2 research outputs found
An ultrahot Neptune in the Neptune desert
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 (Râ), or apparently rocky planets smaller than 2âRâ. Such lack of planets of intermediate size (the âhot Neptune desertâ) 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 4.6âRâ and a mass of 29âMâ, firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite3 revealed transits of the bright Sun-like star LTT 9779 every 0.79âdays. The planetâs mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0+2.7â2.9% of the total mass. With an equilibrium temperature around 2,000âK, it is unclear how this âultrahot Neptuneâ managed to retain such an envelope. Follow-up observations of the planetâs atmosphere to better understand its origin and physical nature will be facilitated by the starâs brightness (Vmagâ=â9.8).<br
TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet
We present the bright (Vmag = 9.12), multiplanet system TOI-431, characterized with photometry and radial velocities (RVs). We estimate the stellar rotation period to be 30.5 ± 0.7 d using archival photometry and RVs. Transiting Exoplanet Survey Satellite (TESS) objects of Interest (TOI)-431 b is a super-Earth with a period of 0.49 d, a radius of 1.28 ± 0.04 R, a mass of 3.07 ± 0.35 M, and a density of 8.0 ± 1.0 g cm-3; TOI-431 d is a sub-Neptune with a period of 12.46 d, a radius of 3.29 ± 0.09 R, a mass of 9.90+1.53-1.49 M, and a density of 1.36 ± 0.25 g cm-3. We find a third planet, TOI-431 c, in the High Accuracy Radial velocity Planet Searcher RV data, but it is not seen to transit in the TESS light curves. It has an Msin i of 2.83+0.41-0.34 M, and a period of 4.85 d. TOI-431 d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterization, while the super-Earth TOI-431 b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431 b is a prime TESS discovery for the study of rocky planet phase curves