Transit Least Squares: Optimized transit detection algorithm to search for periodic transits of small planets

Abstract

We present a new method to detect planetary transits from time-series photometry, the Transit Least Squares (TLS) algorithm. TLS searches for transit-like features while taking the stellar limb darkening and planetary ingress and egress into account. We have optimized TLS for both signal detection efficiency (SDE) of small planets and computational speed. TLS analyses the entire, unbinned phase-folded light curve. We compensate for the higher computational load by (i.) using algorithms like "Mergesort" (for the trial orbital phases) and by (ii.) restricting the trial transit durations to a smaller range that encompasses all known planets, and using stellar density priors where available. A typical K2 light curve, including 80d of observations at a cadence of 30min, can be searched with TLS in ~10s real time on a standard laptop computer, as fast as the widely used Box Least Squares (BLS) algorithm. We perform a transit injection-retrieval experiment of Earth-sized planets around sun-like stars using synthetic light curves with 110ppm white noise per 30min cadence, corresponding to a photometrically quiet KP=12 star observed with Kepler. We determine the SDE thresholds for both BLS and TLS to reach a false positive rate of 1% to be SDE~7 in both cases. The resulting true positive (or recovery) rates are ~93% for TLS and ~76% for BLS, implying more reliable detections with TLS. We also test TLS with the K2 light curve of the TRAPPIST-1 system and find six of seven Earth-sized planets using an iterative search for increasingly lower signal detection efficiency, the phase-folded transit of the seventh planet being affected by a stellar flare. TLS is more reliable than BLS in finding any kind of transiting planet but it is particularly suited for the detection of small planets in long time series from Kepler, TESS, and PLATO. We make our Python implementation of TLS publicly available.

A&A accepted. Code, documentation and tutorials at https://github.com/hippke/tls

References in corpus (19)

• The NumPy array: a structure for efficient numerical computation
• The K2 Mission: Characterization and Early results
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• The effect of red noise on planetary transit detection
• Kepler Flares II: The Temporal Morphology of White-Light Flares on GJ 1243
• A Study of the Shortest-Period Planets Found With Kepler
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• A fast hybrid algorithm for exoplanetary transit searches
• Searching for Exoplanets Using Artificial Intelligence
• Precise time-series photometry for the Kepler-2.0 mission
• Likely Transiting Exocomets Detected by Kepler
• Predicting Photometric and Spectroscopic Signatures of Rings around Transiting Extrasolar Planets
• A study of the performance of the transit detection tool DST in space-based surveys. Application of the CoRoT pipeline to Kepler data
• Direct evidence for an evolving dust cloud from the exoplanet KIC 12557548 b
• Analytic solutions to the maximum and average exoplanet transit depth for common stellar limb darkening laws
• Efficient analysis in planet transit surveys
• The BAST algorithm for transit detection
• Out-of-Transit Refracted Light in the Atmospheres of Transiting and Non-Transiting Exoplanets
• Three small transiting planets around the M dwarf host star LP 358-499

Cited by in corpus (5)

• AutoRegressive Planet Search: Application to the Kepler Mission
• Transit least-squares survey -- II. Discovery and validation of 17 new sub- to super-Earth-sized planets in multi-planet systems from K2
• Analytic solutions to the maximum and average exoplanet transit depth for common stellar limb darkening laws
• Transit least-squares survey - I. Discovery and validation of an Earth-sized planet in the four-planet system K2-32 near the 1:2:5:7 resonance
• Four Small Planets Buried in K2 Systems: What Can We Learn for TESS?