In the vastness of outer space, meetups between celestial bodies can be few and far between. But when they do happen, even the briefest affairs can leave behind permanent scars.
For scientists probing the Solar System’s origins, that’s all good news: Craters on large, rocky bodies immortalize collisions between objects in space—and might comprise a cosmic fossil record stretching billions of years into the past.
Such an archive exists atop the pockmarked surfaces of Pluto and its moon Charon, dating back to the early days of our Solar System, according to a study published today in the journal Science. Within these ancient craters is a time capsule of the past—and now, with this newly generated map, that yarn is finally beginning to unspool.
The study, which analyzed images taken by the New Horizons spacecraft as it hurtled past Pluto in 2015, shows that neither Pluto nor Charon has been bombarded by objects less than a mile or so in diameter. This unexpected deficit hints that the planets in our Solar System were seeded by clouds of dust rapidly collapsing inward, rather than the incremental accumulation of particles gradually mashing together into bigger and bigger bodies.
“This is impressive and dramatic,” says Renu Malhotra, a planetary scientist at the University of Arizona who was not involved in the study. “It could require rethinking theories of [what formed the planets].”
It’s been some 4.5 billion years since the eight planets in our immediate neighborhood sprouted from little more than a swirl of interstellar dust and gas—and in that time, a lot has changed. But a time capsule of the Solar System’s cataclysmic conception lives on. Out in the Kuiper Belt, the region beyond Neptune’s orbit, frigid temperatures have preserved bits and pieces of the raw ingredients thought to have once formed the planets.
Scoping out these planetary starter kits could yield important insights into how the planets first came together, but objects in the Kuiper Belt—especially those of petite stature—aren’t easy to study: Powerful though Earth’s telescopes may be, they still struggle to capture clear images of objects the size of strip malls from 4 billion miles away.
That’s where Pluto and Charon come in. Nestled in the inner fringes of the Kuiper Belt, the pair has a long history of rocky rendezvous with objects that leave craters in their wake. Each blemish commemorates a temporary tryst with another member of the Kuiper Belt—and analyzing these scars can give scientists clues about what struck the surface.
“I think of it as a window into the past,” says study author Kelsi Singer, a planetary scientist at Southwest Research Institute.
During its 2015 flyby, New Horizons snapped high-resolution photos of about 40 percent of the surfaces of both Pluto and Charon. By characterizing the craters and other geologic features captured by these images, the researchers determined that several of these rocky relics were over 4 billion years old, enshrining some of the most ancient days of the Solar System.
But strangely, while both Pluto and Charon had amassed craters galore, they mostly clocked in at a decently large size, with diameters of at least eight miles—indicating they were carved out by objects 1.2 miles across and above. Small craters, however, were conspicuously few and far between.
This struck Singer as odd. “I had to take some time to convince myself that what we were seeing was real,” she says.
No matter which way Singer reassessed the data, the facts didn’t change. The images hadn’t obscured any features; it also didn’t seem that geologic activity was preferentially obliterating tiny craters. “Through all of those tests, we kept seeing the same thing,” Singer says. “No matter what we did, we couldn’t seem to get rid of it.”
Puny cratering may sound trivial, but the implications are, quite literally, huge. A deficit of small craters means a deficit of small impacting objects. In other words, despite billions of years of bombardments, no record of shrimpy space rocks seemed to exist.
This throws quite a wrench into some of the more standard models of planetary formation, Malhotra says.
According to the traditional model of the Solar System’s origins, the planets came together in a stepwise fashion: Bits of dust in that early, Sun-soaked disk coalesced into pebbles, which clumped into hefty boulders, and so on, building up gradually from particles to planets. But if that were the case, we would expect to see Kuiper Belt objects both big and small still intact—the whole hierarchy of sizes, says study author William McKinnon, a planetary scientist at Washington University in St. Louis. “This tells us that the traditional picture can’t be right,” he says.
But there’s an alternative: the “born big” hypothesis. In this model, pebbles leapfrog their way to big-bodied pre-planets, skipping the pint-sized building blocks in between. The process is thought to occur when pockets of high density collapse in on themselves, tugging together massive amounts of material in one fell swoop. In this case, Singer says, teeny space nuggets might have little occasion to form, except as shrapnel when burgeoning worlds collide.
This idea had been hinted at before by the cratering patterns on other bodies in the inner solar system, like Jupiter’s moons, McKinnon says. Kuiper Belt objects are sometimes jettisoned sunward, breaking ground on Jupiter’s moons, for instance. But those results always had to be taken with a grain of salt: This close to the Sun (relatively, at least), objects can get warped by heat, muddying their true nature. Objects bumping up against Pluto and Charon, on the other hand, are more likely to be in their original state.
Nicolle Zellner, a planetary scientist at Albion College who was not involved in the study, notes that though the work is “exciting” and represents “a lot of really good work,” measuring craters is still just a proxy for actually homing in on objects in the Kuiper Belt. “I’m not ready to say that this observation means there are no small Kuiper Belt objects,” she says. “But they’ve presented a really interesting idea that’ll be open for continued testing and debate.”
To this end, Singer doesn’t think the new study has disproved the traditional theory of planetary formation. Models can always be massaged to accommodate new information, she says, and “it’s definitely still an open discussion.”
In the meantime, New Horizons has yet to run out of steam. Now deep in the Kuiper Belt, the spacecraft recently completed a successful flyby of another object in this remote locale: a petite, rocky body nicknamed Ultima Thule. It’s still too early to tell what that data will yield, but with each passing day, researchers are getting a clearer and clearer picture of the vast universe that surrounds us. Like Pluto, Ultima Thule—along with the rest of the Kuiper Belt—is slowly coming into focus.
“You always hope you’ll be able to accomplish something [like this],” Singer says. “It’s really amazing: We found something we weren’t expecting. But that’s why we do this stuff. That’s why we go out there.”