Parts of Earth may originally be from another part of the galaxy, having crossed light years to form the ground beneath our feet. That is the conclusion of a study suggesting that the Milky Way should be full of free-floating rocks like 鈥極umuamua, the interstellar asteroid聽that聽visited our solar system in October 2017, and they may act as seeds to form planets in nascent planetary systems.
Our traditional picture is that planets form聽out of discs of gas and fine dust around a star, but some observations seem to show them being born much faster than that聽model predicts. Interstellar objects聽like 鈥極umuamua may be the solution to this discrepancy.
Researchers have estimated that there should be about 29 trillion 鈥極umuamua-like objects per cubic light year in our galaxy, floating free after having been thrown out of orbit around their home stars. They are likely to be relatively small, dark, and fast-moving, which is why we have only seen one so far.
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Michele Bannister at Queen鈥檚 University Belfast, UK, and Susanne Pfalzner at the J眉lich Supercomputing Center in Germany found that these rocks could play a crucial role in planet formation if they get caught in a聽disc around another star.
Many of the interstellar objects should be moving too fast to get caught, and most that do get trapped are likely to fall into the star. Nevertheless, the pair calculated that there should be at least 10 million 鈥極umuamua-sized (about 100 metres across) and larger objects around each star.
鈥淚t鈥檚 not a very efficient process, but you鈥檙e starting out with so many of them that you still end up with plenty,鈥� says Bannister. Of the 10 million big interstellar objects, thousands are likely to be more than a kilometre across, and a few may be dwarf planets 鈥� similar in size to Ceres or Pluto.
The interstellar exiles could attract dust, pebbles and gas with their gravity and eventually become full-on planets. 鈥淎 fraction of planets could have had an 鈥極umuamua at their heart,鈥� says Bannister. 鈥淵ou鈥檙e not going to have any trace of it anymore, but it鈥檚 a lovely thought.鈥�
This would skip over the inconvenience of building planets out of tiny grains of dust, fixing the problem with the speed of planet formation. 鈥淚f planet-forming discs were seeded with large rocks in this way, then it would accelerate the planet formation process significantly,鈥� says Richard Alexander at the University of Leicester, UK. 鈥淚f it鈥檚 correct then it will have a major impact on our picture of how planets form.鈥�
Hear more about 鈥極umuamua from Michele Bannister:
This mechanism would feed back on itself:聽systems with more planets kick out more rocks, which create more planets in other systems. 鈥淧lanetary systems are helping build planetary systems,鈥� says Bannister.
If that is the case, it could explain why the聽earliest stars seem to have fewer planets than those that formed more recently. Early generations of planets could have formed the conventional way and then gone on to seed other planet-forming discs with discarded 鈥極umuamuas. Planet formation across the galaxy should continually increase as there are more and more stray rocks flying around.
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