Discovery of an Ultra-Dense Super Earth and an Outer Planet Gives Clues to Its Formation Process

An international research team led by Japan has discovered a new multi-planet system orbiting a Sun-like star. The discovery, published in Nature Scientific Reports on November 8, sheds new light on the formation and evolution of planets in extreme environments.

The newly discovered K2-360 system is located approximately 750 light years from Earth. This planetary system consists of two planets orbiting sun-like stars as follows

K2-360 b: An ultra-short-period “super-Earth” (a rocky planet larger than Earth but smaller than Neptune) that is about 1.6 times the size of Earth and orbits its star in 21 hours. With a mass 7.7 times that of the Earth, it is characterized as the densest planet of its kind.
K2-360 c: A larger outer planet at least 15 times the mass of Earth, it orbits in 9.8 days. The exact size of this planet is unknown because it does not pass in front of its star.
K2-360 b is a truly remarkable planet. It packs about eight times the mass of Earth into a sphere about the same size as Earth, and is as dense as lead,” said study leader John Livingston of the Astrobiology Center. “This makes it the densest known ‘ultrashort-period’ planet [for which precise parameters were sought] that orbits its star in less than a day.”

This discovery was made possible by NASA’s K2 mission, which first detected the inner planet passing (transit) in front of its star in 2016; follow-up observations with ground-based telescopes, including HARPS and the HARPS-N spectrograph, confirmed the nature of the planet and revealed the presence of the outer companion star The presence of the outer companion was revealed.

The extreme density of K2-360 b suggests that it may be the remnant central core of what was once a large planet. It appears that the outer layers of the planet were lost due to strong radiation from a nearby star. The planet offers a glimpse into the fate of a world in which the planet and its star are in close proximity. After billions of years of evolution, only a dense rocky core remains,” explains co-author Davide Gandolfi of the University of Turin.

The outer planet K2-360 c adds an even more interesting element to the system. Although it does not transit, its minimum mass could be measured due to its gravitational effect on the star. Computer simulations suggest that this planet may have played an important role in the formation and evolution of the system. While many planets orbiting near stars are thought to have moved inward due to interactions with the primordial gas disk at birth, K2-360 b appears to have arrived at its current orbit in a different manner.

Our dynamical model indicates that K2-360 c may have pushed the inner planet into its current close orbit through a process called ‘high eccentricity transfer,’” explains co-author Alessandro Trani of the Niels Bohr Institute. “This is a process whereby gravitational interactions cause the orbit of the inner planet to first become highly elliptical and then gradually circularize near the star due to tidal forces. Alternatively, the tilt of the planet’s rotational axis could have caused the tidal circularization.”

According to the team’s analysis, K2-360 b has an iron-rich rock composition that is more similar to Earth than Mercury. Using a model based on the chemical composition of the central star, they estimated that a large iron core accounts for about 48% of K2-360 b’s mass. This indicates that despite its extreme density, it is more akin to a “super-Earth” than a “super-Mercury.

Our interior structure model indicates that K2-360 b may have an iron core of substantial size surrounded by a rocky mantle,” explains McGill University doctoral student Mahesh Herath. ‘Its surface may be covered with magma because of the intense heat it receives from the star. Understanding these planets will help us piece together the formation and evolution of terrestrial planets under different conditions throughout the galaxy.”

The discovery of the K2-360 system provides valuable insight into the structure of planetary systems and the processes that shape them. ultrashort-period planets such as K2-360 b are relatively rare, but the discovery of one with a massive outer companion helps constrain theories about their formation. Livingston concludes, “K2-360 is an excellent laboratory for studying the formation and evolution of planets in extreme environments.”