THE WOODLANDS, TEXAS—As data streamed down last month from NASA’s New Year’s flyby of MU69, the most distant planetary object ever explored (above), New Horizons mission scientists got a shock. Rather than the 35-kilometer-long space “snowman” they were expecting, angled images revealed a flatter—not fatter—version, like two lumpy pancakes smooshed together.
“That took us by surprise,” said Alan Stern, the mission’s principal investigator and a planetary scientist at the Southwest Research Institute in Boulder, Colorado, today at the Lunar and Planetary Science Conference here. “We’re looking at something wild and wooly and pristine.”
Scientists believe MU69’s two lobes, with their sparse impact craters and generally smooth features, are primordial planetary building blocks called planetesimals. They still don’t understand why MU69’s two lobes did not form as spheres. But their flat shapes are now the best evidence that MU69, or “Ultima Thule” as the team has nicknamed it, first formed as two small, separate objects, says William McKinnon, a New Horizons team member and planetary scientist at Washington University in St. Louis, Missouri. “This is our strongest evidence that they really did start as an orbiting pair.”
Because the two lobes aren’t spheres, their height, width, and depth can be seen as three distinct axes, and all three axes of the lobes are nearly perfectly aligned, as if they had been laid end-on-end like dominos. This type of alignment would be expected if the duo formerly orbited each other in close proximity, their gravity gently tugging back and forth. “It’s very improbable this would arise completely by chance,” McKinnon says.
The new images support a newer theory of planetary formation, called the streaming instability, as Science reported in January. Fifteen years ago, scientists proposed that boulder-size “pebbles,” built up through static electricity, would clump together like a pack of racing cyclists thanks to the churn of the early solar system’s primordial disk. Those streaming pebbles would eventually gravitationally collapse into planetesimals, leading to pairs of orbiting objects that line up like MU69, McKinnon says. “That comes right out of the streaming instability model.”