TESS-Keck Survey. V. Twin Sub-Neptunes Transiting the Nearby G Star HD 63935
Batalha, Natalie M.; Latham, David W.; Mayo, Andrew W.; Kane, Stephen R.; Stassun, Keivan G.; Huber, Daniel; Claytor, Zachary R.; Schlieder, Joshua E.; Rosenthal, Lee J.; Hirsch, Lea A.; Howard, Andrew W.; Petigura, Erik A.; Behmard, Aida; Chontos, Ashley; Crossfield, Ian J. M.; Dalba, Paul A.; Rubenzahl, Ryan A.; Weiss, Lauren M.; Fetherolf, Tara; Collins, Karen A.; Ricker, George R.; Vanderspek, Roland; Winn, Joshua N.; Seager, S.; Shporer, Avi; Collins, Kevin I.; Ciardi, David R.; Dai, Fei; Nowak, Grzegorz; de Leon, Jerome P.; Fukui, Akihiko; Narita, Norio; Dragomir, Diana; Lund, Michael B.; Jensen, Eric L. N.; Luque, Rafael; Giacalone, Steven; Dressing, Courtney D.; Gonzales, Erica J.; Fulton, Benjamin; Christiansen, Jessie L.; Yahalomi, Daniel A.; Robertson, Paul; Roy, Arpita; Isaacson, Howard; Močnik, Teo; Beard, Corey; Lubin, Jack; Hill, Michelle L.; Scarsdale, Nicholas; Kosiarek, Molly R.; Pallé, Enric; Murphy, Joseph M. Akana; Mori, Mayuko; Rabus, Markus; Twicken, Joe; Wang, Gavin; Jenkins, Jon
United States, Australia, Japan, Spain, Denmark, Chile, China
Abstract
We present the discovery of two nearly identically sized sub-Neptune transiting planets orbiting HD 63935, a bright (V = 8.6 mag), Sun-like (Teff = 5560 K) star at 49 pc. TESS identified the first planet, HD 63935 b (TOI-509.01), in Sectors 7 and 34. We identified the second signal (HD 63935 c) in Keck High Resolution Echelle Spectrometer and Lick Automated Planet Finder radial velocity data as part of our follow-up campaign. It was subsequently confirmed with TESS photometry in Sector 34 as TOI-509.02. Our analysis of the photometric and radial velocity data yielded a robust detection of both planets with periods of 9.0600 ± 0.007 and 21.40 ± 0.0019 days, radii of 2.99 ± 0.14 and 2.90 ± 0.13 R⊕, and masses of 10.8 ± 1.8 and 11.1 ± 2.4 M⊕. We calculated densities for planets b and c consistent with a few percent of the planet mass in hydrogen/helium envelopes. We also describe our survey's efforts to choose the best targets for James Webb Space Telescope atmospheric follow-up. These efforts suggest that HD 63935 b has the most clearly visible atmosphere of its class. It is the best target for transmission spectroscopy (ranked by the transmission spectroscopy metric, a proxy for atmospheric observability) in the so far uncharacterized parameter space comprising sub-Neptune-sized (2.6 R⊕ < Rp < 4 R⊕), moderately irradiated (100 F⊕ < Fp < 1000 F⊕) planets around G stars. Planet c is also a viable target for transmission spectroscopy, and given the indistinguishable masses and radii of the two planets, the system serves as a natural laboratory for examining the processes that shape the evolution of sub-Neptune planets.