How big can a planet be? With very large gas giants, it can be hard to tell

(Press-News.org) Gas giants are large planets mostly composed of helium and/or hydrogen. Although these planets have dense cores, they don’t have hard surfaces. Jupiter and Saturn are the gas giants in our solar system, but there are many other gas giant exoplanets in our galaxy and some are many times larger than Jupiter. The largest gas giants blur the line between planets and brown dwarfs — those substellar objects, sometimes called “failed stars” because they do not fuse hydrogen.

How do these gas giants form? Was it through core accretion, where solid cores gradually grow in a disk by pulling in rocky and icy pebbles until they become massive enough to attract the gas that surrounds young stars — as happened with Jupiter and Saturn? Or was it through gravitational instability, where the cloud of gas surrounding the star rapidly collapses into massive objects like brown dwarfs?

A team of researchers, led by the University of California San Diego, used spectral data from the James Webb Space Telescope (JWST) to probe the HR 8799 star system, leading to a surprising answer to this longstanding astronomical question. Their work appears in Nature Astronomy.

The HR 8799 star system is located approximately 133 light years away in the constellation Pegasus. Each planet around this star is five to ten times the mass of Jupiter and orbits the star HR 8799 at distances of 15-70 astronomical units, meaning the planet closest to the star is 15 times farther away than the Earth is from the sun. The planet masses range from 5−10 MJup, meaning the smallest planet is five times more massive than Jupiter. HR 8799 is a scaled-up version of our own solar system, which also has four outer icy and gas giants stretching from Jupiter to Neptune.

The extreme distances at which these planets orbit around their star and their large masses made astronomers question if this system could have formed through core accretion. Indeed, original models of planet formation based on our solar system predicted that planets would not have time to grow to such larges masses before the star itself would blow away the disk surrounding it.

Harnessing the Power of JWST Astronomers often use spectroscopy — light waves that can reveal the physical properties of exoplanets and provide insight into how they were formed. Prior to JWST, they used ground-based telescopes to measure the amount of water and carbon monoxide in exoplanets. However, today scientists have come to realize that carbon and oxygen-bearing molecules are not the best tracers of planet formation, because it’s not possible to discern their origins. 

They turned away from these “volatile” molecules to more stable ones, called refractories. Refractory elements, like sulfur, are only present in solids in the protoplanetary disk from which planets form. The presence of sulfur is evidence that the gas giant formed through core accretion.

“With its unprecedented sensitivity, JWST is enabling the most detailed study of the atmospheres of these planets, giving us clues to their formation pathways. 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 ten times more massive, which was unexpected,” stated Jean-Baptiste Ruffio, a research scientist at UC San Diego and first co-author of the paper.

HR 8799 is a relatively young star system at around 30 million years old (for reference, our solar system is about 4.6 billion years old). Because planets tend to cool as they age, younger planets are brighter and easier to study via spectroscopy.

JWST has the highest resolution spectrograph available in space, allowing researchers to look at the light of exoplanets without the contamination of molecules from the Earth’s atmosphere. For the first time, astronomers were able to see fine features from a number of rare molecules in the atmospheres of the inner three HR 8799 gas giants, which were previously undetectable.

However, this discovery was not easy. These planets are about 10,000 times fainter than their star and the JWST’s spectrograph was not originally designed for such challenging observations.  Ruffio, who led the analysis, had to develop new data analysis techniques to extract the faint signal and make this discovery possible. Jerry Xuan, a 51 Pegasi b Fellow at UCLA, 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,” he said. “In the end, we detected several molecules in these planets — some for the first time, including hydrogen sulfide.”

The team found very clear evidence of sulfur in the third planet in the system, HR 8799 c, although they believe it is likely present on all three inner planets. They also found that the planets were more enriched in heavy elements, like carbon and oxygen, than their star – further evidence that they formed as planets.

“There are many models of planet formation to consider. I think this shows that older core accretion models are outdated,” stated UC San Diego Professor of Astronomy and Astrophysics Quinn Konopacky, another of the paper’s co-authors. “And of the newer models, we are looking at ones where gas giants can form solid cores really far away from their star.”

Ruffio says HR 8799 is somewhat unique because, thus far, it’s the only imaged system with four massive gas giants, but there are other known systems with one or two even larger companions and whose formation remains unknown.

“I think the question is, how big can a planet be?” he stated. “Can a planet be 15, 20, 30 times the mass of Jupiter and still have formed like a planet? Where is the transition between planet formation and brown dwarf formation?”

For now, the work continues, one star system at a time.

Partial list of authors: Jean‑Baptiste Ruffio, Eve J. Lee and Quinn Konopacky (all UC San Diego); Jerry W. Xuan (California Institute of Technology and UCLA); Dimitri Mawet, Aurora Kesseli, Charles Beichman, Geoffrey Bryden and Thomas P. Greene (all California Institute of Technology); and Yayaati Chachan (UC Santa Cruz). Full list of authors appears in the paper.

This work was supported, in part, by the National Aeronautics and Space Administration (80NSSC25K7300 and FINESST Fellowship award 80NSSC23K1434). Any opinions, findings, and conclusions or recommendations expressed in this work are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration.

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