Constrain the orbital eccentricity of TOI-426 c

Constrain whether the long-period giant planet TOI-426 c has an intrinsically eccentric or circular orbit through additional radial-velocity observations.

Background

TOI-426 c is identified as a long-period giant companion to the highly irradiated sub-Neptune TOI-426 b. The adopted solution assumes a circular orbit because the eccentric model is only slightly disfavoured and does not improve the fit sufficiently, but sparse sampling leaves substantial covariance among the orbital period, periastron parameters, and eccentricity.

The orbital eccentricities of comparable outer giant companions are often moderate to high, making the eccentricity of TOI-426 c relevant to assessing whether the system underwent high-eccentricity tidal migration. The available radial velocities do not distinguish the competing orbital interpretations.

References

Whether TOI-426~c shares this characteristic remains uncertain.

— Two extremely irradiated volatile-rich sub-Neptunes with companions in the TOI-426 and TOI-1839 systems: Insights into arrival and survival near the lower edge of the Neptunian desert  (2609.05413 - Castro-González et al., 4 Sep 2026) in Section 4.3, Discussion, subsection “Outer giant companions to sub-Neptunes near the edge of the Neptunian desert”