Direct Imaging in the Habitable Zone and the Problem of Orbital Motion

TL;DR

This paper discusses the challenge of orbital motion in direct imaging of habitable zone exoplanets, especially with next-generation large telescopes, and proposes methods to mitigate its effects on detection sensitivity.

Contribution

It identifies orbital motion as a significant factor affecting direct imaging of habitable zone planets with large telescopes and provides analytical tools and strategies to correct for it.

Findings

Orbital motion limits signal-to-noise ratio in long exposures.

De-orbiting shorter exposures can mitigate orbital motion effects.

Orbital motion correction is essential for detecting planets around nearby stars.

Abstract

High contrast imaging searches for exoplanets have been conducted on 2.4-10 m telescopes, typically at H band (1.6 microns) and used exposure times of ~1 hr to search for planets with semi-major axes of > ~10 AU. We are beginning to plan for surveys using extreme-AO systems on the next generation of 30-meter class telescopes, where we hope to begin probing the habitable zones (HZs) of nearby stars. Here we highlight a heretofore ignorable problem in direct imaging: planets orbit their stars. Under the parameters of current surveys, orbital motion is negligible over the duration of a typical observation. However, this motion is not negligible when using large diameter telescopes to observe at relatively close stellar distances (1-10pc), over the long exposure times (10-20 hrs) necessary for direct detection of older planets in the HZ. We show that this motion will limit our achievable signal-to-noise ratio and degrade observational completeness. Even on current 8m class telescopes, orbital motion will need to be accounted for in an attempt to detect HZ planets around the nearest sun-like stars alpha Cen A & B, a binary system now known to harbor at least one planet. Here we derive some basic tools for analyzing this problem, and ultimately show that the prospects are good for de-orbiting a series of shorter exposures to correct for orbital motion.