Formation pathway of GJ 523b is undetermined

Determine the formation mechanism of the exoplanet GJ 523b, a 2.55±0.15 R_⊕, 23.5±3.3 M_⊕, high-density (7.8±1.8 g cm⁻³), 170 Myr-old planet on a highly inclined (minimum obliquity ≥71.4°) 17.75-day orbit around the mid-K dwarf GJ 523, given current observational constraints that indicate an apparent lack of a substantial H/He envelope and likely polar orbit.

Background

The authors measure GJ 523b’s mass, radius, bulk density, age, and a minimum orbital obliquity indicative of a likely polar orbit. These properties—particularly the combination of high mass, high density implying little gas envelope, youth, and strong misalignment—are difficult to reconcile with standard formation pathways for sub-Neptune–sized planets.

Several formation scenarios are discussed, including high-eccentricity migration with tidal stripping potentially driven by von Zeipel–Lidov–Kozai torques from an outer companion, misalignment induced by the protoplanetary disk (e.g., via secular resonances or disk misalignment), growth with delayed gas accretion via hybrid pebble–planetesimal accretion, and giant impacts that remove atmospheres. However, current data are insufficient to distinguish between these pathways.

Future observations that could help resolve the formation history include continued radial-velocity monitoring and Gaia DR4/DR5 astrometry to search for massive outer companions, and Rossiter–McLaughlin measurements to determine the sky-projected obliquity and thus fully constrain the true three-dimensional obliquity.