Bloated Hot Neptune TOI-2195 A b: Birth from a Jupiter
We present the discovery and characterization of TOI-2195 A b — a bloated hot Neptune with an orbital period of 4.16 days, a mass of 1.46 Neptune masses, and a radius of 0.79 Jupiter radii, orbiting an early K-star in a wide binary system with a companion at ~600 AU. Using the Magellan/PFS spectrograph, we detected the Rossiter–McLaughlin effect at ~2.6σ, indicating a nearly polar orbit with a projected obliquity λ = 109{+35/-53}°. Joint dynamical and structural modeling reproduces the observed system parameters. The planet likely originated as a cold Jupiter, which was driven onto a highly eccentric orbit via the eccentric Kozai–Lidov mechanism, triggered by the gravity of the stellar companion. At pericenter, it lost up to 90% of its mass through Roche lobe overflow, causing rapid tidal migration and radius inflation due to tidal heating. TOI-2195 A b serves as a test of planet migration theories, and modeling suggests that puffy hot Neptunes may be the stripped remnants of more massive Jupiters that survived mass loss during high-eccentricity migration.
Context
The mystery of intermediate-mass planet formation on short orbits is a key one in the science of exoplanets. The so-called 'Neptune desert' — a scarcity of planets with orbital periods under three days — and the adjacent 'Neptune ridge' (3–6 days) challenge migration theories. The discovery of TOI-2195 A b, a bloated hot Neptune on a polar orbit, sheds light on a possible evolutionary path linking hot Jupiters and Neptunes via high-eccentricity migration and mass loss.
Methods
The system was studied using photometry from the TESS space telescope, ground-based transit observations, and high-resolution spectroscopy with the Magellan/PFS and CHIRON telescopes. The transit method, first widely applied by William Borucki in the Kepler mission, determined the planet's radius, while Doppler spectroscopy measured its mass from the star's radial velocity variations. The key measurement was the Rossiter-McLaughlin (RM) effect, revealing a likely polar orbit orientation. To analyze the dynamical history, three-body numerical simulations were performed, accounting for tidal forces, mass loss via Roche lobe overflow, and the planet's internal structure evolution using MESA grids.
Results
Planet TOI-2195 A b — a new member of the hot Neptune class — orbits an early K-type star every 4.16 days, has a mass 1.46 times Neptune's (~25 M⊕) and a radius 0.79 Jupiter radii (8.9 R⊕). Its low density (0.2 g/cm³) points to a hydrogen-helium envelope inflated by tidal heating. The RM effect measurement at ~2.6σ yields an orbital inclination λ = 109+35/-53°, indicating a nearly polar orientation of the orbital plane relative to the star's spin axis. Evolutionary modeling shows: the planet may have started as a cold Jupiter with mass up to 0.9 MJ at ~6 AU. Interaction with a stellar companion projected at ~600 AU triggered the Eccentric Kozai-Lidov mechanism — high-eccentricity orbital oscillations, during which the planet overflowed its Roche lobe at pericenter, losing up to 90% of its original mass. The remnant Neptunian mass and large radius allowed rapid tidal migration to the current orbit. The present bloated radius is due to residual heat from tidal heating, which increased the planet's entropy, followed by slow cooling.
Implications
This work supports the hypothesis that many hot Neptunes are 'stripped' Jupiters that survived partial disruption during migration. Similarities in metallicity, multiplicity, and obliquity distributions between hot Neptunes and hot Jupiters point to a common formation scenario. Measuring orbital inclinations becomes critical for distinguishing migration channels. The possibility of mass loss without total destruction broadens the likely survival paths for giant planets.
Future development
Future observations with JWST will probe the atmosphere of TOI-2195 A b for water, methane, and other molecules, testing the partial envelope loss model. Measurements of the planet's cooling rate are also important — it can indicate the core content and entropy balance. More precise orbital inclination data from new spectrographs will refine the dynamical history. Statistics on many such systems will reconstruct the population picture of the transition from Jupiters to Neptunes.
Impact
This discovery will impact understanding of exoplanet populations in the Neptune ridge and desert, as well as models of tidal evolution in binary star systems. The results may be key for interpreting data from future PLATO and ARIEL missions.
Next steps
Next steps include high-precision photometric observations to refine the planet's parameters, search for possible transit timing variations, and model atmospheric erosion due to stellar radiation. It is important to conduct similar studies for other Neptune ridge candidates to confirm the universality of the mechanism.
Key open problems
The results address the fundamental problem of short-period planet formation, including the nature of the Neptune desert and ridge, and mass loss mechanisms during high-eccentricity migration. The work also contributes to understanding the role of binary companions in the dynamical evolution of planetary systems.
🎯 The radius of TOI-2195 A b is almost equal to Jupiter's, while its mass is only 1.5 times that of Neptune. If you could put this planet in water, it would float like a giant balloon (density 0.2 g/cm³, less than water's density).
Key numbers
- Orbital period: 4.16 days
- Planet mass: 25.1 Earth masses
- Planet radius: 8.9 Earth radii
- Density: 0.2 g/cm³
- Orbital inclination: 109°