Inner Workings: Was Jupiter Born Beyond the Current Orbits Of

INNER WORKINGS

Was Jupiter born beyond the current orbits of

Neptune and Pluto? INNER WORKINGS

Ken Croswell, Science Writer

Ancient people named the planet Jupiter well. Both gravity stunted the growth of newborn Mars, sculpts its brilliance and its slow, regal movement across the the asteroid belt today, and may even help protect sky evoked a king among gods. Today we know much Earth from catastrophic comet impacts. more about the influence of Jupiter, a planet boasting But how did such a behemoth arise? Conventional more than twice as much mass as the solar system’s theory says that Jupiter formed more or less where it other planets put together. Jupiter’stremendous is now, about five times farther from the Sun than

A new theory suggests that Jupiter formed its core far from the Sun, then moved inward. Image credit: Hubble Space Telescope – NASA, ESA, and Amy Simon (NASA Goddard).

Published under the PNAS license.

www.pnas.org/cgi/doi/10.1073/pnas.2011609117 PNAS Latest Articles | 1of4 Downloaded by guest on September 25, 2021 condense and make planets, whereas at larger distances water molecules, formed from plentiful hydrogen and oxygen, freeze and help create much bigger worlds. Thus, astronomers suspected that the strange close-in giant planet developed farther from its star and then, through interaction with the gas and dust in the disk, spiraled inward. Scientists later applied this idea of planetary mi- gration to our solar system. In the Nice model, named for a city in France where the theory came together, the Sun’s giant planets were once so close together that their gravitational pulls tugged on one another and hurled Uranus and Neptune away from the Sun until they reached their present distances (3). The new theory doesn’t change the Nice model, but it does propose that, while still an infant, Jupiter made a much greater excursion across the solar system. Moreover, whereas the Nice model was silent on Jupiter’s origin, the new theory attacks that issue head on, focusing on a peculiarity in the planet’s atmosphere: It’s rich in nitrogen. “It’s actually something that started bothering me a few years ago,” Öberg says. “I can’t think of any way to explain it if Jupiter formed where it currently is sitting.” Molecular nitrogen is normally a gas, and most gases don’t glom onto planets in large amounts; only Nascent giant planets cut dark circular gaps in the protoplanetary disk around solids—or gases in their frozen state—do. But nitro- the newborn star HL Tauri. In this image from ALMA, the yellow disk’s edge, gen freezes at temperatures far colder than those that ’ located inside the second dark gap, is about the size of Neptune s orbit. The giant prevailed in the protoplanetary disk at Jupiter’s pre- planets sculpting dark gaps in the orange-red region are much farther out from the ’ star than Pluto is from the Sun. Image credit: ALMA (ESO/NAOJ/NRAO). sent position, which means the planet shouldn t have much of the element. Indeed, to find nitrogen as an ice, you have to travel to places such as Neptune’smoon Earth is. At that distance, the disk of gas and dust that Triton or far-off Pluto. In 1995, however, the Galileo swirled around the young Sun was dense enough to probe dove into the Jovian atmosphere and measured give birth to the planetary goliath. a nitrogen-to-hydrogen ratio three to four times higher In 2019, however, two groups of researchers un- than the Sun’s. The larger ratio suggested that Jupiter aware of each other’s work—one in America (1), the had somehow acquired nitrogen in solid form. other in Europe (2)—proposed a literally far-out alter- In both America and Europe, this was the starting native: Jupiter got its start in the solar system’s hinter- point for the new theory. In each case its conception lands, probably beyond the current orbits of Neptune arose from a conversation. Öberg regularly visits with and Pluto, and then moved inward. Harvard planetary scientist Robin Wordsworth. “I “It’s the most fun I’ve had with a paper for some expressed my frustration with not being able to form time,” says Karin Öberg, an astronomer at the Harvard- Jupiter where it currently is,” Öberg recalls. Smithsonian Center for Astrophysics in Cambridge, MA, Meanwhile, a similar discussion was unfolding in The and one of the theory’s originators. “You can explain it to Netherlands. “We started thinking: Well, OK, how can almost anyone in a couple of minutes.” The theory may we get that extra nitrogen into Jupiter?” says Arthur be straightforward, but its consequences are profound: If Bosman, an astrochemist then at Leiden University in it’s right, the solar system’sbiggestplanetwasborn The Netherlands and now at the University of Michigan some 10 times farther from the Sun than it now is, which at Ann Arbor. He and his officemate, Alex Cridland, had means that some of the other giant worlds in our solar both noticed the planet’s high nitrogen level. system and beyond likely arose at vast distances from And it’s not just nitrogen. Argon, a noble gas, also their stars and then moved to their current locations. freezes at frigid temperatures, and it too is abnormally abundant on Jupiter. Ditto for two other noble gases, Nitrogen Provides a Solid Clue krypton and xenon. The idea that planets can rove from far to near was itself once radical. But in 1995, this notion became Ahead of Its Time standard thinking after the shocking discovery of a In a 1999 article, other researchers had noted these giant planet closer to its sun than Mercury is to ours. oddities, proposing possible explanations, including Planets form in so-called protoplanetary disks, pancake- one which suggested that Jupiter migrated to its shaped gatherings of gas and dust that revolve around present position from beyond Neptune’s current orbit newborn stars. Near the star, the protoplanetary disk is (4). But the idea was ahead of its time. For one thing, hot, so only rock and heavy elements such as iron can conventional theory then said planets built up from

