INNER WORKINGS INNER WORKINGS Was Jupiter born beyond the current orbits of Neptune and Pluto? 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’s tremendous 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. First published July 1, 2020. 16716–16719 | PNAS | July 21, 2020 | vol. 117 | no. 29 www.pnas.org/cgi/doi/10.1073/pnas.2011609117 Downloaded by guest on September 29, 2021 Earth is. At that distance, the disk of gas and dust that swirled around the young Sun was dense enough to give birth to the planetary goliath. In 2019, however, two groups of researchers un- aware of each other’s work—one in America (1), the other in Europe (2)—proposed a literally far-out alter- native: Jupiter got its start in the solar system’s hinter- lands, probably beyond the current orbits of Neptune and Pluto, and then moved inward. “It’s the most fun I’ve had with a paper for some time,” says Karin Öberg, an astronomer at the Harvard- Smithsonian Center for Astrophysics in Cambridge, MA, and one of the theory’s originators. “You can explain it to almost anyone in a couple of minutes.” The theory may be straightforward, but its consequences are profound: If it’s right, the solar system’s biggest planet was born some 10 times farther from the Sun than it now is, which means that some of the other giant worlds in our solar system and beyond likely arose at vast distances from their stars and then moved to their current locations. Nitrogen Provides a Solid Clue The idea that planets can rove from far to near was itself once radical. But in 1995, this notion became standard thinking after the shocking discovery of a giant planet closer to its sun than Mercury is to ours. Planets form in so-called protoplanetary disks, pancake- Nascent giant planets cut dark circular gaps in the protoplanetary disk around shaped gatherings of gas and dust that revolve around the newborn star HL Tauri. In this image from ALMA, the yellow disk’sedge, ’ newborn stars. Near the star, the protoplanetary disk is located inside the second dark gap, is about the size of Neptune s orbit. The giant planets sculpting dark gaps in the orange-red region are much farther out from the hot, so only rock and heavy elements such as iron can star than Pluto is from the Sun. Image credit: ALMA (ESO/NAOJ/NRAO). condense and make planets, whereas at larger dis- tances water molecules, formed from plentiful hydrogen and oxygen, freeze and help create much bigger worlds. Triton or far-off Pluto. In 1995, however, the Galileo Thus, astronomers suspected that the strange close-in probe dove into the Jovian atmosphere and measured giant planet developed farther from its star and then, a nitrogen-to-hydrogen ratio three to four times higher through interaction with the gas and dust in the disk, than the Sun’s. The larger ratio suggested that Jupiter spiraled inward. had somehow acquired nitrogen in solid form. Scientists later applied this idea of planetary mi- In both America and Europe, this was the starting gration to our solar system. In the Nice model, named point for the new theory. In each case its conception for a city in France where the theory came together, arose from a conversation. Öberg regularly visits with the Sun’s giant planets were once so close together Harvard planetary scientist Robin Wordsworth. “I that their gravitational pulls tugged on one another expressed my frustration with not being able to form and hurled Uranus and Neptune away from the Sun Jupiter where it currently is,” Öberg recalls. until they reached their present distances (3). Meanwhile, a similar discussion was unfolding in The The new theory doesn’t change the Nice model, Netherlands. “We started thinking: Well, OK, how can but it does propose that, while still an infant, Jupiter we get that extra nitrogen into Jupiter?” says Arthur made a much greater excursion across the solar sys- Bosman, an astrochemist then at Leiden University in tem. Moreover, whereas the Nice model was silent on The Netherlands and now at the University of Michigan Jupiter’s origin, the new theory attacks that issue head at Ann Arbor. He and his officemate, Alex Cridland, had on, focusing on a peculiarity in the planet’s atmo- both noticed the planet’s high nitrogen level. sphere: It’s rich in nitrogen. “It’s actually something And it’s not just nitrogen. Argon, a noble gas, also that started bothering me a few years ago,” Öberg freezes at frigid temperatures, and it too is abnormally says. “I can’t think of any way to explain it if Jupiter abundant on Jupiter. Ditto for two other noble gases, formed where it currently is sitting.” krypton and xenon. Molecular nitrogen is normally a gas, and most gases don’t glom onto planets in large amounts; only Ahead of Its Time solids—or gases in their frozen state—do. But nitro- In a 1999 article, other researchers had noted these gen freezes at temperatures far colder than those that oddities, proposing possible explanations, including one prevailed in the protoplanetary disk at Jupiter’s pre- which suggested that Jupiter migrated to its present sent position, which means the planet shouldn’t have position from beyond Neptune’s current orbit (4). But the much of the element. Indeed, to find nitrogen as an ice, idea was ahead of its time. For one thing, conventional you have to travel to places such as Neptune’smoon theory then said planets built up from dust grains that Croswell PNAS | July 21, 2020 | vol. 117 | no. 29 | 16717 Downloaded by guest on September 29, 2021 slowly grew larger, eventually leading to asteroid-sized their eyes,” says Jonathan Fortney, a planetary sci- bodies which later merged; but this process was slow in entist at the University of California at Santa Cruz. But the tenuous outskirts of the protoplanetary disk, so a thanks to pebble accretion theory, he says, that’sno planet as large as Jupiter wouldn’t have had enough time longer the case. Although it “sounds a little crazy,” it’s to arise there before the disk of gas and dust vanished. not unreasonable that you could create the core of a Since then, however, two new developments have giant planet beyond Pluto’s current orbit, Fortney says. made the bold idea more plausible. First, the Atacama David Stevenson, a planetary scientist at the Cal- ifornia Institute of Technology in Pasadena, is more skeptical. “It’s not that difficult to come up with a va- “It’s such a paradigm shift that I was really happy to see riety of stories, given our lack of understanding,” he that someone else was thinking the same way.“ says. Furthermore, even if Jupiter’s core arose far from —Arthur Bosman the Sun and thus acquired high levels of nitrogen and argon, he says it’s hard to get those elements up to the planet’s atmosphere, where the Galileo probe detected Large Millimeter/submillimeter Array (ALMA) radio them; after all, the core constitutes only a small fraction of telescope started operating in Chile and began imag- the planet. To solve this problem, Öberg and Wordsworth ing protoplanetary disks around newborn stars. The first invoke a giant impact on Jupiter to lift the elements from such image, of a young star named HL Tauri, came out thecoretotheatmosphere(7).“Well,” Stevenson says, in late 2014. “youcanuseagiantimpacttodoanything.” The picture was revolutionary. “It was difficult to believe that was a real image and not an artist’s im- Et Tu, Saturn? pression,” Öberg says. It showed a disk of gas and dust Still, the theory for Jupiter’s distant birthplace does with dark circular grooves. The grooves were pre- make testable predictions. The first concerns oxygen. sumably cut by unseen giant planets orbiting the star. In the protoplanetary disk, oxygen became part of sili- Moreover, some of the grooves were much farther out cates and water. These substances solidified closer to than Neptune and Pluto are from the Sun, a sign that gas the Sun than nitrogen and argon did and also existed at giants such as Jupiter can form even in remote regions. greater distances, where the temperature was even The second advance was theoretical. In recent colder. Thus, if Jupiter’scoreformedbeyondPluto’s years, researchers have formulated a new idea of current orbit, the giant planet incorporated all of these planet formation called pebble accretion, which posits elements as solids and should have similar overabun- ’ — that a giant planet s core builds up as small objects dances of them all with respect to the Sun. Both articles — pebbles flow past a budding core and stick to it (5, therefore predict that Jupiter’s oxygen-to-hydrogen “ ” 6). This process is orders of magnitudes faster than ratio is three to four times greater than the solar value.