An international team of astronomers has recently identified an extraordinary multi-planet system around a star resembling our Sun, offering new insights into the dynamics of planetary formation and evolution in extreme environments. The newly found system, named K2-360, is located approximately 750 light-years away from Earth and includes two distinct planets, one of which exhibits one of the highest densities ever recorded for a planet of its kind. The research findings were published in Scientific Reports on November 8, 2024.
The centerpiece of this discovery is the inner planet, K2-360 b, which belongs to a class known as “ultra-short period” (USP) planets. These planets orbit their stars in less than a single Earth day, subjecting them to intense stellar radiation. K2-360 b is categorized as a “super-Earth,” meaning it is larger than Earth but smaller than Neptune. This particular planet orbits its star at a blistering pace, completing a full orbit in just 21 hours. Despite its relatively small size—only about 1.6 times the diameter of Earth—K2-360 b has a mass approximately 7.7 times that of our planet. This combination of size and mass results in an incredibly high density, making it the densest well-characterized USP planet known.
John Livingston, the study’s lead author from the Astrobiology Center in Tokyo, highlighted the uniqueness of this discovery: “K2-360 b is truly remarkable—it’s as dense as lead, packing nearly 8 Earth masses into a ball only slightly larger than our planet. This makes it the densest known planet among the class of ultra-short period planets with well-constrained parameters.” This density suggests that K2-360 b may be the stripped core of a formerly larger planet, which likely lost its outer layers due to the intense radiation from its close proximity to its host star.
The second planet in the system, K2-360 c, adds further complexity to the dynamics of the system. Unlike its inner companion, K2-360 c does not transit its star, making it impossible to directly measure its size. However, its gravitational influence on the host star allowed researchers to determine its minimum mass to be at least 15 times that of Earth. This outer planet takes about 9.8 days to complete one orbit, positioning it much farther from the star than K2-360 b.
The initial detection of this multi-planet system was made possible by data from NASA’s K2 mission, which discovered the inner planet transiting its star back in 2016. However, it took years of follow-up observations using powerful ground-based telescopes, including the HARPS (High Accuracy Radial Velocity Planet Searcher) and HARPS-N spectrographs, to confirm the nature of K2-360 b and to reveal the existence of the non-transiting outer planet.
One of the most intriguing aspects of K2-360 b is its extremely high density. The planet’s composition suggests it is primarily rocky, with a minimal presence of lighter elements like hydrogen or helium. This has led researchers to speculate that K2-360 b might be the remnant core of a gas giant or a larger planet that was stripped down to its dense interior layers. The intense stellar radiation from the host star, combined with tidal forces, likely contributed to the erosion of the planet’s outer atmosphere over billions of years. Davide Gandolfi, a co-author from the University of Turin, noted, “This planet gives us a glimpse into the possible fate of some close-in worlds, where only the dense, rocky cores remain after billions of years of evolution.”
The discovery of K2-360 c, which does not transit its star, adds another dimension to the system’s complexity. Although its exact size remains unknown, its influence on the star indicates it has a significant gravitational presence. According to computer simulations, this outer planet may have played a crucial role in shaping the current configuration of the system. One theory proposed by the researchers is that K2-360 c could have driven the inner planet, K2-360 b, into its present ultra-tight orbit through a mechanism known as high-eccentricity migration.
Alessandro Trani from the Niels Bohr Institute, who co-authored the study, explained, “Our dynamical models indicate that K2-360 c could have pushed the inner planet into its current tight orbit through a process called high-eccentricity migration. This involves gravitational interactions that initially stretch the inner planet’s orbit into a highly elongated shape. Over time, tidal forces between the planet and its star gradually circularize the orbit, resulting in the ultra-short period we observe today.” Another possibility is that the circularization was driven by tidal interactions related to the axial tilt of the planet, though further study is needed to confirm this hypothesis.
The discovery of K2-360 and its peculiar planetary system contributes to a broader understanding of how planets can form and evolve under extreme conditions. Unlike many other close-in exoplanets that are believed to have migrated inward due to interactions with their original gas disks, the evolution of K2-360 b may have followed a different pathway influenced by gravitational interactions with its companion planet.
The study opens up new questions about the long-term stability and evolution of such compact planetary systems. As researchers continue to analyze data from both space-based and ground-based observatories, they hope to uncover more examples of unusual planetary configurations, deepening our understanding of the diversity of planetary systems in our galaxy. Future observations, particularly with upcoming missions like the James Webb Space Telescope (JWST), could provide further insights into the atmospheres and compositions of similar high-density, short-period planets, shedding light on the processes that sculpt such extreme worlds.