TOI-4129 b sits at the inner edge of the Neptune ridge

TOI-4129 b is a low-density planet about 5.7 times Earth’s radius, orbiting its star every 3.27 days. Its orbit places it at the inner edge of the Neptune ridge, a concentration of intermediate-size planets with short orbital periods. The system’s estimated age and the planet’s potential for atmospheric study make it a useful target for examining how such planets reached their present orbits.

Star system: TOI 4129 | Distance from the Sun: 427 light years

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TOI-4129 b sits at the inner edge of the Neptune ridge

Star system: TOI 4129 | Distance from the Sun: 427 light years

TOI-4129 b has a radius of 5.7 times Earth’s and a mass of about 26 Earth masses. Together, these measurements give it a density of 0.8 grams per cubic centimetre, relatively low compared with Earth. The planet completes an orbit around its orange dwarf star every 3.27 days, placing it at the inner edge of the Neptune ridge: a concentration of intermediate-size planets that orbit their stars every three to six days.

The planet’s size and mass were measured using two methods. Transit observations track the slight dip in a star’s brightness as a planet passes in front of it. Radial velocity measurements track small shifts in the star’s motion caused by the planet’s gravity. The transit data came from the Transiting Exoplanet Survey Satellite and four ground-based telescopes, while the radial velocity data came from the HARPS-N instrument.

The star’s brightness also varies with a 12.7-day rotation period. That rotation rate points to a relatively young age for the system, estimated at about 1.2 billion years. TOI-4129 b’s Transmission Spectroscopy Metric is 112 ± 30. This metric estimates how suitable a planet is for studying its atmosphere as starlight passes through it, making the planet a candidate for observations with the James Webb Space Telescope and Ariel. Its system may also allow future measurements of stellar obliquity, the angle between the star’s rotation axis and the planet’s orbital plane. These observations could help test whether high-eccentricity migration, in which a planet’s orbit becomes stretched before settling closer to its star, explains how planets in the Neptune ridge reached their current locations.

Key findings
TOI-4129 b measures 5.7 ± 0.2 Earth radii and has a mass of 26 ± 6 Earth masses.
Its density is 0.8 ± 0.2 grams per cubic centimetre, and it orbits its star every 3.27 days.
The star’s 12.7-day rotation period indicates an estimated system age of 1.20 billion years, with an uncertainty of +0.16 and -0.30 billion years.
The planet is a potential target for atmospheric observations with the James Webb Space Telescope and Ariel, and for future measurements of the star’s obliquity.
Its position in the Neptune ridge makes the system a test case for the idea that high-eccentricity migration brought these planets into close orbits.

Scientific study

The Hot Neptune Initiative (HONEI): IV. TOI-4129 b, an Adolescent Neptune in the Ridge