TOI-1728b: the Habitable-zone Planet Finder confirms a warm super-Neptune orbiting an M-dwarf host

Shubham Kanodia, Caleb I. Cañas, Gudmundur Stefansson, Joe P. Ninan, Leslie Hebb, Andrea S. J. Lin, Helen Baran, Marissa Maney, Ryan C. Terrien, Suvrath Mahadevan, William D. Cochran, Michael Endl, Jiayin Dong, Chad F. Bender, Scott A. Diddams, Eric B. Ford, Connor Fredrick, Samuel Halverson, Fred Hearty, Andrew J. MetcalfAndrew Monson, Lawrence W. Ramsey, Paul Robertson, Arpita Roy, Christian Schwab, Jason T. Wright

    Research output: Contribution to journalArticlepeer-review

    19 Citations (Scopus)


    We confirm the planetary nature of TOI-1728b using a combination of ground-based photometry, near-infrared Doppler velocimetry and spectroscopy with the Habitable-zone Planet Finder. TOI-1728 is an old, inactive M0 star with Teff = 3980-32+31 K, which hosts a transiting super-Neptune at an orbital period of ∼3.49 days. Joint fitting of the radial velocities and TESS and ground-based transits yields a planetary radius of 5.05-0.17+0.16 R, mass 26.78-5.13+5.43 M, and eccentricity 0.057-0.039+0.054. We estimate the stellar properties, and perform a search for He 10830 Å absorption during the transit of this planet and claim a null detection with an upper limit of 1.1% with 90% confidence. A deeper level of He 10830 Å absorption has been detected in the planet atmosphere of GJ 3470b, a comparable gaseous planet. TOI-1728b is the largest super-Neptune - the intermediate subclass of planets between Neptune and the more massive gas-giant planets - discovered around an M dwarf. With its relatively large mass and radius, TOI-1728 represents a valuable data point in the M-dwarf exoplanet mass-radius diagram, bridging the gap between the lighter Neptune-sized planets and the heavier Jovian planets known to orbit M dwarfs. With a low bulk density of 1.14-0.24+0.26 g cm-3, and orbiting a bright host star (J ∼ 9.6, V ∼ 12.4), TOI-1728b is also a promising candidate for transmission spectroscopy both from the ground and from space, which can be used to constrain planet formation and evolutionary models.

    Original languageEnglish
    Article number29
    Pages (from-to)1-15
    Number of pages15
    JournalAstrophysical Journal
    Issue number1
    Publication statusPublished - 10 Aug 2020


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