Astronomers using the Immersion GRating INfrared Spectrograph (IGRINS) instrument on the Gemini South telescope have examined the atmosphere of WASP-121b, one of the most extensively studied ultrahot Jupiters.
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This artist’s illustration shows WASP-121b, an alien world that is losing magnesium and iron gas from its atmosphere. Image credit: NASA / ESA / J. Olmsted, STScI.
Discovered in 2016 by astronomers with the WASP-South survey, WASP-121b is 1.87 times bigger than Jupiter and 1.18 times more massive.
Its host star, WASP-121 (TYC 7630-352-1), is an active F6-type main-sequence star about 1.5 times the size of the Sun.
The WASP-121 system is located about 881 light-years away in the constellation of Puppis.
WASP-121b is a so-called ‘hot Jupiter’ and takes just 1.3 days to orbit WASP-121. It is so close to the parent star that if it got any closer, the star’s gravity would start ripping it apart.
Astronomers estimate the planet’s temperature to be about 2,500 degrees Celsius (4,600 degrees Fahrenheit), hot enough to boil some metals.
The new IGRINS observations uncovered something unexpected about the formation history of WASP-121b.
With these observations, Arizona State University astronomer Peter Smith and his colleagues demonstrated for the first time measuring the rock-to-ice ratio for a transiting planet using a single instrument.
“Ground-based data from Gemini South using IGRINS actually made more precise measurements of the individual chemical abundances than even space-based telescopes could have achieved,” Dr. Smith said.
The spectroscopic data show that WASP-121b has a high rock-to-ice ratio, indicating that it accreted an excess of rocky material while it was forming.
This suggests the planet formed in a region of the protoplanetary disk where it was too hot for ices to condense, which is a surprising discovery since it’s typically believed that gas giants need solid ices to form.
“Our measurement means that perhaps this typical view needs to be reconsidered and our planet formation models revisited,” Smith said.
The astronomers also found remarkable characteristics of WASP-121b’s atmosphere.
“The climate of this planet is extreme, and nothing like that of Earth,” Smith said.
The planet’s dayside is so hot that elements typically thought of as ‘metal’ are vaporized into the atmosphere, making them detectable via spectroscopy.
Strong winds blow these metals to the planet’s permanent nightside, where it is cool enough for them to condense and rain out — an effect that was observed on WASP-121b in the form of calcium rain.
“Our instrument sensitivity is advancing to the point where we can use these elements to probe different regions, altitudes, and longitudes to see subtleties like wind speeds, revealing just how dynamic this planet is,” Smith said.
The findings appear in the Astronomical Journal.
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Peter C. B. Smith et al. 2025. The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-volatile Ratios. AJ 168, 293; doi: 10.3847/1538-3881/ad8574
