Worth Revisiting: a young Neptune sits on a planetary traffic jam

Sureena Lakhanpal and collaborators confirm TOI-4129 b using HARPS-N radial velocities, TESS transits, and observations from four ground-based telescopes. The planet is about 5.7 Earth radii and 26 Earth masses, producing a low density near 0.8 grams per cubic centimeter while orbiting its K-dwarf host every 3.27 days.

That period places the system at the inner edge of the Neptune ridge, an excess of intermediate-size planets on three-to-six-day orbits whose origin remains uncertain. The host's rotation suggests an age near 1.2 billion years, and the planet's expected atmospheric signal makes it attractive for Webb and Ariel. Future atmospheric and spin-orbit measurements could test high-eccentricity migration; the present discovery does not establish that history.

Why it matters

A young, observable planet located at a demographic boundary can connect the final architecture of close-in systems to the migration processes that produced it.

What the source establishes

Worth Revisiting. The preprint was submitted November 2 and resubmitted to The Astronomical Journal. Planet mass, radius, density, and orbit are measured; age and migration history remain model-dependent.