Lead-like Exoplanet May Be Core Remnant of a Giant Planet That Orbit Too Close to Its Star
Named K2-360 b, this exoplanet has a mass 7.7 times that of Earth, while its size is only 1.6 times larger than our planet. This exoplanet has a density of about 11 g/cm³, which means it is as heavy as lead and twice the density of Earth.
Among the super-Earths with extremely short orbital periods, K2-360 b is the most densely packed planet.
Close-up illustration of K2-360 b (in red) orbiting its star (Image: Astrobiology Center).
The orbital period of a planet is what we often refer to as a year, or the time it takes for a planet to complete one full orbit around its star. K2-360 b is nicknamed “ultra-short” because its year is only 21 hours on Earth.
Due to its proximity to its star, this exoplanet has been easily detected by astronomers, leading to discoveries regarding its unusual density.
In 2016, when NASA’s K2 project discovered the shadow of this planet transiting its star, scientists immediately investigated and uncovered that this was an entirely new exoplanet.
Further research revealed the measurements of its mass and radius, allowing scientists to calculate the density of K2-360 b. In contrast, some exoplanets have a density as low as 0.03 g/cm³, which could be imagined as being light and fluffy like cotton candy.
To determine what makes K2-360 b so solid, the research team created a model simulating the core of this super-Earth, based on observations of it and its host star.
The model indicates that this planet has a large iron core that constitutes about 48% of its mass. K2-360 b may represent the dead core of a world that was once much larger and located farther from its star.
Over time, it migrated closer, where intense radiation stripped away the gases in its atmosphere, leaving behind a solid rock mass that could have been covered in lava.
Another discovery reinforced the team’s conclusions. They found that farther away from K2-360 b is a much larger planet, named K2-360 c, which has a size and density similar to that of Neptune.
Dynamical models suggest that due to an eccentric migration, K2-360 c may have pushed the smaller planet K2-360 b into its current close orbit around the star.
The gravitational interaction between these celestial bodies causes K2-360 b’s orbit to be elliptical, followed by tidal forces that bring it spiraling closer to the star.
The findings of this research provide further evidence that the universe is home to many strange planets that seem to exist only in science fiction.
