Stellar Activity and Exclusion of the Outer Planet in the HD 99492 System

TL;DR

This study updates the HD 99492 system analysis, revealing that stellar activity cycles can mimic planetary signals, leading to the exclusion of the outer planet hypothesis based on new radial velocity and photometric data.

Contribution

The paper demonstrates that stellar activity cycles can produce false planetary signals, providing revised orbital parameters and emphasizing the importance of activity analysis in exoplanet detection.

Findings

The host star has a ~13-year activity cycle.

The activity cycle correlates with the outer planet's purported orbital period.

The outer planet is likely not present, as activity mimics its signal.

Abstract

A historical problem for indirect exoplanet detection has been contending with the intrinsic variability of the host star. If the variability is periodic, it can easily mimic various exoplanet signatures, such as radial velocity variations that originate with the stellar surface rather than the presence of a planet. Here we present an update for the HD~99492 planetary system, using new radial velocity and photometric measurements from the Transit Ephemeris Refinement and Monitoring Survey (TERMS). Our extended time series and subsequent analyses of the Ca II H\&K emission lines show that the host star has an activity cycle of $\sim$13 years. The activity cycle correlates with the purported orbital period of the outer planet, the signature of which is thus likely due to the host star activity. We further include a revised Keplerian orbital solution for the remaining planet, along with a new transit ephemeris. Our transit-search observations were inconclusive.