Ice and rocks booted from their star during the planet-formation process may serve as the seeds for new worlds in other systems. Stripped from their parent suns, these objects can find themselves passing close by unfamiliar stars. Yet unlike the solar system’s first known interstellar visitor, ‘Oumuamua, which whizzed past our sun in 2018, such bits of interstellar flotsam may settle down to stay—while also catalyzing the formation of new worlds.
Planet formation is a messy process in which worlds emerge from the embryonic disks of gas and dust that give birth to stars themselves. As debris clumps together and grows, whirling around the central star, its gravity pushes and scatters smaller clumps throughout the disk. Gas giant planets make the biggest bullies, hurling material into their star or out of the system entirely. Jupiter is thought to have tossed out tens of Earth-masses of debris early in the life of the solar system. Material at the edge of a planetary system can also be stripped away by the gravity of a passing star.
Multiple theories postulate that such processes are responsible for sending ‘Oumuamua our way. Researchers have proposed that the strange object may have been thrown out of its young system after a brush with a giant planet there.
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While pondering the origins of ‘Oumuamua over breakfast during a workshop on the curious visitor, Susanne Pfalzner of the Max Planck Institute for Radio Astronomy and Michele Bannister of Queen’s University Belfast raised an intriguing question: what would happen if something like ‘Oumuamua dropped in on a system still in the midst of building planets?
Previous research has suggested that ejected objects can travel at speeds anywhere between the orbital velocity of their star to well in excess of the escape velocity of their galaxy; that latter group of objects usually leaves the galaxy completely and do not interact with planetary formation. Using these estimates, the pair found that most kicked-out debris would be moving far too fast to be captured gravitationally elsewhere, instead simply cruising through star systems, much as ‘Oumuamua did in our own.
But according to their analysis, roughly 10 million 10- to 1,000-meter objects out of the billions intersecting each disk would travel at the right speed to become permanent residents, with bigger planet-forming disks being more likely to accept such interstellar immigrants. At such a disk’s outskirts, where the central star’s gravitational grip is relatively weak, clumps of native material can gravitationally capture passing intruders.
That is when things start to get really interesting. Just like native-born rocks, the interstellar visitors will begin to gather material, growing into larger and larger objects, the researchers found. Yet when sizable interstellar interlopers settle down in particularly young systems that have yet to form their own coterie of kilometer-scale planetary building blocks, they can act to accelerate the entire process of assembling worlds, rapidly gobbling up stray material to grow very large, very fast. “Having those present during planet formation would basically speed up the whole thing,” Pfalzner says.
A Catalyst for Planet Formation?
This idea of interstellar debris acting as an accelerant for planet formation could potentially help solve several lingering mysteries. For one thing, giant planets seem to take too long to form according to many models. A giant planet’s multiple-Earth-mass solid core must grow very quickly, before the gas in a disk disappears—otherwise it will not be able to scoop up enough material to build a gas giant’s thick atmosphere. Observations of protoplanetary disks around young stars suggest such gas reserves dissipate after only a few million years. Yet if such disks are seeded with materials hurled from other planetary systems, that could theoretically jumpstart the process. Pfalzner and Bannister’s ideas remain preliminary; researchers still need to model how much time could be shaved off by a peppering of interstellar immigrants.
Alessandro Morbidelli, who studies planet formation at the Côte d’Azur Observatory in France, says that such a theory only serves to push the problem backward. In other words, the universe’s very first planetary systems would have formed without seeding from external sources, and would thus still face a time crunch when trying to form giant planets.
Pfalzner argues instead that it is entirely possible those first planet-building disks lasted longer than the ones observed today. One possibility relies on the fact that the very first stars had extremely low levels of elements heavier than hydrogen and helium as compared with subsequent stellar generations. This relative paucity of heavy elements could easily lead to differences in early stars’ planet-forming disks that allowed them to produce planets faster.
More Questions Than Answers
It is also possible that the supposed time limit on the formation of gas giants is not as clear-cut and universal even for modern-day disks. Key aspects of planet-forming disks remain mysterious for researchers. For example, although most disks vanish in three to five million years, a few oddballs appear to be as old as 10 or 20 million years. No one knows what causes gas in such disks to stick around. “We still don’t have a good handle on what makes disks disappear and how long that takes place,” Chambers says. The gas in a disk may be swept up by planets, blown off by stellar winds, ionized by a star’s ultraviolet or x-ray light, or even channeled away by magnetic fields.
Morbidelli also wonders why scientists are not seeing much evidence of interstellar intrusion in solar system samples preserved from our sun’s epoch of planet formation. But according to Conel Alexander, a meteorite researcher at the Carnegie Institution of Science, an early intruder would no longer stand out. “The exotic objects would have been thoroughly mixed with the more abundant solar system materials and we would be hard-pressed to detect the isotopic anomalies,” he says.
Edward Young, a geochemist at the University of California, Los Angeles, agrees with Alexander. Young says that exotic materials would not look too different from the familiar rocks of the solar system. He points to observations of dying stars called white dwarfs, which from time to time can be seen devouring rocky material from expired planets. According to Young, the ratios of major elements for that alien debris are similar to those for rocks around the sun, although not exactly the same. “It’s not obvious to me that they would be that different,” Young says.
Although preliminary, the new idea is “a very provocative idea that deserves some attention,” says Sean Raymond, an astronomer at the Laboratory of Astrophysics of Bordeaux in France who models the early solar system. By drawing attention to the need to include ‘Oumuamua-like objects in models of planet formation, Pfalzner and Bannister have acted as catalysts themselves, fostering what will very likely become a long-term discussion. “It’s going to take a while to work through the implications,” Chambers says.