An ultrahot Neptune in the Neptune desert
Jackman, James A. G.; Bieryla, Allyson; Latham, David W.; Quinn, Samuel N.; Jordán, Andrés; Espinoza, Néstor; Brahm, Rafael; Wheatley, Peter J.; Tamura, Motohide; Bayliss, Daniel; Moyano, Maximiliano; Ziegler, Carl; Lissauer, Jack J.; Collins, Karen A.; Vanderspek, Roland; Seager, Sara; Winn, Joshua N.; Jenkins, Jon M.; Beichman, Charles A.; Günther, Maximilian N.; Collins, Kevin I.; Ciardi, David R.; Cabrera, Juan; Eigmüller, Philipp; Erikson, Anders; Narita, Norio; Palle, Enric; Rauer, Heike; Casewell, Sarah L.; Gill, Samuel; Goad, Michael R.; Armstrong, David J.; Burleigh, Matthew R.; Gillen, Edward; Jenkins, James S.; McCormac, James; Udry, Stéphane; Vines, Jose I.; West, Richard G.; Mann, Andrew W.; Smith, Alexis M. S.; Bryant, Edward M.; Gänsicke, Boris T.; Isopi, Giovanni; Mallia, Franco; Queloz, Didier; Watson, Christopher A.; Bouchy, François; Chaushev, Alexander; Foxell, Emma; Louden, Tom; Nielsen, Louise D.; Raynard, Liam; Titz-Weider, Ruth; Lopez, Eric D.; Christiansen, Jessie L.; McCauliff, Sean; Ricker, George; Díaz, Matías R.; Law, Nicholas; Vezie, Michael; Henze, Christopher E.; Burke, Christopher J.; Walker, Simon. R.; Pollacco, Don; Mori, Mayuko; Acton, Jack S.; Tilbrook, Rosanna H.; Nishiumi, Taku; de Leon, Jerome Pitogo; Klaus, Todd C.; Turner, Oliver; Hooton, Matthew J.; King, George W.; Cooke, Benjamin F.; Torres, Pascal; Belardi, Claudia; Villaseñor, Jesus Noel; Cortés-Zuleta, Pía; Soto, Maritza G.; Ercolino, Andrea; Kurtovic, Nicolás T.; Rojas, Pablo A. Peña; Petrovich, Cristobal
Chile, United States, United Kingdom, Belgium, Japan, Spain, Italy, Canada, Switzerland, Germany
Abstract
About 1 out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultrashort-period planet1,2. All of the previously known ultrashort-period planets are either hot Jupiters, with sizes above 10 Earth radii (R⊕), or apparently rocky planets smaller than 2 R⊕. Such lack of planets of intermediate size (the `hot Neptune desert') has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here we report the discovery of an ultrashort-period planet with a radius of 4.6 R⊕ and a mass of 29 M⊕, firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite3 revealed transits of the bright Sun-like star LTT 9779 every 0.79 days. The planet's mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0-2.9+2.7?% of the total mass. With an equilibrium temperature around 2,000 K, it is unclear how this `ultrahot Neptune' managed to retain such an envelope. Follow-up observations of the planet's atmosphere to better understand its origin and physical nature will be facilitated by the star's brightness (Vmag = 9.8).