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 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 (V_mag = 9.8).

在200个类似太阳的恒星中，大约有1个行星的轨道周期短于一天：超短周期行星^1,2。所有先前已知的超短周期行星要么热木星，具有尺寸高于10地球半径（ř _⊕），或明显岩石行星小于2  - [R _⊕。缺乏中等大小的行星（“热海王星沙漠”）被解释为低质量行星无法在强烈的恒星辐射下保留任何氢/氦（H / He）包膜。在这里，我们报告的超短周期行星的发现具有4.6的半径 [R _⊕和29块 中号_⊕，坚定地在炎热的海王星沙漠中。来自转轨系外行星调查卫星^3的数据显示，每0.79天，类似太阳的明亮恒星LTT 9779的运行一次。行星的平均密度类似于海王星，并且根据热演化模型，它具有富含H / He的包膜，占总质量的9.0 \（{} _ {-2.9} ^ {+ 2.7} \）％ 。由于平衡温度约为2,000 K，因此尚不清楚这种“超热海王星”如何保持这样的外壳。恒星的亮度（V _mag  = 9.8）有助于对行星大气的后续观测，以更好地了解其起源和物理性质。