The history of the solar system is a dish best served cold. And it is so very cold on Ultima Thule.
That is the message beaming back to Earth from NASA’s New Horizons probe now that it has completed its historic exploration of a small body in the Kuiper Belt, the sprawling population of dwarf planets and cometlike objects out beyond Neptune. When New Horizons flew past at 12:33 AM Eastern time on January 1, Ultima was a hair over four billion miles from the sun. It is by far the most distant object ever visited by spacecraft, and correspondingly one of the coldest: about 35 kelvins, or nearly 400 degrees below zero Fahrenheit.
At such low temperatures, Ultima (more formally known by its scientific designation, 2014 MU69) preserves its initial, ancient composition. Ultima is also cold in another, more specialized and intriguing way. It is dynamically cold, part of what’s known as the “cold classical” Kuiper Belt, meaning that it circles the sun in a settled orbit that was undisturbed by all the chaotic events that buffeted Earth and other planets as they came together more than four billion years ago.
Prior to the encounter, the great hope of the New Horizons team was that they would see an intact survivor from the solar system’s birth. The first images of Ultima, showing its delicately stacked snowman shape, fully vindicate those hopes. It matches up exactly with models of how clouds of gas and dust around young stars clump together into larger and larger objects—a process that has been well studied in theory, but never observed in reality until now. “We’re looking at one of the first building blocks that came together to form the planets and moons,” says Jeffrey Moore, a research scientist at NASA’s Ames Research Center. “It looks like somebody left it out in the back of God's freezer for the last four-and-a-half billion years.”
Long way to the Kuiper belt
Being left out in the cold is an all-too-familiar feeling for Alan Stern, principal investigator of New Horizons. He had started campaigning for NASA to mount a mission like the one to Ultima in the 1980s, long before anyone even knew Ultima existed. His original goal was to visit Pluto, completing the exploration of the then-nine planets, but the plans went nowhere. Befitting their outsider status, Stern and other like-minded outer-solar system fans called themselves the “Pluto Underground.”
Scientific interest in Pluto exploded in the 1990s, when astronomers discovered it is just one part of the much grander Kuiper Belt. Even so, Stern watched three different mission concepts wither and die. It took another decade, a cancellation, and a rare resurrection before New Horizons launched on January 19, 2006.
The probe’s nominal objective was only to visit Pluto and its moons, but from the start Stern had a second act in mind. “New Horizons is a very healthy spacecraft, and it has the power to operate for another 15 or 20 years,” he says. Why waste a once-in-a-lifetime opportunity to explore more of the Kuiper Belt? A 2015 flyby revealed Pluto as an excitingly dynamic world, but in some ways it is too dynamic: Its active geology (plutology?) has erased much of its early history. A visit to a much smaller, more pristine follow-on target would fill in a huge missing piece of the reconnaissance of the solar system.
The huge catch was that there was no suitable target in astronomers’ catalogues; the New Horizons team would have to find one themselves, something the spacecraft could reach on its dwindling stores of fuel. That meant marshaling the world’s most powerful telescopes, searching for a dim dot of light moving slowly among a thicket of background stars, located in exactly the right spot for an easy interception. Marc Buie of Southwest Research Institute (SwRI) describes his work leading that search. “I spent 10 years looking for an object for New Horizons to go to,” he says with admirable sangfroid. “I’ll let you in on a little secret: When you take photographs through a telescope, they don’t come with arrows on them to tell you what to look at.”
Finally, Buie spotted it. The telltale blip consisted of just 144 photons in a survey image from the Hubble Space Telescope, “but the instant I saw it, I got a chill. I knew it was The One.” As recognition of its importance increased, the object originally catalogued as 1110113Y became Potential Target 1, then Kuiper Belt object 2014 MU69, and most recently it was informally dubbed Ultima Thule (a name which has generated some unfortunate controversy).
When the first high-resolution images of Ultima arrived from New Horizons, all those past agonies vanished from the scientists’ minds, replaced by backslaps and oversize, unshakable grins.
“It’s a snowman!” Stern laughed as he looked at Ultima’s iconic, double-lobed shape. Or in scientific terms, he notes, it is a “contact binary,” two objects that formed separately but are now held together by their mutual gravity. The larger lobe is three times the size of the smaller one; stacked together, they measure 21 miles long.
That snowman structure is precisely what the New Horizons team was hoping to find in the Kuiper Belt. “The number-one thing I wanted to get out of this mission was incontrovertible evidence that we're looking at a primordial, unaltered object,” Buie says. More specifically, he wanted to witness an object frozen time from the moment when small bodies in the solar system began accreting together and building up bigger ones.
On planets and moons all evidence of that process was wiped clean by subsequent geologic activity. Even on relatively primitive comets like 67/P Churyumov-Gerisamenko—the “rubber duck” comet studied until 2016 by the European Space Agency’s Rosetta probe—the surface has been extensively cooked and altered by the sun’s heat. Moore breezily dismisses such objects as “post-toasties.”
On Ultima it is as if the birth of the solar system happened yesterday, with two planetesimals (the first large conglomerations of rock, dust and ice to coalesce around a young star) still lumped together right before New Horizon’s digital eyes. “I looked at it and thought, ‘I see accretion happening’,” Buie says. “This is going to revolutionize our view of where we came from and how this whole process works.”