2of4 | www.pnas.org/cgi/doi/10.1073/pnas.2011609117 Croswell Downloaded by guest on September 25, 2021 dust grains that slowly grew larger, eventually leading David Stevenson, a planetary scientist at the Cal- to asteroid-sized bodies which later merged; but this ifornia Institute of Technology in Pasadena, is more process was slow in the tenuous outskirts of the pro- skeptical. “It’s not that difficult to come up with a va- toplanetary disk, so a planet as large as Jupiter riety of stories, given our lack of understanding,” he wouldn’t have had enough time to arise there before says. Furthermore, even if Jupiter’s core arose far from the disk of gas and dust vanished. the Sun and thus acquired high levels of nitrogen and Since then, however, two new developments have made the bold idea more plausible. First, the Atacama Large Millimeter/submillimeter Array (ALMA) radio “It’s such a paradigm shift that I was really happy to see telescope started operating in Chile and began imag- that someone else was thinking the same way.“ ing protoplanetary disks around newborn stars. The first — such image, of a young star named HL Tauri, came out Arthur Bosman in late 2014. The picture was revolutionary. “It was difficult to argon, he says it’s hard to get those elements up to the believe that was a real image and not an artist’s im- planet’s atmosphere, where the Galileo probe detected pression,” Öberg says. It showed a disk of gas and dust them; after all, the core constitutes only a small fraction of with dark circular grooves. The grooves were pre- the planet. To solve this problem, Öberg and Wordsworth sumably cut by unseen giant planets orbiting the star. invoke a giant impact on Jupiter to lift the elements from Moreover, some of the grooves were much farther out the core to the atmosphere (7). “Well,” Stevenson says, than Neptune and Pluto are from the Sun, a sign that gas “you can use a giant impact to do anything.” giants such as Jupiter can form even in remote regions. The second advance was theoretical. In recent Et Tu, Saturn? years, researchers have formulated a new idea of Still, the theory for Jupiter’s distant birthplace does planet formation called pebble accretion, which posits make testable predictions. The first concerns oxygen. ’ — that a giant planet s core builds up as small objects In the protoplanetary disk, oxygen became part of sili- — pebbles flow past a budding core and stick to it (5, cates and water. These substances solidified closer to “ ” 6). This process is orders of magnitudes faster than the Sun than nitrogen and argon did and also existed at the conventional scenario, Bosman says. As a result, a greater distances, where the temperature was even ’ giant planet s core can spring forth even in the sparse colder. Thus, if Jupiter’s core formed beyond Pluto’s outer reaches of a protoplanetary disk. Then, when the current orbit, the giant planet incorporated all of these core grows massive enough, its gravitational pull attracts elements as solids and should have similar overabun- a topping of hydrogen and helium, which were the two dances of them all with respect to the Sun. Both articles main gases in the protoplanetary disk and are the two therefore predict that Jupiter’s oxygen-to-hydrogen dominant elements composing Jupiter today. And if the ratio is three to four times greater than the solar value. planet had arisen in the coldest parts of the disk, Jupiter In February, Cheng Li, a planetary scientist at the should have acquired plenty of nitrogen and argon ice, University of California at Berkeley, and his team de- explaining why the atmosphere is rich in these elements. livered a preliminary verdict. “Their prediction and our Even as the American and European teams were observations agree with each other,” Li says, although readying their papers for publication, neither knew some uncertainty remains. His team reported that the about the other—until Bosman saw the American Juno spacecraft now orbiting Jupiter had measured an team’s paper on ArXiv just a day before he had plan- oxygen-to-hydrogen ratio near the equator between ned to submit his own work. “It’s such a paradigm shift one and five times solar, compatible with the prediction that I was really happy to see that someone else was (8). But this range also means that Jupiter could still thinking the same way,” he says. “On the other hand, have the same oxygen-to-hydrogen level as the Sun, someone else was first, and that was a bit of a bum- which would contradict the new theory. Li’s team aims mer. But that’s how things go in science.” When to use Juno to refine the measurement in coming years. Öberg learned of the other paper, she was “very Both Öberg and Bosman point to another test the happy,” she says. “It sort of gave me some confidence theory faces: Saturn, which should have formed beyond that maybe this idea wasn’t so strange after all.” Jupiter. “That means we should see a similar pattern in Other researchers have noticed the radical new the abundances in Saturn as we’re seeing in Jupiter,” theory. “Ten years ago, people would have just rolled Bosman says. Future probes to the ringed planet can their eyes,” says Jonathan Fortney, a planetary sci- therefore search for high levels of nitrogen and argon. entist at the University of California at Santa Cruz. But Meanwhile, Öberg is pleased with the theory’s thanks to pebble accretion theory, he says, that’sno reception. “I met much less resistance than I expec- longer the case. Although it “sounds a little crazy,” it’s ted,” she says. “The vast majority of reactions have not unreasonable that you could create the core of a been ‘Yeah, that kind of makes sense.’” And she can giant planet beyond Pluto’s current orbit, Fortney says. even explain the theory to her nonscientist neighbors.

