When interstellar asteroid ‘Oumuamua was seen speeding out of our solar system last October, astronomers knew it was something special. Traveling too fast to be captured by our sun’s gravity, the strangely elongated asteroid obviously didn’t originate in our interplanetary neighborhood.
But how ‘Oumuamua left its home star system is a puzzle that astronomers are just beginning to understand and, in doing so, they’re getting a unique look into how planets may form around other stars.
«This object was likely ejected from a distant star system,» said astrophysicist Elisa Quintana, who works at NASA’s Goddard Space Flight Center, in the press release. «What’s interesting is that just this one object flying by so quickly can help us constrain some of our planet formation models.»
How to Make Comets and Asteroids
When it was initially discovered by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) on Oct. 19, 2017, ‘Oumuamua was assumed to be a comet since it was on a comet-like hyperbolic trajectory, traveling fast enough to escape the sun’s gravitational pull. By tracing its path back through the solar system, astronomers realized that the object had zoomed past the sun the previous month. Usually when comets approach the sun, their ices erupt from the nucleus, creating a gaseous coma and tails. In this instance, however, there was no evidence of any cometary activity. ‘Oumuamua didn’t have the ices that you would typically find on a comet. With complementary spectroscopic analyses, astronomers concluded that it was an ancient asteroid from another star.
Before we continue, a bit of background: Comets and asteroids arise from the protoplanetary disks of dust, ice and gas around young stars. But whether a comet or an asteroid clumps together from the primordial material depends on the proximity to its star. If it forms close, where stellar heating is so intense that it bakes away the ices, an asteroid is created. Beyond a certain distance, however, it’s cold enough for these objects to retain their ices and, voila, you wind up with a comet. The distance from a star beyond which it’s cold enough for comets to form is known as the «snow line» (for obvious reasons). For our solar system, the snow line is located approximately three times farther away than the sun-Earth distance.
This is where things get puzzling: Comets are loosely bound to the gravity of their star since they typically develop farther away. Therefore, it’s comparatively easy to kick a comet out of a given star system. This is one of the reasons why ‘Oumuamua was thought to be an interstellar comet at first; comets are susceptible to being perturbed by a stellar encounter. Asteroids, on the other hand, typically form in orbits near their stars, and it takes more energy to boot them out of a star system. Something cataclysmic must happen to hurl asteroids out of the gravitational well.
«It’s harder for that stuff to get ejected because it’s more gravitationally bound to the star. It’s hard to imagine how ‘Oumuamua could have gotten kicked out of its system if it started off as an asteroid,» said astrophysicist Sean Raymond, at the French National Center for Scientific Research and the University of Bordeaux, in a statement.
Where does this leave ‘Oumuamua? Well, it’s an asteroid, with a strange shape, and it comes from another star system. And that tells us that either our theories as to how asteroids, comets and planets form aren’t correct, or that ‘Oumuamua must have experienced an energetic event to be propelled out of its home star system.
In our solar system, the gas giant Jupiter is an interplanetary bully. As the largest planet, its gravity plays an important role in the evolution of planetary orbits, and it vacuums up errant asteroids and comets. Jupiter basically acts as the sun’s «fixer,» making sure the system stays in balance. Some theories even suggest if it weren’t for Jupiter’s clout, life may not have had a chance to gain a foothold on Earth. (Though other theories suggest that it might hurl space rocks at us, so it also could be the antihero of planetary science.)
Early in our solar system’s history, there was a lot of dynamic instability, and Jupiter would have careened around its orbit, scattering any smaller bodies with its massive gravity. It’s therefore thought that the solar system lost a lot of planetesimals in its formative years, and asteroids would have been slingshot and ejected into interstellar space.
So, if other star systems contain gas giant exoplanets, it stands to reason that they would have the same effect on bodies orbiting close to their star. These massive worlds would exert a herculean tidal force on any small objects in their vicinity, and many would be ripped to shreds — like Comet Shoemaker-Levy 9 that was pulled apart by Jupiter’s gravity and then spectacularly impacted the planet in 1994.
In their study, published on March 27 in the journal Monthly Notices of the Royal Astronomical Society (MNRAS), the researchers speculate that this could have been the origin of ‘Oumuamua: It was slingshot out of its home star system by a giant planet after being stretched into its peculiar cigar-like shape by extreme tidal pressures.
In an earlier study, also published in MNRAS, researchers concluded that small bodies such as asteroids are more likely to be flung out of binary star systems than systems with single stars (like the solar system). As most stars in the galaxy are in binary systems, there might be a surprising number of asteroids ejected as interstellar vagabonds if their orbits are being perturbed by the orbital dance of two suns.
The upshot is that ‘Oumuamua delivered a surprisingly detailed message about its possible origins. Now we just need to detect more of these objects that occasionally zip through our solar system, so we can build a better picture of the alien environment they evolved in.