Gas Giants May Form Larger Than Previously Thought

For many years, the conventional narrative about the slow formation of a planet such as Jupiter was almost comforting. Dust particles clash and adhere to one another. Boulders are formed from pebbles. Boulders create a rocky core big enough to begin sucking up gas when they have enough time and gravity. According to textbooks, the process took three to five million years. Bottom-up, tidy, and patient.

That narrative is beginning to seem unfinished. Although the fissures have been apparent for some time, they have recently gotten worse to the point where the field is unable to ignore them. In addition to forming more quickly than anticipated, gas giants may also form larger, in locations, and at times in cosmic history when no one previously believed it was possible, according to new observations from the James Webb Space Telescope and a new reexamination of earlier data.

Think about what Hubble discovered in 2003. A planet orbiting two dead stars simultaneously—a pulsar and a white dwarf, the remnants of a supernova and a long-faded sun—was nestled inside Messier 4, a globular cluster located more than 6,000 light-years away. PSR B1620-26b was an ancient planet. Amazingly old. Its formation is estimated to have occurred approximately 12.7 billion years ago, or just one billion years after the Big Bang. This shouldn’t be possible, according to theory. Simply put, heavy elements—the very components that planets are meant to require—had not yet been cooked up in large enough quantities. Nevertheless, Hubble observed the planet. Theorists shrugged and continued.

Webb is making it more difficult to shrug. Its Near-Infrared Spectrometer has now discovered evidence that planet-forming disks can endure in low-metallicity environments, where there shouldn’t be enough material for large planets to form in a timely manner by all conventional measures. Reading the latest papers gives the impression that the field is getting close to one of those awkward times when six beloved models all need to be adjusted at once.

Ji Wang, an Ohio State assistant professor of astronomy, came to a similar conclusion in a different way. He computed the amount of solid material that each of the seven gas giants outside our solar system had vacuumed up during formation using historical data. The figures were startling. Each planet in the sample had accumulated fifty Earth masses of solids on average. He contends that only when a protoplanetary disk is younger than two million years old is that type of inventory accessible. The total budget of solids in our solar system was most likely between thirty and fifty Earth masses. In other words, those exoplanets were wealthy and had an early start.

The array of radio dishes high in Chile’s Atacama Desert, known as ALMA, is the other recent surprise. Astronomers monitored thirty planet-forming disks over a range of ages using the AGE-PRO survey, and they discovered something surprising: gas disappears from these disks more quickly than dust. Young disks have a shorter window than anyone thought for a true gas giant to bulk up because they first blow off their gas violently and then more gradually. One of the co-investigators, Ilaria Pascucci, pointed out that part of the reason the image took so long to appear was because observing gas requires a lot more telescope time than observing dust.

The frequency with which this occurs in astronomy is difficult to ignore. After a theory has been refined through twenty years of use, one strange detection suddenly doesn’t fit. One of them was the “impossible” planet in Messier 4. Another is the inventories of dust-rich exoplanets. Nothing in our own backyard quite compares to hot Jupiters, those sweltering giants whipping around their stars in days or even hours, which were a sort of insult to the old picture. The discovery of HIP 67522 b orbiting a star that is only seventeen million years old suggests how early the migration story may start.

It’s still unclear exactly what all of this could mean. Perhaps the slow core-accretion route was less common in the early universe than gravitational instability, where disks broke apart into giants in a single, dramatic collapse. Perhaps both routes are active, and the equilibrium changes under circumstances that astronomers have not yet figured out how to quantify. Here, there is a genuine ambiguity that is almost invigorating. As we watch this develop, it seems less like a field in crisis and more like one that has quietly come to terms with the fact that the universe got a head start on planet-building without bothering to explain how.