Ultra-hot super Neptune orbiting the subgiant star TOI-5646 is a desert survivor

Star system: TOI-5646 | Distance from the Sun: 1352 light years

A rare, ultra-hot super Neptune planet named TOI-5646b has been confirmed orbiting the subgiant star TOI-5646. This planet resides within the Neptune desert, a region where such planets are seldom found due to intense stellar radiation. TOI-5646 is notable for being the largest star discovered to date that hosts a Neptune-sized planet in this harsh environment, offering new insights into how planetary systems evolve and survive extreme conditions.

The planet orbits its star every 2.4 days and maintains an extremely high equilibrium temperature of approximately 1935 Kelvin. With a mass 45.9 times that of Earth and a radius 6.6 times larger, it is classified as a super Neptune. Despite its extreme environment, the planet exhibits a relatively low density of 0.87 grams per cubic centimeter.

To confirm the existence of the planet, radial velocity measurements were conducted using the HARPS-N instrument. This method detects the tiny gravitational tugs a planet exerts on its parent star, causing the star to wobble slightly. Subsequent structural modeling and simulations suggest that the planet is highly enriched in metals and has likely lost between 6 and 16 percent of its total mass due to intense X-ray and extreme ultraviolet radiation from its host.

These findings suggest that TOI-5646b has a unique formation history. Its current state may be the result of early disk dispersal, a past planetary collision, or a late-stage gravitational interaction that pushed it into its current, precarious orbit.

Key findings
- TOI-5646b is a super Neptune with a mass of 45.9 Earth masses and a radius of 6.6 Earth radii.
- The planet maintains an equilibrium temperature of 1935 Kelvin.
- TOI-5646 is the largest star known to host a Neptune-sized planet in the Neptune desert, measuring 1.8 solar radii.
- The planet has lost between 6 and 16 percent of its mass due to intense radiation from its host star.
- Internal structure models indicate the planet is highly enriched in metals.

Scientific study

The Hot Neptune Initiative (HONEI) III. An ultra-hot super Neptune orbiting the metal-rich subgiant star TOI-5646: the largest host for a desert dweller (external link)