Do you want to build a planet
In fact, the revolution is already well underway. On the first three days after the Ultima flyby, the members of the New Horizons analysis teams huddled near mission control at Johns Hopkins University Applied Physics Laboratory to make sense of the first data downloads. After all those decades of waiting, Stern is eager to get going. “We will begin writing our first science papers next week!” he promises. But really, nobody was waiting even that long to start digging into the lessons from Ultima.
One of the first orders of business is interpreting how Ultima’s two lobes came together so gently, with a tidy little necklace of brighter material rimming the place where they meet. Bill McKinnon, a planetary dynamics expert at Washington University in Saint Louis, notes objects in the Kuiper Belt are moving so slowly that typical encounter velocities would be around 600 miles per hour, which limits the amount of damage an impact can create.
Ultima apparently came together even more lightly than that. Most likely the two lobes initially formed as separate bodies in orbit around each other, spiraling together until they touched. “It would have been like a parking lot fender bender, walking speed. Also, these are probably very porous objects; they have energy-absorbing bumpers on them,” McKinnon says. As a result, the two lobes stuck together rather than shattering when they touched.
The jammed-together lobes of Ultima provide the most direct evidence yet of the standard hierarchical theory of how planets form: First dust grains stick together into pebbles, which gather into planetesimals. Those accrete into protoplanets, continuing all the way up to bigger things like Earth and Jupiter.
Another notable detail is that, other than the difference in size, the two lobes of Ultima look extremely similar in shape and color. “That’s also consistent with forming as a result of the merger of two objects that formed close together in the protoplanetary disk,” says Silvia Protopapa of the SwRI. In such a chemically and dynamically cold region two neighboring planetesimals should have aggregated from an essentially identical mix of raw materials.
Information still stored onboard New Horizons will reveal much about exactly what those materials are. “We’ll be looking for ammonia, water, carbon monoxide and organics,” Protopapa says. Ultima’s overall red tint already suggests it is covered with tholins (organic compounds processed by eons of radiation), similar to what is seen on many other outer-solar system objects, including Pluto’s moon Charon.
Moore is relieved, if maybe a little disappointed, that nothing about Ultima directly confounds expectations — as would be the case if, say, the two lobes had totally different colors. “To zeroth order, our ideas of how these things formed seem to be vindicated,” he says. “But there's a whole range of specific explanations that people had. It's like going into a restaurant, you can choose from any number of things on the menu. Now that we see Ultima, we know what the good choices on the menu are.”
Ultima's next act
These rapid-fire readings of Ultima are all the more impressive considering they were made by highly caffeinated scientists operating on little or no sleep, working with an initial download of just 1 percent of the total data New Horizons collected during its flyby.
Perhaps the most egregious missing data element so far is a clear look at the topography of Ultima: We can see it but we can’t really feel it. The best views released so far were taken with the sun almost directly behind the spacecraft, making it difficult to distinguish hills from depressions on the alien landscape below. Significantly sharper pictures, including ones showing deep shadows, are still sitting on New Horizons’ solid-state recorders. “It’ll take 20 months to empty the recorders of all the data we’ve taken, including hundreds of images and spectra,” Stern says. Blame the extremely limited bandwidth of a spacecraft six light-hours from home.
As the stored information trickles in, you can expect a lot more history lessons. One big hanging question for the planetary scientists is how Ultima’s individual lobes themselves were assembled. Somewhat surprisingly, there’s no unambiguous evidence for craters. There certainly are things that resemble craters, but in the calm environment where Ultima formed those poorly resolved formations might actually be hills that were built up rather than gouged-out pits. “We may find that the texture is dominated by slow-motion [accretionaary] processes,” Moore says. If so, that would reveal a lot about the early stages of planetary formation.
With that thought in mind, Cathy Olkin of SWrRI, a deputy project scientist on New Horizons, is eager to examine how the composition of Ultima varies from spot to spot. Spectral measurements from New Horizons should make it possible to identify the individual splats from ancient impact that stuck to the surface like flies on a windshield. Then she’ll do a census of the splats to understand the objects that created them. “So we will be able to look at the smallest particles in the Kuiper Belt, a population we could not see any other way,” she says.
Perhaps the most subtle but significant surprise about Ultima is that we were able to reach it at all. Recent studies of newborn stars using the Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile show many of them seem to be forming planets across a much larger scale than what we see in our solar system, with their versions of the Kuiper Belt taking shape at far greater distances. The vast distance between our sun and Ultima does not look so vast in comparison.
McKinnon throws out another possible history lesson in there: The outer edge of our solar system may have been clipped off by radiation from neighboring stars at a very early stage, he suggests, well before even Ultima formed. “It’s probably defined by the other stars in the sun’s birth cluster, its lost brothers and sisters,” he says.
In the debate over whether our solar system is normal or a cosmic outlier, then, Ultima tips the balance a tiny bit further in the weirdo direction. “Then you consider that the sun is a single star, which is not the norm, and solar-type stars are not the most common stars,” McKinnon says. “You add it up and you think, ‘We’re not exactly a garden variety planetary system.’”