1 K. I. Öberg, R. Wordsworth, Jupiter’s composition suggests its core assembled exterior to the N2 snowline. Astron. J. 158, 194 (2019). 2 A. D. Bosman, A. J. Cridland, Y. Miguel, Jupiter formed as a pebble pile around the N2 ice line. Astron. Astrophys. 632, L11 (2019).

Croswell PNAS Latest Articles | 3of4 Downloaded by guest on September 25, 2021 3 S. Ornes, News Feature: Space fossils. Proc. Natl. Acad. Sci. U.S.A. 112, 3849–3851 (2015). 4 T. Owen et al., A low-temperature origin for the planetesimals that formed Jupiter. Nature 402, 269–270 (1999). 5 A. Johansen, M. Lambrechts, Forming planets via pebble accretion. Annu. Rev. Earth Planet. Sci. 45, 359–387 (2017). 6 N. T. Redd, Inner Workings: Newborn stars don’t have enough dust to build planets. What are the missing ingredients? Proc. Natl. Acad. Sci. U.S.A. 116, 7605–7607 (2019). 7 S.-F. Liu et al., The formation of Jupiter’s diluted core by a giant impact. Nature 572, 355–357 (2019). 8 C. Li et al., The water abundance in Jupiter’s equatorial zone. Nat. Astron. 4, 609–616 (2020).

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