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A Faster Way to Form Planets

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Artist's impression of a planetesimal
This artist's impression imagines how the Sun would look to an icy planetesimal on the outskirts of the solar system. Planetesimals ejected from their nascent star systems could be the seeds for planet growth in other systems. [NASA, ESA, and G. Bacon (STScI)]

When interstellar asteroid ‘Oumuamua sped through our solar system in 2017, it revealed that small bodies might make a habit of visiting other planetary systems. Could such objects occasionally be responsible for jump-starting planet formation?

Planet Formation in the Works

1I/2017 U1 ('Oumuamua)

An artist’s impression of interstellar asteroid 1I/2017 U1 (‘Oumuamua). [European Southern Observatory / M. Kornmesser]

One of the leading models of planet formation is core accretion, in which dust grains come together to form pebbles, planetesimals, and planets. While the core accretion model explains many features that we observe in protoplanetary disks, it’s not yet clear exactly how dust grains clump together to form larger objects.

It’s also expected to take up to tens of millions of years to form a planet, which clashes with observations of planets forming in as little as a few hundred thousand years in disks that dissipate after just a few million years.

Many models of planet formation show that once planetesimals in a disk reach a certain size, they rapidly accrete gas to form planets. What if core accretion could get a head start from long-lost planetesimals ejected from other stellar systems?

Pfalzner & Bannister 2019 Fig. 1

A qualitative illustration of the timeline and mechanisms of interstellar object production. The times here are typical for a solar-mass star. Higher-mass stars would begin the final stage of interstellar object production earlier. Click to enlarge. [Pfalzner & Bannister 2019]

One System’s Trash, Another System’s Treasure?

To estimate how the population of interstellar objects affects the formation of new planets, Susanne Pfalzner (Jülich Supercomputing Center and Max Planck Institute for Radio Astronomy, Germany) and Michele Bannister (Queen’s University Belfast, UK) calculated the number of interstellar objects that survive the journey from their nascent star systems and become incorporated into new systems.

They began with an estimate of ‘Oumuamua-sized objects based on 30 years of observations from the Pan-STARRS1/Catalina Sky Survey: 1015 objects in a cubic parsec (about 35 cubic light-years).

The authors estimated that only a few percent of these objects can be easily captured by molecular clouds, and even fewer make it into the surroundings of a young star, which consumes all but 1–10% of the interstellar objects capable of acting as seeds for planet formation.

Plentiful Planetesimals

Pfalzner & Bannister 2019 Fig. 2

Density of 100-meter interstellar objects over the course of the star and planet formation process. [Pfalzner & Bannister 2019]

Of the initial 1015 interstellar objects found in a single cubic parsec, the authors estimate that more than 10 million are able to be captured by molecular clouds, survive collisions, withstand heating, and become incorporated into a protoplanetary disk to participate in accretion. Most are about 100 meters in size, but as many as 105 are kilometer sized and a thousand are 100 km or larger.

It’s not yet clear what role the abundant 100-meter objects play in planet formation, but simulations have shown that planetesimals larger than ~200 km rapidly accrete gas from their surroundings, forming both terrestrial and gas-giant planets in a blazingly fast 1,000 years.

Since the number of interstellar objects increases over time, the number of potential seeds for planet formation increases as well, meaning that planet formation is likely faster now than it was billions of years ago. There’s still plenty of work to be done, but one thing is clear: we can’t ignore the importance of interstellar objects in the planet formation process.

Citation

“A Hypothesis for the Rapid Formation of Planets,” S. Pfalzner & M. T. Bannister 2019 ApJL 874 L34. doi:10.3847/2041-8213/ab0fa0