Aliasing in the Radial Velocities of YZ Ceti: An Ultra-Short Period for YZ Ceti c?

TL;DR

This study reanalyzes radial velocity data of YZ Ceti, suggesting that what was thought to be a 3.06-day period for one planet may actually be an alias, implying a much shorter true orbital period of 0.75 days.

Contribution

It reveals the potential aliasing issue in the orbital periods of YZ Ceti's planets, proposing a revised period for YZ Ceti c and highlighting the need for further observations.

Findings

The 3.06-day period for YZ Ceti c may be an alias.

The true period of YZ Ceti c could be 0.75 days.

Revised minimum mass of YZ Ceti c is less than 0.6 Earth masses.

Abstract

Mid-late M stars are opportunistic targets for the study of low-mass exoplanets in transit because of the high planet-to-star radius ratios of their planets. Recent studies of such stars have shown that, like their early-M counterparts, they often host multi-resonant networks of small planets. Here, we reanalyze radial velocity measurements of YZ Ceti, an active M4 dwarf for which the HARPS exoplanet survey recently discovered three exoplanets on short-period (P = 4.66, 3.06, 1.97 days) orbits. Our analysis finds that the orbital periods of the inner two planets cannot be uniquely determined using the published HARPS velocities. In particular, it appears likely that the 3.06-day period of YZ Ceti c is an alias, and that its true period is 0.75 days. If so, the revised minimum mass of this planet is less than 0.6 Earth masses, and its geometric transit probability increases to 10%. We encourage additional observations to determine the true periods of YZ Ceti b and c, and suggest a search for transits at the 0.75-day period in TESS lightcurves.