NEWS FEATURE

“Celestial snowman” starts to reveal its secretsThe distant rock has offered clues about planet formation and the state of the early

solar system.

Nola Taylor Redd, Science Writer

Within the cloud of icy rocks at the edge of the solarsystem lie objects that have remained virtually un-touched since their formation more than four billionyears ago. Last January, NASA’s New Horizons space-craft made the first flyby of one such primitive sample,an object known as 2014 MU69 and nicknamed “Ul-tima Thule” (although that label has proved controver-sial*). After New Horizons’ successful flyby of Pluto in2015, researchers were keen to study a primordialbody that was within the craft’s reach. With MU69, thatdream became a reality. The tiny object, one of onlythree possible destinations discovered after the mis-sion launched, turned out to be an incredible target. “Ireally think we hit the jackpot,” says New Horizonsprincipal investigator Alan Stern, of the Southwest Re-search Institute (SwRI) in Boulder, CO.

At first glance, MU69 looked much as researchershad imagined a pristine Kuiper Belt object (KBO) wouldappear, with a dark surface, rich in water ice and organicmaterial, and relatively unscarred by craters. But whenthey looked closer, it offered plenty of surprises. Fromits shape to its spin to its composition, the distant rock isproviding planetary researchers with a wealth of in-formation about the conditions in the vicinity of the sun4.5 billion years ago, and it’s even helping solve adecades-old puzzle about how the planets formed.

A Squashed SnowmanResearchers revealed their first results at the annualLunar and Planetary Sciences conference in theWoodlands, TX, in March.† MU69 stretches roughly35 kilometers (20miles) from tip to tip and spins on its axisevery 15.9 hours. Unlike most solar system inhabitants,

Researchers have started to map out the various geological features of 2014 MU69, including troughs (black lines),scarp crests (notched lines), a feature circling unit mh dubbed “The Road to Nowhere,” and a large crater (lc) dubbedthe Maryland Crater. Here, the magenta areas labeled pm are patterned material; green areas labeled rm are roughmaterial, and the blue areas labeled um are undifferentiated material. Image credit: Wikimedia Commons/NASA/JohnsHopkins University Applied Physics Laboratory/Southwest Research Institute/ESA.

Published under the PNAS license.*After a naming campaign led by the SETI Institute labeled 2014 MU69 “Ultima Thule,” some astronomers and others objected. The age-old term,which essentially means the farthest point or far to the north or a land beyond known lands, was co-opted by the Nazi party during Adolf Hitler’s riseto power—part of that party’s vision of a mythical land of Aryan purity. But others, including Alan Stern of SwRI, supported the label, suggesting thatit evoked exploration and that researchers shouldn’t let the Nazis hijack the term.

†S. Porter et al., “A contact binary in the Kuiper Belt: The shape and pole of (486958) 2014 MU69” in Proceedings of the 50th Lunar and PlanetaryScience Conference. LPI Contribution No. 2132, id.2737 (2019).

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which spin with their equator aimed at the sun,MU69 lies on its side, pole pointing sunward. Thetwo lobes are roughly the same color, a reddish huesimilar to other KBOs observed from Earth, whichlikely comes from materials known as tholins, formedwhen radiation from the sun modifies organicmolecules.

Although the first image of MU69 suggested a pairof spheres, later observations revealed a big surprise.Thule is very thin, and Ultima even flatter: only 7 kmthick and 22 km across. “If it’s not the flattest body in thesolar system, it’s up there,” says team member WilliamMcKinnon, of Washington University in St. Louis, MO.

Most solar system objects this size are roughlyspheroidal, with only a handful of exceptions. About ayear before the flyby, Jeff Moore, New Horizons co-investigator at NASA Ames Research Center in MoffettField, CA, had discussed the possibility that MU69could be like some of the exceptions, showing imagesof the flattened moons of Saturn in a talk to fellow re-searchers. Forming in the disk of Saturn’s rings, ratherthan a more spherical cloud of particles, some of thesemoons have the appearance of a squashed walnut.“People scoffed,” Moore says. “They said, ‘whateverwe’re seeing isn’t going to look like that.’”

But then New Horizons revealed the flattenedhamburger lobes of Ultima and Thule. The similarity toSaturn’s moons suggests that they, too, were createdin a swarm of particles spinning fast enough to form adisk. What’s more, the original axes of rotation forUltima and Thule are almost parallel, only a few de-grees apart.‡ The similar orientation suggests that the

pair formed from a single cloud of material beforejoining together.

