WASP-178b

Lore & Background

The planet was independently discovered by two teams in 2019. One team, led by Coel Hellier, published their findings in November 2019 alongside three other planets (WASP-184b, WASP-185b, WASP-192b) using photometric transit data from WASP-South collected over eight years (May 2006 to August 2014), combined with follow-up observations from the CORALIE spectrograph and EulerCam on the Swiss 1.2-metre Leonhard Euler Telescope. Another team, headed by Romy Rodríguez Martínez, announced the planet in December 2019 as part of the Kilodegree Extremely Little Telescope (KELT) survey, labeling it KELT-26b. KELT-South observed the host star from September 2013 to September 2015, and confirmation came from TESS, the Perth Exoplanet Survey Telescope (PEST), and the CHIRON spectrograph on the SMARTS 1.5 m telescope at Cerro Tololo Inter-American Observatory. This was the 26th and final planet discovered by the KELT survey before it was decommissioned in 2020.

The host star, WASP-178, is a likely Am star and possibly a Delta Scuti variable with spectral type A1IV-V, placing it between a main sequence star and a subgiant. It is about twice as massive as the Sun, with a radius of 1.67 or 1.80 solar radii, an effective temperature of roughly 9,000 K, and around 20 times the Sun's luminosity. A 2019 estimate made it the second-hottest host to a hot Jupiter ever discovered, behind KELT-9 (10,170 K) and ahead of MASCARA-2 (8,980 K), though a lower estimate of 8,640 K from another paper may place it below MASCARA-2.

Reader's Guide

WASP-178b is notable for its extreme physical conditions and its role in atmospheric chemistry studies. Its dayside temperature of 2,250–2,750 K is sufficient to evaporate silicate rock and, above 2,500 K, to break down hydrogen molecules into individual atoms. The planet's tidal locking drives a constant supersonic wind of about 2000 mph from the dayside to the nightside, where atomic hydrogen recombines into molecular H₂ and evaporated minerals cool and condense into rock rain. In 2022, the discovery of silicon monoxide was reported on WASP-178b, the first time the compound was detected in an exoplanet atmosphere, consistent with theoretical predictions of silicate minerals at high temperatures. However, a follow-up study in 2024 concluded that the atmosphere is more likely dominated by ionized magnesium and iron rather than silicon monoxide. Emission signals from the dayside and eclipse observations strongly suggest atmospheric super-rotation and an uneven chemical composition on the dayside. The planet's geometric albedo was measured between 0.1 and 0.35 using CHEOPS photometry and further constrained to below 0.23, indicating a poorly reflective surface typical of gas giants. Its low density (0.37 g/cm³ or 0.238 g/cm³) is comparable to cork, and its inflated size places it among the largest exoplanets alongside other hot Jupiters such as WASP-12b and Ditsö̀.

Did You Know?

• The planet's equilibrium temperature of 2470 K is comparable to the boiling point of silver (2,162 °C).
• Silicon monoxide was detected in its atmosphere in 2022, the first such detection on an exoplanet.
• The host star WASP-178 is about 20 times more luminous than the Sun.