Samuel Grunblatt1, Nicholas Saunders2, Daniel Huber2,3, Daniel Thorngren1, Shreyas Vissapragada4 1. Department of Physics and Astronomy, Johns Hopkins University, 2. Institute for Astronomy, University of Hawaii, 3. Sydney Institute for Astronomy, University of Sydney, 4. Center for Astrophysics | Harvard & Smithsonian An unlikely survivor: A low-density, hot Neptune orbiting a red giant star A targeted visual search for giant planets transiting giant stars with the giants pipeline (Saunders+ 2022) revealed a shallow (~400 ppm), long-duration (~9 hr) transit signal at a 4.2 day period. This signal was not found by other pipelines until 120-second cadence TESS data was available and a SPOC light curve was produced. An atmosphere saved by re-inflation? Key Points A needle in the haystack The first hot Neptune with an evolved host ❖The shallow, long-duration transit signal of this planet could initially be found only with the giants FFI pipeline, but has now been confirmed with a high-cadence, SPOC light curve detection and ground-based RV followup. ❖With a mass of 19.2 +/- 4.2 Earth masses and 6.2 +/- 0.8 Earth radii, this is the first hot Neptune found around an evolved star. ❖ The planet is predicted to have lost ~65% of its mass to atmospheric mass loss, which should have stripped its atmosphere entirely, unless the star was unusually inactive, or the planet experienced late-stage migration and/or re-inflation. Using exoplanet (Foreman-Mackey+ 2018), we modeled the available data for this system to determine the planet mass and radius. References 1. Saunders, N., Grunblatt, S., Huber, D., Collins, K., Jensen, E., et al. “TESS Giants Transiting Giants. I. A Noninflated Hot Jupiter Orbiting a Massive Subgiant,” AJ, 163, 53, 2022. 2. Foreman-Mackey, D., Luger, R., Agol, E., Barclay, T., Bouma, L, et al. “exoplanet: Gradient-based probabilistic inference for exoplanet data and other astronomical time series,” JOSS, 2021. 3. Watson, A.J., Donahue, T.M., Walker, J.C.G. "The dynamics of a rapidly escaping atmosphere: Applications to the evolution of Earth and Venus,” Icarus, 48, 150, 1981. Integrating the XUV flux expected for this and all other well- characterized planets with ages, we determine the cumulative expected atmospheric mass loss as a fraction of total planet mass. This planet is expected to have lost 65% of its mass, more than any other gaseous planet, despite models suggesting the atmosphere is only 20-30% of the planet mass (Lopez & Fortney 2014, Chen & Rogers 2016). How did this planet retain its atmosphere? Subsequent Keck/HIRES RV followup detected a signal at the same period and phase as giants and SPOC. Taking stellar parameters into account, we found that this is the first well-characterized hot Neptune known orbiting an evolved star. Rp = 6.21 +/- 0.76 REarth Mp = 19.2 +/- 4.2 MEarth Atmospheric mass loss depends on the planet density (Watson+ 1981): Thus, if the planet’s density was higher in the past, it can retain significantly more atmosphere. A 16% reduction in early planet radius results in a 50% reduction in atmospheric mass loss, allowing this planet to survive. 0 2 4 6 Age (Gyr.) 4 5 6 7 8 Radius (R©) Re-inﬂation Mass Loss 16% difference in radius at 1 Gyr = 50% difference in mass loss rate! Link to arXiv: https://arxiv.org/abs/2303.06728 (or use QR code above!)