The final clue came from how the two lobes haveretained their rounded shapes. If Ultima and Thule hadslammed together at collision speeds typical of theKuiper Belt, a few hundred meters per second, thatwould have been enough to deform or shatter them.Instead, they seem to have drifted gently into oneanother. Stern compares them to docking ships thatcame together without the violence common in therest of the solar system.

This gentle docking maneuver makes sense if thepair started as part of the same cloud of material. Thenthey would have had similar speeds and wound uporbiting one another. As the pair orbited, other nearbymaterial could have been ejected, reducing the sys-tem’s angular momentum and pulling the pair to-gether; or something else entirely could have causedthe two to become one. “That’s the bit I don’t thinkpeople have worked out yet,” says planetary forma-tion researcher Harold Levison (SwRI), who is not onthe New Horizons team but was present at the flyby.

A slow collision would mean that MU69’s interiorprobably remains loosely packed, a fluffy aggregate ofice and rock similar to the interior of comets (1). Buthad the lobes instead collided at high speed, theirinteriors would have been compacted.

Meanwhile, small lumps on the surface may beremnants from the birth cloud, the last pieces to accreteonto the KBO. Co-investigator Will Grundy, of LowellObservatory in Flagstaff, AZ, says that the lumps appearto be homogeneous across the pair, again suggestingthat they originated in the same cloud of debris.

Missing CratersThe early life of MU69 was a flurry of excitement. Thetwo lobes formed and merged, and residual heat fromthe formation may have powered a bit of surface ac-tivity. But after half a billion years, most of the com-motion had subsided. MU69 is too far out for the sun’sheat to change its surface. Apart from the occasionalcollision with a small piece of debris, the celestialsnowman has remained essentially the same for thelast four billion years. Although that may sound like aboring existence, it’s exactly why researchers wantedto visit the Kuiper Belt. Visiting MU69 is almost liketraveling back in time to the birth of the solar system—

with a few caveats.When New Horizons flew by Pluto, researchers

were stunned to see how few intermediate-sized cra-ters§ covered the surface of the dwarf planet and itslarge moon, Charon. MU69 shows the same paucity,with only a handful of midsized excavations. “We seesome very nice-looking craters on MU69, just not verymany of them,” says co-investigator Kelsi Singer (SwRI).

In 2006, NASA launched the New Horizons spacecraft, here being inspected atthe Kennedy Space Center in Merritt Island, FL, a year prior to launch. After its2015 Pluto encounter, New Horizons headed for MU69. Image credit: NASA.

‡W. McKinnon et al., “A pristine “contact binary” in the KuiperBelt: Implications from the New Horizons encounter with 2014MU69 (“Ultima Thule”)” in Proceedings of the 50th Lunarand Planetary Science Conference. LPI Contribution No. 2132,id.2767 (2019).

§K. Singer et al., “Impact craters on 2014 MU69: Implications forthe geologic history of MU69 and Kuiper Belt population size-frequency distributions” in Proceedings of the 50th Lunarand Planetary Science Conference. LPI Contribution No. 2132,id.2239 (2019).

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The dearth reveals a lot about MU69’s virtuallyunchanged environment over the last 4.5 billion years.Objects that would generate such craters would betens to hundreds of meters across. These could formdirectly from gas and dust or be chipped off a largerbody in a collision, but the shortage of craters suggeststhat neither of these processes prevailed in the KuiperBelt. “It looks like it just didn’t initially make a lot of smallobjects and just kept not making them,” Singer says.New Horizons’ resolution made it difficult to definitivelyidentify craters, but there aren’t many potential midsizedscars that could have come from medium impacts. Thelargest potential crater scars Thule, stretching abouthalfway across the lobe’s diameter. Nicknamed Mary-land, the crater has raised questions on the timing of itsformation.

Although the colliding object would have beenabout 10 times smaller than the crater it excavated,the collision would still have been significant. One wayto avoid breaking Ultima and Thule apart would be tohave Maryland smash into the smaller lobe before thepair came together, but it’s hard to determine whichcame first. Based on her previous work with impact cra-ters, Singer doesn’t think that the Maryland smackdownwould have been that destructive. Moore agrees,pointing out that the survival of MU69 would dependon its interior. The porous interiors of comets act likeStyrofoam, absorbing the impact energy of a collision,he says. If MU69 is similarly porous on the inside, itcould have absorbed the blow from the impact withoutflying apart. “The impact that formed Maryland mighthave had essentially no effects on the two lobes of thebinary,” Moore says.

