A high-contrast search for variability in HR 8799bc with VLT-SPHERE

Abstract

The planets HR8799bc display nearly identical colours and spectra as variable young exoplanet analogues such as VHS 1256-1257ABb and PSO J318.5-22, and are likely to be similarly variable. Here we present results from a 5-epoch SPHERE IRDIS broadband-H search for variability in these two planets. HR 8799b aperture photometry and HR 8799bc negative simulated planet photometry share similar trends within uncertainties. Satellite spot lightcurves share the same trends as the planet lightcurves in the August 2018 epochs, but diverge in the October 2017 epochs. We consider Δ(mag)b − Δ(mag)c to trace non-shared variations between the two planets, and rule out non-shared variability in Δ(mag)b − Δ(mag)c to the 10-20% level over 4-5 hours. To quantify our sensitivity to variability, we simulate variable lightcurves by inserting and retrieving a suite of simulated planets at similar radii from the star as HR 8799bc, but offset in position angle. For HR 8799b, for periods <10 hours, we are sensitive to variability with amplitude $>5\%$. For HR 8799c, our sensitivity is limited to variability $>25\%$ for similar periods.

Let the Great World Spin: Revealing the Stormy, Turbulent Nature of Young Giant Exoplanet Analogs with the Spitzer Space Telescope

“…These results are encouraging for future high-contrast searches for variability in directly-imaged exoplanets with JWST. While a number of ground-based searches have been carried out for the HR8799 planets (Apai et al 2016;Biller et al 2021) without detecting variability, JWST will provide unprecedented sensitivity and precision. The results of these searches will be directly informed by the variability studies of isolated low-gravity brown dwarfs such as this one.…”

“…Due to their close proximity to their host star and relative faintness, variability searches on directly-imaged exoplanets such as HR8799 bcde and β Pic b present considerable challenges. To date, there have been two groundbased attempts to detect variability in HR8799 bc using the Very Large Telescope (VLT) (Apai et al 2016;Biller et al 2021), however neither study reached the sensitivity to detect variability, placing near-infrared upper limits of 5 − 10% on the variability amplitudes of the planets. Searching for variability in these exoplanets will likely require next-generation telecopes such as the James Webb Space Telescope (JWST) and 30-m class telescopes such as the European-Extremely Large Telescope (E-ELT), the Thirty Metre Telescope (TMT) and the Giant Magellan Telescope (GMT), which will provide unprecedented sensitivity for variability searches in directly-imaged exoplanets.…”

“…Indeed, there have already been a number of variability detections in wide-orbit companions with masses in the range ∼ 10 − 30 M Jup (Zhou et al 2016(Zhou et al , 2018(Zhou et al , 2020Naud et al 2017). For the closer in directly-imaged exoplanets, there have been two attempts to detect photometric variability in the HR8799 bc planets (Apai et al 2016;Biller et al 2021).…”

On the Detection of Exomoons Transiting Isolated Planetary-mass Objects

“…Using high-contrast imaging techniques, it has proven difficult to achieve a photometric precision better than the 1% level in 1 hr due to photon noise from the host star and time-variable image artifacts (Sahoo et al 2020). For example, attempts have been made to search for variability of the HR 8799 planets, but the upper limits on any variability are only at the 5%-10% level (Apai et al 2016;Biller et al 2021).…”

Dynamical Mass of the Exoplanet Host Star HR 8799

“…During the course of this analysis,Biller et al (2021) andWahhaj et al (2021) presented new astrometry from VLT/SPHERE-IRDIS. While these data were not included in this study, continued astrometric monitoring will be essential for future studies of HR 8799.3 We also include the 2010.55 and 2012.83 Keck/NIRC2 epochs presented inCurrie et al (2014), which were inadvertently omitted in theBowler (2016) compilation.…”