Unexplained patterns in the colors of certain space rocks suggest scientists still have much to learn about the solar system’s origins and early evolution

Colorful Asteroids Near Neptune Reveal a Solar System Conundrum

Planets and moons often get the spotlight, but there is much to learn about our solar system from its vast numbers of smaller bodies such as comets and asteroids. That is the mind-set of David Jewitt, a planetary scientist at the University of California, Los Angeles, who found a cosmic conundrum when he observed a group of asteroids near Neptune.

These objects are conventionally thought to derive from another group of asteroids known as the Kuiper Belt, which forms a ring well beyond Neptune. But Jewitt showed the asteroids near the planet have distinctly different colors than their supposed parents in the belt. Stranger still, the Neptunian asteroids are so far from the sun that their surfaces should have stayed almost pristine since birth, scarcely altered by our nearest star’s warming rays—and yet they appear remarkably similar to a group of sunbaked asteroids near Jupiter. These puzzling observations—asteroids that look nothing like their putative parents in the sedate solar hinterlands but strongly resemble their supposed relatives that have been toasted by the sun—call into question scientists’ best theories for the origin and evolution of the solar system.

Asteroids, which range in size from pebbles to large metropolitan areas, are relics of our star system’s formation just over 4.5 billion years ago. They represent material that was not swept up into planets, and they are scattered throughout the solar neighborhood today. Millions orbit the sun between Mars and Jupiter in the Asteroid Belt, but others known as Trojan asteroids occupy the same orbits as Jupiter and Neptune, albeit in twin clumps either 60 degrees ahead of or behind their host planets. (Mars also has a few Trojans, but they are an “inconsequential” population, Jewitt says.)

Astronomers have long debated the origin of these so-called Jovian and Neptunian Trojans. Are they holdouts from the disk of gas and dust that formed the planets, primordial hangers-on that from the start have accompanied their respective planetary hosts? Or did they form farther afield, perhaps in the distant Kuiper Belt, only to later fall in toward the sun to be gravitationally captured by Jupiter and Neptune?

Jewitt argues the Trojans formed near Jupiter and Neptune as the solar system coalesced from its birth disk—an epoch in which asteroids would have been more plentiful and moving at lower relative velocities compared with each planet, facilitating their capture. “If you throw me a ball, I can catch it,” he says. “If you shoot a bullet at me, I’m not going to catch it.”

But numerous independent simulations of the first few hundred million years of the solar system’s history have painted a dramatically different picture. These simulations suggest the giant planets—Jupiter, Saturn, Uranus and Neptune—underwent drastic changes in their orbits, each at turns moving millions of kilometers in toward the sun or out toward the Kuiper Belt. According to most theories, the ensuing gravitational ruckus from this “planetary migration” should have swept away any Jovian and Neptunian Trojans that formed near their host worlds. Consequently, many astronomers suspect the Trojans must be migrants from the Kuiper belt flung in to their present-day locations during this period of interplanetary tumult.

To resolve the mystery of the Trojans’ origins, Jewitt conducted what he calls the “dumbest test”—he compared the colors of the Trojans with those of Kuiper Belt asteroids. If the colors matched, he says, then one could conclude that both populations likely came from the same place—the Kuiper belt.

The colors of a large number of Jovian Trojans were already known, so Jewitt used one of the twin 10-meter Keck telescopes in Hawaii to study the Neptunian Trojans. Such an enormous telescope is necessary to collect the light from these objects, which are six times farther away than the Jovian ones and roughly 10 trillion times fainter than the full moon. Jewitt measured the optical colors of six Neptunian Trojans and combined his data with published measurements of seven others.

He found the colors of the Jovian and Neptunian Trojans were practically identical. This similarity suggests each planet’s Trojans were either captured from a common source or exposed to the same surface-modifying processes, Jewitt says. “This is really puzzling,” says Chad Trujillo, an astronomer at Northern Arizona University who was not involved in the research. “How could the two populations, the Neptunian and Jovian Trojans, have similar surfaces?” says Trujillo, who measured the colors of four Neptunian Trojans used in Jewitt’s study. “They are really far apart and have very different temperatures.”

To Jewitt’s surprise, the data also revealed both groups of Trojans to be far less red than the asteroids in the Kuiper Belt, challenging the theory that the two populations originated from that far-off clime. Perhaps, Jewitt reasoned, each planet’s Trojans had in fact been imported from the Kuiper belt, but had experienced surface alterations at some point in their histories that imbued them with nearly identical colors.

This work-around, however, is not without flaws, due to Jupiter’s and Neptune’s vastly different distances from the sun, Jewitt notes. Neptune is much farther out than Jupiter—and therefore much colder. Temperatures there average about a frigid–223 degrees Celsius—too low to drive any thermally induced resurfacing, he says. In contrast, Jupiter’s vicinity hovers at a relatively balmy–148 degrees C—warm enough to liberate gas from ices on the planet’s Trojans. “You have a big burst of gas, which drives the ejection of material from the surface,” he says. This resurfacing might alter an asteroid’s color. “The conundrum is that [Neptunian Trojans] don’t look the same as their source population, but there’s no obvious process by which their surfaces could be modified,” says Jewitt, who presented these results in October at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences. Earlier this year he also published his findings in The Astronomical Journal.

Identifying a process that could resurface Neptunian Trojans would offer a potential solution to the mystery. One possibility involves collisions—perhaps asteroids crashing together change their colors as the impacts redistribute material across their surfaces. But this proposal has its problems: The rate of impacts is predicted to be much too small to produce any observable signature. “We wouldn’t expect to see color changes,” Jewitt says.

“These findings are very intriguing,” says Alex Parker, a planetary astronomer at the Southwest Research Institute who did not take part in the work. But the mystery of the Neptunian Trojans may run even deeper, says Parker, who measured the colors of three Neptunian Trojans used in Jewitt’s investigation. Earlier this year a study submitted for publication found a Neptunian Trojan with a very red surface, hinting “the story must be even more complex than the one presented by Jewitt,” he says. “It may be that the Neptune Trojans do not all share a common origin.”

Additional observations of large numbers of Neptunian Trojans are clearly necessary, researchers agree. Taking a closer look at Jovian Trojans would be wise, too—and an interplanetary NASA mission, Lucy, is launching in October 2021 to do just that. Jewitt and other astronomers are also eagerly awaiting the first observations from the Large Synoptic Survey Telescope (LSST), an 8.4-meter telescope in Chile that will twice weekly survey the entire night sky beginning in the early 2020s. “We’ll get a lot of Trojans and Kuiper Belt objects measured,” Jewitt says. “LSST will do a number on the whole solar system.”