A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered by TESS

Abstract

We present the discovery of HD 221416 b, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. HD 221416 b (HIP 116158, TOI-197) is a bright (V=8.2 mag), spectroscopically classified subgiant that oscillates with an average frequency of about 430 μHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2 minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (R å =2.943±0.064 R e), mass (M å =1.212±0.074 M e), and age (4.9±1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (R p =9.17±0.33 R ⊕) with an orbital period of ∼14.3 days, irradiance of F=343±24 F ⊕ , and moderate mass (M p =60.5±5.7 M ⊕) and density (ρ p =0.431±0.062 g cm −3). The properties of HD 221416 b show that the host-star metallicity-planet mass correlation found in sub-Saturns (4-8 R ⊕) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ∼15%, HD 221416 b is one of the best characterized Saturn-size planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.

Two New HATNet Hot Jupiters around A Stars and the First Glimpse at the Occurrence Rate of Hot Jupiters from TESS∗

“…We do note, though, that more than half of the TESS stars brighter than 10th magnitude are evolved. Eventually, this will be a rich hunting ground (e.g., Huber et al 2019;Rodriguez et al 2019). Figure 15 shows the color-magnitude diagram (CMD) of the 120,000 stars brighter than T mag =10 that were observed by TESS.…”

The TESS–Keck Survey. I. A Warm Sub-Saturn-mass Planet and a Caution about Stray Light in TESS Cameras*

“…Much of the follow-up effort has been focused on confirming and characterizing the small planets (e.g., Huang et al 2018;Dragomir et al 2019;Vanderburg et al 2019;Winters et al 2019). Several giant exoplanets have been confirmed as well (e.g., Huber et al 2019;Jones et al 2019;Rodriguez et al 2019;Wang et al 2019). However, due to observational biases, the TESS primary mission strategy, and the ease of characterizing planets that transit frequently, most of the TESS planets announced to date orbit relatively close to their stars.…”

TIC 278956474: Two Close Binaries in One Young Quadruple System Identified by TESS

“…The main purpose of the Transiting Exoplanet Survey Satellite, TESS (Ricker et al 2014), is to identify nearby planets ≤ 4 R ⊕ which can be fully characterised. However, a great deal of complementary science has come from the mission, particularly in stellar science (see for example Fausnaugh et al (2019), Holoien et al (2019), arXiv:2006.08979v1 [astro-ph.SR] 16 Jun 2020 Huber et al (2019); Schofield et al (2019), Zhan et al (2019), Ahlers et al (2020)).…”