A European-Japanese research team has identified a solar-type star, K2-360, hosting multiple planets, including one with an exceptionally short orbital period. This planet, K2-360 b, is among the densest known, comparable to lead, condensing nearly eight Earth masses into a sphere slightly larger than Earth.
John Livingston from the Astrobiology Center in Tokyo, the lead author of the study published in Scientific Reports, noted, "K2-360 b is the densest known ultrashort-period planet with precise parameters."
The discovery was facilitated by NASA's K2 mission, which first observed the inner planet transiting its star in 2016. Follow-up observations with ground-based telescopes, including HARPS and HARPS-N spectrographs, confirmed the presence of the planet and revealed another orbiting body.
K2-360 b's extraordinary density suggests it may have once been larger, with its outer layers stripped away by the nearby star, leaving only its core. Davide Gandolfi from the University of Turin explained, "This provides insight into the fate of close-in planets, whose dense rocky cores may persist for billions of years."
The outer planet, K2-360 c, presents another intriguing aspect. Although a transit was not observed, researchers calculated its minimum mass based on its gravitational influence on the star. Computer simulations indicate it may have played a significant role in the system's formation and evolution.
Most planets close to their stars migrate inward due to interactions with the gas disk, but K2-360 b appears to have followed a different path. Alessandro Trani from the Niels Bohr Institute stated, "Our dynamical models suggest that K2-360 c may have pushed the other planet inward through high eccentricity migration. This process involves gravitational interactions that first elongate the inner planet's orbit, which gradually becomes circular due to tidal forces."
Analysis indicates that K2-360 b's iron-rich core resembles Earth's more than Mercury's. Based on the observed chemical composition of the star, models estimate that K2-360 b likely has a significant iron core comprising about 48% of its mass, making it more akin to a super-Earth than a super-Mercury, despite its extreme density.
Mahesh Herath from McGill University, a co-author of the study, remarked, "Our internal structure models suggest that K2-360 b likely has a substantial iron core surrounded by a rocky mantle. Its surface may be covered in magma due to the immense heat from its proximity to the star. Studying such planets can reveal how terrestrial bodies form in various galactic environments."
The discovery of the K2-360 system contributes valuable information to our understanding of planetary system formation and the processes involved. Ultrashort-period planets like K2-360 b are relatively rare, and this finding, along with the massive outer planet, aids in refining our theories regarding their formation.
Livingston concluded, "K2-360 offers an excellent laboratory for studying the formation and evolution of planets in extreme environments."