[Paper Review] Expected Planet and False Positive Detection Rates for the Transiting Exoplanet Survey Satellite

Timothy M. Brown, David W. Latham|ArXiv.org|Dec 7, 2008

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This paper estimates the expected number of transiting exoplanets and astrophysical false positives detectable by the Transiting Exoplanet Survey Satellite (TESS), using simulations based on known exoplanet distributions and stellar population models. It quantifies ground-based follow-up observing requirements, projecting 1,400 telescope-nights for imaging, 300 for low-precision radial velocities, and 380 for high-precision radial velocity measurements to confirm and characterize planets, especially Super Earths.

ABSTRACT

The proposed Transiting Exoplanet Survey Satellite (TESS) will survey the entire sky to locate the nearest and brightest transiting extrasolar planets with orbital periods up to about 36 days. Here we estimate the number and kind of astrophysical false positives that TESS will report, along with the number of extrasolar planets. These estimates are then used to size the ground-based follow-up observing efforts needed to confirm and characterize the planets. We estimate that the needed observing resources will be about 1400 telescope-nights of imaging with 0.5m to 1m-class telescopes, 300 telescope-nights with 1m to 2m-class telescopes for the classification of the host stars and for radial velocity measurements with roughly 1 km/s precision, and 380 telescope-nights with 2m to 4m-class telescopes for radial velocity studies with precision of a few m/s. Follow-up spectroscopy of the smallest planets discovered by TESS at the best possible velocity precision will be limited by the number of telescope nights available on 4m to 10m class telescopes with instruments such as HARPS and HIRES, but the pay-off of such efforts will be the determination of masses for Super Earths with sufficient accuracy to distinguish rocky desert planets from water worlds.

Motivation & Objective

• To estimate the number and types of astrophysical false positives that TESS will detect, which can mimic planetary transits.
• To determine the ground-based follow-up observing resources required to distinguish true planets from false positives.
• To project the telescope time needed for radial velocity measurements to determine planetary masses, especially for Super Earths.
• To assess the impact of pre-selecting target stars (e.g., removing giants) on reducing follow-up observing costs.
• To evaluate the feasibility of measuring masses of small planets with high-precision spectroscopy and the implications for characterizing planetary composition.

Proposed method

• Adapted a simulation code from Brown (2003) to model line-of-sight triple star systems and their transit-like light curves.
• Modified the code to account for TESS’s sensitivity, observing strategy, and the full Galactic distribution of stars.
• Used observed period and radius distributions of known exoplanets to calibrate the simulation of true planetary transits.
• Modeled false positive sources including grazing eclipsing binaries, eclipsing binaries with giant stars, and diluted eclipsing binaries with bright third stars.
• Estimated observing time requirements for imaging, low-precision radial velocities, and high-precision radial velocity measurements based on expected detection rates.
• Assessed the potential time savings from pre-selecting target stars by removing giant stars before transit search.

Experimental results

Research questions

• RQ1 How many transiting exoplanets and false positives can TESS expect to detect over a 2-year sky survey?
• RQ2 What is the expected distribution of false positive types, and how do they differ from planetary transits in depth, duration, and orbital period?
• RQ3 How much ground-based follow-up observing time is required to confirm and characterize TESS-discovered planets?
• RQ4 To what extent can pre-selection of target stars (e.g., removing giants) reduce the total follow-up observing effort?
• RQ5 What is the achievable yield of mass measurements for Super Earths using high-precision radial velocity spectroscopy?

Key findings

• TESS is expected to detect approximately 1,400 transiting exoplanets, with a significant fraction of false positives due to astrophysical phenomena such as eclipsing binaries and line-of-sight triples.
• The total ground-based follow-up observing effort is estimated at 1,400 telescope-nights for imaging with 0.5m–1m telescopes, 300 telescope-nights for low-precision radial velocities (1 km s⁻¹), and 380 telescope-nights for high-precision radial velocities (a few m s⁻¹).
• Pre-selection of target stars to remove giants could reduce imaging follow-up by about 5% and low-precision radial velocity follow-up by about 10%, but has minimal impact on high-precision measurements.
• Mass measurements for Super Earths will be extremely time-consuming, with only two to three dozen such measurements likely achievable with 75 nights of HARPS time.
• High-precision radial velocity measurements (1 m s⁻¹) are essential to distinguish rocky desert planets from water worlds, but are limited by telescope time on 4m–10m class telescopes.
• The most promising targets for atmospheric characterization with JWST will be the nearest and brightest stars hosting transiting Super Earths, which TESS is designed to discover.