A Hot Super-Neptune on the Edge: Unveiling TOI-5795 b

In their latest paper, Manni et al. present the discovery and detailed analysis of TOI-5795 b, a super-Neptune planet found orbiting a metal-poor star. This study is part of the Hot Neptune Initiative (HONEI), a project aiming to uncover the mysteries of Neptune-sized exoplanets that reside unusually close to their host stars, regions traditionally sparse in such planets and known as the "Neptune desert." The paper follows a thorough investigation into how TOI-5795 b formed, its current state, and the role that stellar radiation may have played in shaping its atmosphere.

From Candidate to Confirmed Planet

TOI-5795 b was first identified by NASA’s TESS mission, which scans the sky looking for periodic dips in starlight that might indicate a planet transiting in front of its star. TESS observations showed regular dips every 6.14 days, suggesting a possible Neptune-sized planet. To confirm this, the team used precise radial velocity measurements from the HARPS spectrograph, which allowed them to detect the gravitational pull of the planet on its host star. Additional high-resolution imaging ruled out nearby stellar companions that might have produced a false positive. Their combined observations confirmed TOI-5795 b as a true planet, 23.7 times the mass of Earth and 5.6 times its radius, placing it firmly in the category of a hot super-Neptune.

Meet the Host Star

The planet orbits a G-type star (similar to our Sun in temperature) located about 162 light-years away. What makes this star particularly interesting is its metal-poor nature ([Fe/H] ≈ −0.27), meaning it has fewer elements heavier than hydrogen and helium compared to the Sun. This is unusual for stars that host gas-rich planets, which tend to be more metal-rich. The star is also quite old, around 10 billion years, and shows little stellar activity, which makes it ideal for precision measurements and long-term monitoring.

Physical Properties and Orbital Details

TOI-5795 b’s short orbital period places it at the edge of a region astronomers call the "Neptune ridge," a transition zone between the dry "desert" and a more populated "savanna" of Neptune-sized planets at longer periods. The planet’s density is low, like Saturn, suggesting a thick atmosphere. Its relatively high transmission spectroscopy metric (TSM ≈ 100) makes it an exciting target for future atmospheric studies with telescopes like JWST.

A Formation Puzzle

To understand how TOI-5795 b formed, the team ran thousands of computer simulations using planet formation models based on pebble accretion, a process where small particles gradually grow into larger planets. Surprisingly, nearly none of the simulations could reproduce a planet with TOI-5795 b’s exact mass and orbit. Even when varying the disk properties and pebble sizes, only a handful of simulations came close. This suggests that the planet may not have formed in its current position or size through smooth growth processes. Instead, it could have migrated inward or formed through dramatic events like planet-planet collisions.

A History of Evaporation

The paper also investigates the history of the planet’s atmosphere. Close-in planets like TOI-5795 b receive intense ultraviolet and X-ray radiation from their host stars, especially when the stars are young and active. This radiation can strip away a planet’s atmosphere over time. The authors modeled how much mass TOI-5795 b might have lost through this process. They found that, depending on the strength of the stellar radiation, the planet could have originally been up to 33 Earth masses and has likely lost 4 to 9 Earth masses over billions of years. Its radius has also shrunk significantly due to gravitational contraction and atmospheric loss.

Final Thoughts

TOI-5795 b stands as a compelling example of a planet that challenges our understanding of how Neptune-like worlds form and evolve. Its location at the boundary of the Neptune desert, combined with its low density and metal-poor host star, makes it a rare find. The planet’s survival, despite billions of years of high-energy irradiation, offers a unique opportunity to explore the atmospheric and structural evolution of exoplanets under extreme conditions. As part of the broader HONEI effort, TOI-5795 b joins a growing catalog of exotic Neptunes helping astronomers piece together the diverse architectures of planetary systems.

Source: Manni