Securing the Legacy of TESS through the Care and Maintenance of TESS Planet Ephemerides

Abstract

TESS has begun fulfilling its promise of delivering thousands of new transiting planets orbiting nearby bright stars. The mission's legacy will fuel exoplanet science for many years to come, but much of this science relies on precisely predicted transit times that are needed for many follow-up characterization studies. We investigate the severity of ephemeris deterioration for TESS planets, and find that most will have uncertainties greater than 1 hour just one year after their TESS observations. It is the mission's relatively short observing baseline that drives this fast deterioration. We identify the parameters that have the strongest impact on this deterioration. We recommend that one or two follow-up transits be observed three and/or nine months after the end of a planet's TESS observations, in order to refresh its ephemeris for two years past the follow-up observations. We find that the longer the baseline between the TESS and the follow-up observations, the longer the ephemerides will stay fresh, facilitating the scheduling of future observations with expensive facilities such as the Hubble Space Telescope, the James Webb Space Telescope, the ELTs, and Ariel.

Transits of Known Planets Orbiting a Naked-eye Star

“…One pressing concern is that, despite long-term constraints from RVs and from the single transit, the period of planet c is still relatively uncertain. Therefore, further observations, either during the TESS extended mission or by the CHEOPS mission (Broeg et al 2014), are needed to ensure that the ephemeris of this planet can be refined to enable followed-up observations (Dragomir et al 2020). As noted in Section 4.1, planet d did not pass through inferior conjunction during the TESS observations, so follow-up photometric campaigns could reveal whether planet d also transits.…”

CHEOPS observations of the HD 108236 planetary system: a fifth planet, improved ephemerides, and planetary radii

“…Starting from CHEOPS last observations, the reference timescales for the ephemerides' drifts vary from ∼29 yr for planet f (358 orbits) up to ∼108 yr for planet e (2013 orbits). Dragomir et al (2020) evaluated that ∼98% of TESS target stars re-observed by a follow-up mission nine months after TESS observations keep their ephemerides fresh (that is the uncertainty on T 0 is lower than 30 min) for at least two years. In our case CHEOPS observations occur ∼1 yr after TESS observations, therefore our results may be comparable with the ephemerides deterioration estimated by Dragomir et al (2020).…”

Science Extraction from TESS Observations of Known Exoplanet Hosts

“…The continuous time series photometry of these stars may be used to achieve multiple science goals that have an over-arching theme of unprecedented characterization of these planetary systems. These science goals include the detection of transits for known planets (Dalba et al 2019), the discovery of additional planets (Brakensiek & Ragozzine 2016), the detection of phase variations and secondary eclipses (Mayorga et al 2019), refinement of transit ephemerides (Dragomir et al 2020), and asteroseismology of host stars (Campante et al 2016). Each of these science goals have been realized to various degrees through the course of the TESS primary mission, providing significant insight into the physical properties of the known planets and the architectures of those systems.…”