A comet-like interior could also explain somegeological differences between the two lobes. Al-though Ultima is relatively smooth, Thule has morefeatures on its surface. Moore thinks many of thosefeatures may have formed when the two lobesmerged. He says that the orientation of grooves, re-gional uplifts, and surface depressions on Thule make

them more likely to be tied to its gentle bump withUltima than the hit from Maryland. Because it’ssmaller, Thule would have been harder hit by the im-pact than the larger Ultima. “Thule basically mighthave been split at the seams,” Moore says, allowingmethane, nitrogen, and other volatile material toescape its interior. Movement of that material couldhave driven the processes that shaped Thule’slandscape.

Planet BuildingFully understanding the images of MU69 will takesome time, but the first view is already helping shedsome light on how planets formed. As rocky piecesmove through the gas and dust around a newbornstar, they should collide with one another to formlarger and larger bodies. It is thought that theywould have built up to objects like MU69, which thencame together to form larger objects known asplanetesimals, which in turn built the planets, fromMercury to Pluto (2). But there is a missing chapter tothis story.

Simulations can turn bits of dust into so-calledpebbles up to a meter in size and transform larger,kilometer-sized objects into planetary cores, but theyhad a problem in the middle, going from small pebblesto rocks kilometers across (3). The computer modelssuggested collisions would blow apart growing plane-tary embryos in this size range. Known as the meter-sized barrier, the problem has plagued planetary for-mation for decades.

In recent years, new theories have emerged to helpwith the problem. One idea, known as the streaminginstability (4), suggests that gas in the disk createsdrag, forcing these small objects to concentrate intoclumps. Gravity then pulls the clumps together tocreate kilometer-sized objects in a matter of tensof thousands to hundreds of thousands of years,depending on the solar distance no midsized materialis necessary. A popular extension of this theory, known

Astronomers are starting to piece together how Ultima and Thule joined together, as explained in this graphic composed by postdoc JamesTuttle Keane at California Technical Institute in Pasadena. Image credit: NASA/JHUAPL/SwRI/James Tuttle Keane.

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as pebble accretion, suggests that smaller objectscontinue to fall onto the kilometer-sized objectsto rapidly build the cores of giant planets.

Both the pancake shape of MU69 and its dearth ofmidsized craters seem to validate these theories. Iflarge objects form via the streaming instability, therewill be few projectiles in the range of tens of meters tocreate those craters. And according to David Nesvorny,who models early solar system formation at SwRI, theflat lobes of MU69 are also telling. The shapes are tooflat to have come from the initial spin of a cloud ofmaterial. Such a large cloud would have had too muchangular momentum to hold together and, instead,split into two objects. A loose collection of material inthe outer solar system could have gathered together,as proposed by the streaming instability. Gravitywould have pulled most of it together, creating ob-jects flatter than a sphere but not quite pancake-like,then pebble accretion would have piled most of theremaining pieces on top, creating the flattened lobesvisible today. Eventually, Ultima and Thule wouldhave drawn closer together, gently docking withone another.

The researchers haven’t yet simulated the processfor MU69 specifically, although they hope to. If correctit could solve the problem of how to build a planet.“That meter-sized barrier that we all stressed out

about, we just jumped right over it,” says astronomerKevin Walsh of SwRI. “We all suspected somethinglike that but these observations from the Kuiper Beltreally seem to clinch it.”

Previous models have always worked with spher-ical bodies rather than pancakes, in part because oflimited computer power, according to McKinnon.“[Simulations] might have tracked angular momen-tum, but they certainly didn’t try to build shapes,” hesays. The new results fromMU69 have likely changedthat. “Now I’m sure people will [attempt to simulatefinal shapes] because we have the capabilities to dothis sort of stuff.”

MU69 may live at the edge of the solar system,but the processes that formed it probably workedthroughout, Nesvorny says. Earth and other planetslikely formed from similarly flattened objects. And theprocess is not limited to our planetary neighborhood.“Elsewhere in exoplanetary systems, it might be quitecommon to form pancake and contact binaries,” he says.

New Horizons will be sending back data onMU69 until the end of 2020, and plenty of furtherstudy will be needed, but it already has provided awealth of insight about both the Kuiper Belt and theentire solar system. “We had no right to expect thatwe would get so much from this one small body,”Stern says, “but we have.”

1 S. A. Stern et al., Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object. Science 364, 6441 (2019).2 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).

3 S. Weidenschilling, Aerodynamics of solid bodies in the solar nebula. Mon. Not. R. Astron. Soc. 180, 57–70 (1977).4 A. Youdin, J. Goodman, Streaming instabilities in protoplanetary disks. Astrophys. J. 620, 459 (2005).

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