The Origin Story of Rosetta Comet Is Weird

November 1, 2018

The Rosetta Space Probe’s “rubber duck” comet seems likely to have come with a gentle kiss in the cold blackness of outer space. And this comet may have a secret to tell you about Neptune.

The European Space Agency (ESA) landed Rosetta Probe’s Comet 67P here back in the summer of 2016, and it’s oddly shaped. It’s fairly small, only about 2.5 miles (4.1 kilometers) across at its widest point, and consists of two raised lobes connected by a narrow neck. In an unpublished paper awaiting peer review published in the preprint journal arXiv, astronomers detail how the comet may have formed and migrated into Jupiter’s orbit. And this history has important implications for the early history of the solar system, especially for Neptune, the researchers say.

Astronomers don’t have a time machine; they can’t travel back in time and see how 67P formed. But they can use the information they do have about comets and the solar system to model the object’s history. The model came to some fascinating conclusions about what the solar system looked like years ago, when 67P might have formed.

The solar system is a vortex of orbiting objects tugging at each other under the influence of gravity, so there’s a limit to how precisely astronomers can track 67P backward through time. Researchers already know that the comet passed near Jupiter on Feb. 4, 1959, and Oct. 2, 1923. Scientists say it’s much more difficult to peer further into the past, though.

But by simulating dozens of paths that could point the comet to its current location, the researchers concluded that the object likely moved into the inner solar system more than 1,000 years ago. That was after it formed, circling the sun for centuries in a huge cloud of debris 20 to 30 times farther away than where Earth is located.

But that trajectory poses a problem, because unless 67P separates from a much larger comet, it almost certainly won’t survive in the version of the early solar system that scientists have long imagined, the researchers of the new study say.

Most histories of the early solar system feature a disk of dust and debris orbiting at 20 to 30 times the Earth’s current distance from the sun. In this cloud of material, billions of years ago, small clumps of dirt formed and smashed into each other, sometimes condensing into larger objects and sometimes falling apart.

In the most common version of this story, that cloud lasted about 400 million years. But astronomers in the new study found that if that were true, 67P would almost certainly not have survived. In all that time, the researchers say, the comet would have crashed into other celestial bodies and fallen apart.

So 67P’s survival lends credence to another hypothesis. Maybe Neptune had been orbiting closer to the sun and drifted out through that disk of matter shortly after the Neptune disk formed 4.5 billion years ago. The new study says that Neptune’s drift may have caused the disc to disperse, sparing 67P from collisions in the rough and tumbling environment of the debris disc that gave birth to the comet.

If Neptune’s volume absorbed or dispersed most of the debris 10 million years after the disc’s formation, it might explain how 67P survived to this point, the study says. 400 million years? Lethal. But 10 million years? The researchers wrote in their study that it was a short enough span of time spent in the dirt cloud that 67P was able to survive intact.

The Kiss of the Universe.
Astronomers have not given a definitive answer as to how 67P itself formed. They write that it’s possible that a pile of debris simply formed its current rubber duck shape from the start, although there are reasons to doubt that hypothesis.

The study says the lobes could also have been formed by two smaller comets orbiting another comet, known as binaries, coming together, and there are two possibilities for how this could have happened. Perhaps a slow-running binary grazed a planet and was pushed to form a comet. Or perhaps tiny collisions with other rocks pushed the two lobes together until they met.

The researchers found it unlikely that a brush with a planet would push a binary star together. Such close encounters with planets would likely destroy most comets.

But perhaps, in that cloud of debris, the pebbles slammed into 67P’s two orbital blocks over and over again. Each collision drained a bit of energy from the binary star’s orbit, slowing down the orbital blocks, the study reports.

The authors write that the debris was already moving very slowly.67P’s lightweight lobes would have been orbiting each other at a few feet per second at the time. As the chunks experienced collisions, this speed would have decreased over time until the fragments gathered together relatively gently.