First Doppler Limits on Binary Planets and Exomoons in the HR 8799 System

Andrew Vanderburg,Joseph E Rodriguez

doi:10.3847/2041-8213/ac33b4

Abstract

Abstract We place the first constraints on binary planets and exomoons from Doppler monitoring of directly imaged exoplanets. We model radial velocity observations of HR 8799 b, c, and d from Ruffio et al. and determine upper limits on the m sin i of short-period binary planets and satellites. At 95% confidence, we rule out companions orbiting the three planets more massive than m sin i = 2 M J with orbital periods shorter than 5 days. We achieve our tightest constraints on moons orbiting HR 8799c, where with 95% confidence we rule out out edge-on Jupiter-mass companions in periods shorter than 5 days and edge-on half-Jupiter-mass moons in periods shorter than 1 day. These radial velocity observations come from spectra with resolution 20 times lower than typical radial velocity instruments and were taken using a spectrograph that was designed before the first directly imaged exoplanet was discovered. Similar data sets from new and upcoming instruments will probe significantly lower exomoon masses.

Highlights

Three decades of exoplanet discovery have resulted in the detection of over 4500 planets outside the solar system1 (Akeson et al 2013) and the knowledge that planets outnumber stars in our galaxy (e.g. Swift et al 2013; Dressing & Charbonneau 2015)
At 95% confidence, we rule out companions orbiting the three planets more massive than m sin i = 2 MJ with orbital periods shorter than 5 days
We achieve our tightest constraints on moons orbiting HR 8799 c, where with 95% confidence we rule out out edge-on Jupiter-mass companions in periods shorter than 5 days and edge-on half-Jupiter-mass moons in periods shorter than 1 day. These radial velocity observations come from spectra with resolution 20 times lower than typical radial velocity instruments and were taken using a spectrograph that was designed before the first directly imaged exoplanet was discovered

Summary

INTRODUCTION

Three decades of exoplanet discovery have resulted in the detection of over 4500 planets outside the solar system (Akeson et al 2013) and the knowledge that planets outnumber stars in our galaxy (e.g. Swift et al 2013; Dressing & Charbonneau 2015). We place the first limits on exomoons and binary planets from radial velocity measurements of directly imaged exoplanets. This method has been mentioned occasionally in literature since 2005 (Hook 2005; Cabrera & Schneider 2007; Heller 2016; LilloBox et al 2018) and was recently discussed in detail by Vanderburg et al (2018) who showed that existing and forthcoming instruments could yield strong constraints on moons orbiting bright directly imaged planets. We show that these radial velocity observations are sensitive to massive exomoons in short-period orbits and use the time series to place upper limits on the presence of exomoon and binary companions to the HR 8799 planets. This Letter is organized as follows: Section 2 describes the observations and our radial velocity analysis, Section 3 describes the constraints we are able to place, and Section 4 discusses the implications of our upper limits and the prospects for improved constraints or exomoon detections with future datasets

Keck OSIRIS Radial Velocities

MCMC Radial Velocity Analysis

RESULTS

DISCUSSION