Astronomers Determine How "Super Jupiters" Around Remote Star Took Shape

The planets in our solar system grew out of a disk of material that swirled around our Sun. Inner rocky planets formed as tiny grains stuck together, becoming pebbles, then boulders, and, ultimately, full-grown planets. The outer gas and ice giant planets also accumulated cores of rocky material, which then attracted halos of cooler gas and ice around them.

Planets around other stars are thought to form in a similar fashion, called core accretion, but astronomers have wondered if this still holds true for planets that are even bigger and farther from their star than Jupiter. It is possible that these "super Jupiters" form more like stars in a process called gravitational instability rather than like planets via core accretion.

A team of researchers have used spectral data from NASA's James Webb Space Telescope (JWST) to find answers. They probed the HR 8799 star system, located approximately 133 light-years away in the constellation Pegasus, which consist of four super Jupiters. Each planet around this star has a mass five to 10 times greater than the mass of Jupiter, and they orbit at distances between 15 to 70 astronomical units, meaning the planet closest to the star is 15 times farther away from that star than Earth is from our Sun.

The results, published in Nature Astronomy, reveal that the third planet, HR 8799 c, contains sulfur. That is significant because unlike carbon and oxygen-containing molecules, sulfur-containing molecules would be solid, not gaseous, in a planet-forming disk. And this indicates the planet formed via core accretion. The scientists say it is likely that sulfur is present on all of HR 8799's three innermost planets. These inner three planets were also found to be more enriched in heavy elements, like carbon and oxygen, than their star—further evidence that they formed as planets.

"With its unprecedented sensitivity, JWST is enabling the most detailed study of the atmospheres of these planets, giving us clues to their formation pathways," says co-lead study author Jean-Baptiste Ruffio, a research scientist at UC San Diego and a former postdoctoral scholar at Caltech, where he began the research project. "With the detection of sulfur, we are able to infer that the HR 8799 planets likely formed in a similar way to Jupiter despite being five to 10 times more massive, which was unexpected."

Charles (Chas) Beichman, a co-author of the study and a senior faculty associate at IPAC, a science and data center for astronomy at Caltech, says it was not previously clear how far out a gas giant could be from its star and still form a rocky core. "This sets a new marker for where the planetary disk processes favor core accretion."

Isolating the spectral data from the planets was a tremendous challenge because the planets are 10,000 times fainter than their star, and JWST's spectrograph was not originally designed for these types of observations. Ruffio, who led the analysis, had to develop new techniques to extract the faint signal and make this discovery possible. Co-lead author Jerry Xuan (PhD '25), a 51 Pegasi b Fellow at UCLA and a former postdoc at Caltech, created detailed atmospheric models that could be compared to the JWST spectra to see if sulfur was present.

"The quality of the JWST data is truly revolutionary, and existing atmospheric model grids were simply not adequate. To fully capture what the data were telling us, I iteratively refined the chemistry and physics in the models," Xuan says. "In the end, we detected several molecules in these planets—some for the first time, including hydrogen sulfide."

Beichman says the observations will give theorists something new to think about. "Astronomy is driven by observations, and then the theorists have to explain it. The theorists come up with new ideas that go back to the experimentalists, and the process starts all over. This is how we expand our knowledge, and it is happening every day with JWST and telescopes around the world."

Read a longer story on this work from UC San Diego.

Other Caltech authors on the paper include Dimitri Mawet, the David Morrisroe Professor of Astronomy at Caltech and senior research scientist at JPL, Heather Knutson, professor of planetary science at Caltech, Geoffrey Bryden of JPL, and Thomas Greene, executive director of IPAC.

This work was supported by NASA.