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Drifting Interstellar Worlds Like ʻOumuamua Could be the Seeds of New Planets

Jülich/Belfast, 8 April 2019 – Interstellar objects the size of skyscrapers such as ʻOumuamua, discovered two years ago, could help new star systems to quickly form planets. This is the result of a recent study by the Jülich Supercomputing Centre together with Queen’s University Belfast. Myriads of such asteroids are likely drifting through our Milky Way. Once part of newly developing star systems, these huge objects between the stars could accelerate the formation of new planets.

A boulder from a planetary system light years away might have kick-started the development of the planets in our solar system. This was revealed in a recent study conducted by astrophysicists Prof. Dr. Susanne Pfalzner from the Jülich Supercomputing Centre in Germany and Dr. Michele Bannister from Queen’s University Belfast in Northern Ireland. Our Milky Way might be full of drifting interstellar objects such as the asteroid ʻOumuamua, which paid a brief visit to our solar system in October 2017. Planetary systems form and subsequently scatter trillions of tiny worlds into interstellar space, just like dandelions spreading their seeds. These drifting boulders can serve as a kind of seeds from which entire planets may eventually form.

Interstellare Objekte wie Oumuamua könnten das Wachstum neuer Planeten beschleunigenInterstellar objects such as ʻOumuamua might accelerate the growth of new planets.
Copyright: ESA/Hubble, NASA, ESO, M. Kornmesser, cc by

“According to existing models, planets form slowly from micrometre-sized gas and dust particles in protoplanetary disks surrounding stars. These disks become increasingly dense over millions of years,” explains Pfalzner. However, observations have been made that paint a different picture. Some planets must have developed in a much shorter time than the standard model allows. Interstellar objects like ʻOumuamua could reconcile these discrepancies.

The two researchers estimate that the Milky Way, our home galaxy, contains septillions (that’s a 1 with 24 zeros!) of objects similar to ʻOumuamua, and around 29 trillion (12 zeros) within a volume of space of one cubic light year. By comparison, Proxima Centauri, the star closest to our own Sun, is more than four light years away. These planetesimals – which are likely relatively small, dark, and fast – move freely in space after having been thrown out of their orbit around their home stars. These interstellar “exiles” might play a decisive role in the formation of planets if captured by a protoplanetary disk surrounding a different star.

“Many of these objects probably move too quickly to be captured by protoplanetary disks,” explains Pfalzner. “And the ones that do get captured will most likely be swallowed by the star.” And yet, according to the astrophysicists’ calculations, there should be at least 10 million of these interstellar objects surrounding every star. “Most of these get lost during the capturing process. But since there are so many of these objects, many of them will still be there in the end,” explains Bannister. “Thousands of these objects are likely more than one kilometre in size. A few could even be the size of dwarf planets like Ceres or Pluto – or like our Moon.”

With their gravity, the planetesimals might attract matter – gas, dust, rocks – and thus finally grow into full planets. This scenario would solve the problem regarding the speed of planet formation. “According to the usual accretion model, it would take up to tens of thousands of years for microscopic dust particles to form matter particles just a few millimetres or centimetres in size,” says Bannister. “The formation of planets like Earth then takes many millions of years, and that of gas giants like Jupiter even longer.” And yet, younger star clusters have been found to contain planets that are only one million years old.

“If planets did not have to form slowly from micrometre-sized dust and gas particles, then this would greatly accelerate their creation process,” says Pfalzner. “When the idea was first put forward, it just appeared so plausible. I hope that many other researchers will take it up and test the model.” The mechanism would also impact on itself: systems containing more planets eject more boulders like ʻOumuamua, which then create more planets in other systems – planetary systems thus help in creating more planetary systems.

“If our model turns out to be correct, this would also explain why the oldest stars have fewer planets than we observe in younger star systems,” says Pfalzner. “Early planet generations would have developed according to the conventional model – and would then have provided the seed for new protoplanetary disks by ejecting such asteroids.” Planet formation in the entire galaxy could thus increase exponentially since more and more lost boulders would be zooming around in space.

Further information:

Work on this study began at a workshop concerning ʻOumuamua at the International Space Science Institute in Bern, Switzerland. The scientists’ results were published in the journal Astrophysical Journal Letters on Monday .
Original publication: A hypothesis for the rapid formation of planets, Susanne Pfalzner and Michele T. Bannister, Astrophysical Journal Letters, April 2019, DOI: 10.3847/2041-8213/ab0fa0, preprint available at arXiv:1903.04451

Jülich Supercomputing Centre


Prof. Dr. Susanne Pfalzner
Jülich Supercomputing Centre
Tel.: +49 2461 61-85420

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