Journal Article

An ultrahot Neptune in the Neptune desert

Winn, Joshua N., Armstrong, David J., de Leon, Jerome Pitogo, Pollacco, Don, Smith, Alexis M. S., West, Richard G., Acton, Jack S., Rauer, Heike, Cooke, Benjamin F., Latham, David W., Burleigh, Matthew R., Collins, Kevin I., Isopi, Giovanni, Goad, Michael R., Jackman, James A. G., Cortés-Zuleta, Pía, Vezie, Michael, Chaushev, Alexander, Ciardi, David R., Seager, Sara, Christiansen, Jessie L., Belardi, Claudia, Ziegler, Carl, Casewell, Sarah L., Lopez, Eric D., Foxell, Emma, Henze, Christopher E., Gill, Samuel, Collins, Karen A., Queloz, Didier, Tilbrook, Rosanna H., Hooton, Matthew J., Ricker, George, McCauliff, Sean, Walker, Simon. R., Petrovich, Cristobal, Watson, Christopher A., Bryant, Edward M., Titz-Weider, Ruth, Mallia, Franco, Vines, Jose I., Narita, Norio, Erikson, Anders, Mann, Andrew W., Torres, Pascal, Jenkins, Jon M., Jordán, Andrés, Lissauer, Jack J., Cabrera, Juan, Günther, Maximilian N., Rojas, Pablo A. Peña, McCormac, James, Wheatley, Peter J., Nishiumi, Taku, Bayliss, Daniel, Tamura, Motohide, Ercolino, Andrea, Eigmüller, Philipp, Moyano, Maximiliano, Gänsicke, Boris T., Gillen, Edward

2020

Overview

About 1 out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultrashort-period planet 1 , 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 Satellite 3 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 ( V mag  = 9.8). LTT 9779 b is Neptune-sized planet rotating around its star with a period of 0.79 days and an equilibrium temperature of 2,000 K. It is not clear how it retained its atmospheric envelope, which contains ~10% of H/He, as it should have been photoevaporated by now.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/34/862 / 639/33/445/862 / Astronomy / Astrophysics and Cosmology / Deserts / Helium / Irradiation / Letter / Neptune / Physics / Physics and Astronomy / Planets / Stars