Asymmetric helium outflow detected from the atmosphere of planet TOI-3884 b

Researchers have identified an evaporating atmosphere of helium escaping from the super-Neptune exoplanet TOI-3884 b. The planet, which orbits a Red dwarf star in a polar path, exhibits an unusual asymmetric outflow of gas that precedes the planet during its transit. This finding provides new insights into how stellar radiation drives the loss of planetary atmospheres and how magnetic fields interact with orbiting worlds.

Star system: TOI 3884 | Distance from the Sun: 141 light years

The super-Neptune exoplanet TOI-3884 b is losing parts of its atmosphere to space, creating a distinctive plume of helium that precedes the planet as it orbits its host star. This phenomenon, known as an evaporating exosphere, reveals complex interactions between the planet and the intense radiation emitted by its host star. Unlike many other exoplanets that display a trailing tail of escaping gas, TOI-3884 b features a pre-transit headwind, where gas is detected moving ahead of the planet's main body before it passes in front of the star.

Measurements were conducted by observing five separate transits of the planet using the Habitable Zone Planet Finder spectrometer. The team tracked a specific spectral line of helium at 10830 angstroms, which indicated a 6.8 percent excess in the transit depth. The outflow signatures were detected starting as early as five hours before the optical transit began, but no significant tail was observed afterward. This specific asymmetric pattern suggests that the mass loss is primarily driven by irradiation from the star rather than heat originating from the planet's core.

The unique observation of this pre-transit headwind allows for a deeper understanding of star-planet interactions, specifically the potential for sub-Alfvenic interactions, where magnetic fields and plasma influence the planet's environment. This data offers a rare opportunity to study planetary magnetic fields and the forces that shape the evolution of atmospheres on distant worlds. By analyzing the shape of the escaping helium, researchers can better constrain the theoretical models of how planets survive or lose their gaseous envelopes over time.

Key findings
TOI-3884 b exhibits an evaporating exosphere characterized by a helium outflow.
The observed helium shows an asymmetric pre-transit headwind instead of a post-transit tail.
A 6.8 percent excess transit depth was measured in the helium 10830 angstrom line.
The outflow morphology supports irradiation-driven mass loss as the primary mechanism.
The interaction likely involves complex magnetic or sub-Alfvenic processes between the star and the planet.

Scientific study

A Headwind-like Helium Outflow from TOI-3884b (external link)