We apply for the first time the Transit Least-Squares ( TLS ) algorithm to search for new transiting exoplanets . TLS has been developed as a successor to the Box Least-Squares ( BLS ) algorithm , which has served as a standard tool for the detection of periodic transits .
In this proof-of-concept paper , we demonstrate how TLS finds small planets that have previously been missed .
We showcase TLS ’ s capabilities using the K2 EVEREST -detrended light curve of the star K2-32 ( EPIC 205071984 ) , which has been known to have three transiting planets . TLS detects these known Neptune-sized planets K2-32 b , d , and c in an iterative search and finds an additional transit signal with a high signal detection efficiency ( SDE _ { TLS } ) of 26.1 at a period of 4.34882 _ { -0.00075 } ^ { +0.00069 } d. We show that this additional signal remains detectable ( SDE _ { TLS } = 13.2 ) with TLS in the K2SFF light curve of K2-32 , which includes a less optimal detrending of the systematic trends .
The signal is below common detection thresholds , however , if searched with BLS in the K2SFF light curve ( SDE _ { BLS } = 8.9 ) as in previous searches .
Markov Chain Monte Carlo sampling with the emcee software shows that the radius of this candidate is { 1.01 } _ { -0.09 } ^ { +0.10 } R _ { \oplus } .
We analyze its phase-folded transit light curve using the vespa software and calculate a false positive probability { FPP } = 3.1 ~ { } \times~ { } 10 ^ { -3 } .
Taking into account the multiplicity boost of the system , we estimate an { FPP } < 3.1 ~ { } \times~ { } 10 ^ { -4 } , which formally validates K2-32 e as a planet .
K2-32 now hosts at least four planets that are very close to a 1:2:5:7 mean motion resonance chain .
The offset of the orbital periods of K2-32 e and b from a 1:2 mean motion resonance is in very good agreement with the sample of transiting multiplanet systems from Kepler , lending further credence to the planetary nature of K2-32 e. We expect that TLS can find many more transits of Earth-sized and even smaller planets in the Kepler and K2 data that have hitherto remained undetected with algorithms that search for box-like signals .