TOI-6434 b — a hot Neptune on the edge of a desert

TOI-6434 b is a Neptune-like exoplanet confirmed in 2026, orbiting a star 417.5 light-years from Earth. Its radius of 29,403 km — 4.62 times Earth’s radius and 0.41127736 times Jupiter’s radius — and its mass of 19.2 Earth masses place it squarely between the super-Earth and gas-giant classes, in the same size neighbourhood as Neptune. What makes it notable is where it sits: a circular orbit just 0.033 AU from its host star, completed in 2.8 days — a configuration that places it inside the region of orbital parameter space where planetary theory predicts most Neptune-sized worlds should not survive.

Key facts

Mass: 19.2 Earth masses (19.2× Earth)
Body type: exoplanet
Host star: TOI-6434
Radius: 29,403 km (4.62× Earth)
Density: 1,080 kg/m³ (0.2× Earth)
Temperature: 832 K (559 °C)
Orbital period: 2.8 days
Orbital eccentricity: 0
Distance: 417.5 light-years
Insolation: 108,618 W/m²
Discovered: 2026
Discovery method: Transit

Physical characteristics

TOI-6434 b has a radius of 29,403 km — roughly 4.62 times Earth’s radius — and a mass of 19.2 Earth masses. Together those two numbers produce a mean bulk density of 1,080 kg/m³, which is 0.2 times Earth’s mean density. That low density is consistent with a substantial gas or ice envelope surrounding a denser core.

Its orbital eccentricity is measured at 0.0, meaning the orbit is circular to within measurement precision. The planet holds a fixed distance of approximately 0.033 AU from its host star TOI-6434 throughout its 2.8-day year. At that proximity, the temperature at the planet’s dayside reaches an estimated 832 K (559 °C). The insolation — the flux of stellar energy striking each square metre of the planet’s outer atmosphere — is recorded at 108,618 W/m², placing TOI-6434 b in an extreme irradiation regime.

With a radius 0.41127736 times that of Jupiter, TOI-6434 b sits above the super-Earth range but well below the gas-giant threshold. It belongs to the Neptune-like size class: worlds that are too large to be purely rocky, yet too small and insufficiently massive to have accreted the thick hydrogen–helium envelopes that define Jovian planets.

Discovery

TOI-6434 b was found via the transit method: each time the planet crosses the face of its host star as seen from Earth, it dims the starlight by a measurable fraction. The TESS spacecraft — launched in April 2018 as a Medium-Explorer-class (MIDEX) mission to survey the sky for transiting planets around the brightest nearby stars — is capable of detecting transit depth signals as small as 3 × 10⁻⁴ of the parent star’s brightness, which is sufficient to catch Neptune-sized planets passing Sun-like stars.

The host star TOI-6434 has a TESS-band apparent magnitude of 7.1462 ± 0.0060 and a visual magnitude of 7.72 ± 0.03. Both values place the star in the brightest and most observationally favourable range for TESS transit detection. In the TOI naming system, the integer portion of the identifier designates the target star; the decimal portion identifies a specific transiting object associated with it.

The planet’s confirmation was announced on October 8, 2026, as part of a batch of 70 newly confirmed TESS planets added to the NASA Exoplanet Archive on that date. That addition brought the archive’s total confirmed planet count from all missions to 6,445. TOI-6434 b was one of ten planets specifically highlighted in that milestone announcement alongside TOI-6450 b, TOI-6647 b, TOI-6662 b, TOI-6729 b and c, TOI-7182 b, TOI-7389 b, TOI-7391 b, and TOI-7550 b. As of October 8, 2026, nearly 5,000 additional TESS candidate planets remained in the Exoplanet Follow-up Observing Program (ExoFOP), operated by the NASA Exoplanet Science Institute (NExScI) within IPAC at Caltech, awaiting confirmation.

