TOI-561 b: A Low-density Ultra-short-period "Rocky" Planet around a Metal-poor Star

Creators: Brinkman, Casey L.; Weiss, Lauren M.; Dai, Fei; Huber, Daniel; Kite, Edwin S.; Valencia, Diana; Bean, Jacob L.; Beard, Corey; Behmard, Aida; Blunt, Sarah; Brady, Madison; Fulton, Benjamin; Giacalone, Steven; Howard, Andrew W.; Isaacson, Howard; Kasper, David; Lubin, Jack; MacDougall, Mason; Akana Murphy, Joseph M.; Plotnykov, Mykhaylo; Polanski, Alex S.; Rice, Malena; Seifahrt, Andreas; Stefánsson, Guðmundur; Stürmer, Julian

Abstract

TOI-561 is a galactic thick-disk star hosting an ultra-short-period (0.45-day-orbit) planet with a radius of 1.37 R_⊕, making it one of the most metal-poor ([Fe/H] = −0.41) and oldest (≈10 Gyr) sites where an Earth-sized planet has been found. We present new simultaneous radial velocity (RV) measurements from Gemini-N/MAROON-X and Keck/HIRES, which we combined with literature RVs to derive a mass of M_ = 2.24 ± 0.20 M_⊕. We also used two new sectors of TESS photometry to improve the radius determination, finding R_b = 1.37 ± 0.04 R_⊕ and confirming that TOI-561 b is one of the lowest-density super-Earths measured to date (ρ_b = 4.8 ± 0.5 g cm⁻³). This density is consistent with an iron-poor rocky composition reflective of the host star's iron and rock-building element abundances; however, it is also consistent with a low-density planet with a volatile envelope. The equilibrium temperature of the planet (∼2300 K) suggests that this envelope would likely be composed of high mean molecular weight species, such as water vapor, carbon dioxide, or silicate vapor, and is likely not primordial. We also demonstrate that the composition determination is sensitive to the choice of stellar parameters and that further measurements are needed to determine whether TOI-561 b is a bare rocky planet, a rocky planet with an optically thin atmosphere, or a rare example of a nonprimordial envelope on a planet with a radius smaller than 1.5 R_⊕.

Additional Information

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. We recognize the cultural significance and sanctity that the summit of Maunakea has within the indigenous Hawaiian community. We are deeply grateful to have the opportunity to conduct observations from this mountain, while acknowledging the impact of our presence there and the ongoing efforts to preserve this special place in the universe. C.L.B. is supported by the National Science Foundation Graduate Research Fellowship under grant No. 1842402. C.L.B., L.M.W., and D.H. acknowledge support from National Aeronautics and Space Administration (80NSSC19K0597) issued through the Astrophysics Data Analysis Program. D.H. also acknowledges support from the Alfred P. Sloan Foundation. C.L.B. and L.M.W. also acknowledge support from NASA Keck Key Strategic Mission Support grant No. 80NSSC19K1475. M.B. is supported by the National Science Foundation Graduate Research Fellowship grant No. DGE 1746045. J.M.A.M. is supported by the National Science Foundation Graduate Research Fellowship Program under grant No. DGE-1842400. J.M.A.M. acknowledges the LSSTC Data Science Fellowship Program, which is funded by LSSTC, NSF Cybertraining grant No. 1829740, the Brinson Foundation, and the Moore Foundation; his participation in the program has benefited this work. We thank the time allocation committee of the University of Hawaii for supporting this work with observing time at the W. M. Keck Observatory and Gemini Observatory. We gratefully acknowledge the efforts and dedication of the Keck Observatory and Gemini Observatory staff and representatives for observation support. The development of the MAROON-X spectrograph was funded by the David and Lucile Packard Foundation, the Heising-Simons Foundation, the Gemini Observatory, and the University of Chicago. The MAROON-X team acknowledges support for this work from the NSF (award No. 2108465) and NASA (through the TESS Cycle 4 GI program, grant No. 80NSSC22K0117). This work was enabled by observations made from the Gemini North telescope, located within the Maunakea Science Reserve and adjacent to the summit of Maunakea. We are grateful for the privilege of observing the universe from a place that is unique in both its astronomical quality and its cultural significance. Facilities: Transiting Exoplanet Survey Satellite (TESS) - , W. M. Keck Observatory - , Gemini Observatory - . Software: Exoplanet (Foreman-Mackey et al. 2021), Batman (Kreidberg 2015), Lightcurve (Lightkurve Collaboration et al. 2018), RadVel (Fulton et al. 2018), emcee (Foreman-Mackey et al. 2013), BurnMan 0.9 (Cottaar et al. 2016), SuperEarth (Valencia et al. 2007; Plotnykov & Valencia 2020) NumPy (Harris et al. 2020), Matplotlib (Hunter 2007), pandas (Wes 2010), Astropy (Astropy Collaboration et al. 2013; Astropy Collaboration, et al. 2018; Astropy Collaboration et al. 2022), SciPy (Virtanen et al. 2020).

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