“Super-Earth” in planetary system shedding new light on planetary evolution
image: The black star represents the position of the star. The colored lines are the orbits of two planets. The yellow one is the outer, and it is highly inclined. The inner one is the red one. As time passes, the orbit becomes more and more stretched because of the influence of the outer one, and then suddenly migrates inward to a much smaller orbit around the star.
Credit: Alessandro Alberto Trani
An international team led by researchers in Japan and Europe has discovered a new multi-planet system around a Sun-like star, including an ultra-short period planet with one of the highest densities ever measured. The findings, published in Nature Scientific Reports, shed new light on the formation and evolution of planets in extreme environments.
The results could act as a toolbox for determining evolution of planetary systems more precisely in the future, through using the models applied here on new planetary discoveries.
The newly discovered system, named K2-360, is located approximately 750 light-years from Earth. It consists of two planets orbiting a star similar to our Sun:
- K2-360 b: An ultra-short period "super-Earth" - a rocky planet larger than Earth but smaller than Neptune - about 1.6 times the size of Earth, orbiting its star every 21 hours. With a mass 7.7 times that of Earth, it is the densest well-characterized planet of its kind discovered to date.
- K2-360 c: A larger outer planet at least 15 times more massive than Earth, orbiting every 9.8 days. This planet does not transit its star, so its exact size is unknown.
A super-Earth that lost its mantel
The extreme density of K2-360 b suggests it may be the stripped core of a once-larger planet, having lost its outer layers due to intense radiation from its nearby host star.
"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," explains co-author Davide Gandolfi from the University of Turin.
The key to understanding the system is the interplay between the two planets
"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," says co-author Alessandro Alberto Trani, postdoc at the Niels Bohr Institute.
"This involves gravitational interactions that first make the inner planet's orbit very elliptical, before tidal forces gradually circularize it close to the star. Alternatively, tidal circularization could have been induced by the spin-axial tilt of the planet."
Planetary system history writing starts with a prediction and ends in the present
Alessandro Trani was involved in creating the dynamical models helping explain the evolution of the planetary system: “When our goal is to understand the system’s origin, we have to assess its initial configuration and then see which setups naturally evolve into what we observe now.
Because not all initial conditions will reproduce the present-day system, we run many simulations across a broad parameter space. Our knowledge of the key evolutionary mechanism (in this case, tidal migration) guides which initial conditions we choose to explore.
Finally, we compare the simulation outputs (e.g., each planet’s orbital parameters, mass, and evolution over time) with observational data from radial velocity and transit methods. If the simulated system matches the actual measurements and remains stable, it bolsters our understanding that these initial conditions and physical processes accurately describe the real planetary system.
In this way, we can not only put a constraint on the parameters that we cannot measure, but also assemble a complete narrative: not just what the planets are “right now,” but how they likely got there over time.
The planetary system can be viewed as a toolbox
This planetary system is an exceptional laboratory for a few reasons. First, we can observe K2-360b through both transit measurements, where measurements are made from the planet’s passing in front of the star to determine its size and orbital period. Radial-velocity data – the pull of gravity between planet and star – can be made to determine its mass.
These observations let us calculate the planet’s density—and by extension, its composition. It turns out K2-360b is this extremely dense super-Earth, rich in iron, suggesting it may once have had a thick, gaseous atmosphere that was stripped away by tidal forces and stellar radiation. What we see now could be the planet’s exposed core.
Second, the presence of an outer planet, K2-360c, provides crucial clues about multiple formation and evolution pathways within the same system. We can test different migration scenarios and see whether they match our observations. Without evidence for K2-360c, we could only guess at how K2-360b ended up so close to its star, and its migration history would remain largely mysterious.
Development of new, even more detailed modelling tools is the way forward
From a theoretical modeling perspective, our results show that we need even more sophisticated tidal-migration models. For example, incorporating photoevaporation—how intense starlight can strip away a planet’s atmosphere—into our simulations would allow us to track changes in a planet’s mass and radius over time. By merging these effects (tides + UV radiation) into a single framework, we can gain a clearer, more precise view of how planets like K2-360b evolve. And applying these enhanced models to new planetary discoveries could reveal whether what we’ve seen here is common or a cosmic rarity.