WASP-178 has a spectral type of A1IV-V, indicating that it is in an evolutionary stage between a main sequence star and a subgiant. The star is comparable to Sirius A in mass and radius, but slightly cooler, older, and less luminous. It is about twice as massive as the Sun and has a radius of 1.7 R☉,[4] with an effective temperature of roughly 9,000 K. A 2019 estimate of 9,360±150 K makes WASP-178 the second-hottest host to a hot Jupiter ever discovered, behind KELT-9 (10,170 K) and ahead of MASCARA-2 (8,980 K),[3] though a lower estimate (8,640+500 −240K) provided by another paper[5] may put it below MASCARA-2. The star is around 20 times brighter than the Sun and is about 140 million years old.[4] For comparison, Sirius A has a mass of 2.063 M☉,[8] a radius of 1.711 R☉,[9] an effective temperature of 9,940 K,[10] a luminosity of 25.4 L☉,[9] and an age of 242 Myr.[8]
Much like Sirius A, the star is a likely Am star and a slow rotator, with a rotational velocity of 8.2–12.2 km/s.[3][5] For comparison, Sirius A has a rotational velosity of 16 km/s,[11] while typical A-type stars rotate much faster at around 160 km/s.[12] It has a near-[5] or above-solar[3]metallicity. The star is rich in chromium, nickel, yttrium, and barium, while being slightly poor in calcium and scandium.[3]
Variability
Aside from periodic dimming caused by the transiting planet, the star experiences regular oscillations in brightness by a few thousandths of a magnitude. The period at which the oscillations occur is measured to be 0.185 days, almost exactly one-eighteenth of WASP-178b's orbital period. The planet's mass is likely too small to cause periodic swaying of the host star, therefore it remains to be known whether this is merely coincidental.[5]
Significant excess noise in the astrometry, totaling to 0.18 milliarcseconds in 254 astrometric measurements, is reported for WASP-178 in the Gaia DR2 catalogue. This may suggest a previously unresolved and invisible binary companion.[3]
In 2019, two teams, part of the WASP and KELTplanet surveys respectively, independently reported the discovery of an exoplanet orbiting the star using the transit method.[3][5] The planet was revealed to be an ultra-hot Jupiter revolving around the star every 3.3 days a mere 0.0558 AU (8,350,000 km) away, heating its surface up to a white-hot 2,470 K (2,200 °C; 3,990 °F).[3] As a result of intense stellar radiation it receives, some of the highest known in an ultra-hot Jupiter,[13] the planet's atmosphere is inflated to a radius of 1.81±0.09RJ[3] or 1.940+0.060 −0.058RJ,[5] placing it among the largest planets discovered so far.
Photometric observations at the CHEOPSspace telescope revealed that the planet has a low geometric albedo of 0.1–0.35, typical of giant planets.[14] Based on this, the dayside temperature of WASP-178b is estimated at 2,250–2,800 K,[14] more than enough to vaporize silicate rock.[15] As one side of the planet always faces the star (tidal locking), the atmosphere on the heated daytime side blows across the planet toward the nighttime side in winds reaching upwards of 2,000 miles per hour (3,200 km/h).[15] On the nightside of the planet, minerals that evaporated on the dayside may cool and condense into rock that pours down from clouds as rain.[15]Silicon monoxide in particular was reported to have been discovered on WASP-178b in 2022, the first time the compound was detected in an exoplanet, but consistent with theoretical models on silicate minerals at high temperatures.[16] In 2024, however, a follow-up study found that the atmosphere was instead more likely dominated by ionizedmagnesium and iron.[17]
^Adelman, Saul J. (8–13 July 2004). "The Physical Properties of normal A stars". Proceedings of the International Astronomical Union. Vol. 2004. Poprad, Slovakia: Cambridge University Press. pp. 1–11. Bibcode:2004IAUS..224....1A. doi:10.1017/S1743921304004314.
