific class called ultra-short period exoplanets. They include super-Earths and hot Jupiters but such a proximity to their star that their year is so short is a rare case so far such as for example EPIC 228732031b.

The interest in these exoplanets is also due to the fact that, according to current models of planetary formation, it’s not possible that they formed so close to their stars. The most likely hypothesis is that they formed at a greater distance and then moved closer following migration mechanisms that are also under study.

It’s possible that originally rocky-type ultra-short period planets were mini-Neptune whose atmosphere was swept away by their star when they got closed to it. In the case of K2-141b, what remains is an exoplanet with a diameter that is about 1.5 times the Earth’s and a mass that is about 5 times the Earth’s, with a considerable amount of iron in its composition.

The exoplanet K2-141b, also referred to as EPIC 246393474b, orbits a star that has a mass and a size that are about 70% the Sun’s. This means that the temperature on its surface is also a bit lower than the Sun’s but that planet is so close that the researchers estimated that the temperature on its surface can reach 3,000 Kelvin on the face illuminated by its star. In fact, a planet so close to its star is certainly tidally locked, meaning that the same hemisphere is always facing its star like the Moon to the Earth.

The estimates of the temperature on the hidden side of the exoplanet K2-141b, where there’s a perennial night, depend a lot on its characteristics and on the possible presence of an atmosphere capable of transferring heat from the other hemisphere. There may be a significant temperature difference in the two hemispheres but at least part of the surface should be molten, in fact rocky-type ultra-short period exoplanets generally fall in the category of lava planets as well.

The star K2-141 also has at least one other planet, called K2-141c, probably similar to Neptune but its size and density haven’t been measured yet. Its orbital period was measured in 7.7 Earth days. Its presence may have had a gravitational influence on its neighbor pushing it towards their star. To obtain more information on both exoplanets further observations will be needed with other telescopes such as Hubble and the ones that will enter service in the coming years.

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