The Neptunian desert and why TOI-6434 b is unusual

The Neptunian desert is defined as a pronounced deficit of exoplanets with masses between Neptune and Saturn on orbital periods shorter than a few days. The scarcity is not an artefact of detection bias: surveys have confirmed many planets with similar masses and radii to Neptune on much longer periods, which are geometrically harder to detect via transits. The deficit at short periods is therefore real, and TOI-6434 b, with its 2.8-day orbit, sits inside or at the inner boundary of this depleted zone.

The leading physical explanation is that at such short separations, intense X-ray and extreme-ultraviolet (XUV) irradiation drives rapid photoevaporation of a planet’s atmosphere. Neptune-mass worlds have lower gravitational potentials than their Jovian-mass counterparts, so they cannot hold on to their envelopes against the same irradiation flux that a Jupiter-mass planet would easily withstand. Roche-lobe overflow — where a planet’s atmosphere spills into the gravitational domain of the star — can accelerate envelope loss further. The combined effect is thought to deplete the hot-Neptune population over timescales much shorter than stellar ages.

Researchers have also proposed that the desert’s upper boundary is sculpted by a different process: high-eccentricity migration followed by tidal disruption during close pericenter passages, where a planet temporarily overflows its Roche lobe. The lower boundary, by contrast, is more consistently attributed to photoevaporation. These two mechanisms operating at different mass regimes may together carve the observed shape of the desert.

Sitting adjacent to the desert are two related structures in orbital-period space. The Neptunian ridge is an overdensity of Neptune-sized planets at periods between roughly 3.2 and 5.7 days, identified by an international team including researchers from the University of Geneva using bias-corrected Kepler data published in Astronomy and Astrophysics. Beyond 5.7 days lies the Neptunian savanna, where Neptune-sized planets become common again. TOI-6434 b’s 2.8-day period places it just inside the desert boundary, shorter than the 3.2-day inner edge of the ridge.

Atmospheric prospects

For hot Neptune-like planets, published research shows that atmospheric compositions could range from hydrogen-dominated ice-giant envelopes to more exotic mixtures, depending on the planet’s mass, effective temperature, formation history, and irradiation environment. TOI-6434 b’s temperature of 832 K (559 °C) and its insolation of 108,618 W/m² place it in a regime where atmospheric evolution is expected to be ongoing and rapid by planetary standards.

One outcome proposed in the literature for Neptune-sized planets at very short orbital periods is the formation of helium-dominated atmospheres. Because hydrogen escapes faster than helium through hydrodynamic outflow — a process in which the upper atmosphere effectively flows away from the planet — a world that began hydrogen-rich could gradually shift toward a helium-dominated composition. Such a state has no counterpart among the ice giants of the Solar System. Whether TOI-6434 b is currently losing its envelope, has already lost most of it, or is somehow retaining it remains an open question.

TESS and the wider survey context

TESS, the Transiting Exoplanet Survey Satellite, launched in April 2018 and is classed as a Medium-Explorer (MIDEX) mission.

A paper published in The Astrophysical Journal on April 28, 2026, based on analysis led by a Princeton University team, identified over 10,000 new exoplanet candidates from a single sweep of TESS data, illustrating the scale of the pipeline feeding future confirmation campaigns. As of October 8, 2026, nearly 5,000 TESS candidates were in ExoFOP awaiting confirmation. The newly confirmed planets from the October 8, 2026 batch — including TOI-6434 b — are cited as targets for further study by the James Webb Space Telescope, which can probe atmospheric composition at a level of detail TESS itself cannot provide.

Did you know

As of October 8, 2026, almost 5,000 more TESS planet candidates remained in the Exoplanet Follow-up Observing Program awaiting confirmation. At the 1,000-confirmed-planet milestone that the October 8 batch helped mark, that ratio meant roughly five additional candidates were still queued for every one planet that had been formally confirmed. (ipac.caltech.edu)
One outcome proposed in published research for Neptune-sized planets on very short orbits is a helium-dominated atmosphere. As hydrogen escapes faster than helium through hydrodynamic outflow, a planet that began with a hydrogen-rich envelope could end up with an atmosphere composed mainly of helium — a chemistry that has no equivalent among the planets and ice giants of the Solar System. (iopscience.iop.org)

Data sources

Data updated: 2026-10-26