Asteroids from Outside

In the past few years we have discovered dozens of objects in our solar system that most likely came from outside.  It has long been assumed that there are ancient objects that were ejected from other solar systems and subsequently captured by our sun and planets.  But until this century, no such objects were identified.  Now many are reported every year.

This summer, researchers from France and Brazil report a study of the orbits of 17 objects that they conclude came from other solar systems [2].  In particular, they argue that 4.5 billion years ago, when our solar system was forming, there were many other star systems forming nearby, within gravitational interaction.

At that time, the mass that would become our solar system was a flat disk.  This means that any objects orbiting off the plane of the main disk probably wandered in from nearby systems.

The study itself uses statistical methods that, frankly, are beyond my meager understanding.  The basic idea is to compute zillions of ‘clones’ of the object’s orbit, reflecting the uncertainty of the current estimates.  This population of orbits can extends back in time, so it is possible to derive a distribution of probable orbits circa 4.5 million years ago.

The results indicate that the 17 objects in question had high inclination orbits or were outside the main disk of the solar system at the time in question.  Notably, the objects all ended up in polar orbits after a million years or so.

Like Monte Carlo methods, this sucks down huge amounts of computation!  But it’s really neat to be able to play the simulation back and forth in time, to visualize the history of the objects.

we built a computer simulation that works like a time machine, running their trajectories backwards by 4.5 billion years.” (Professor Maria Helena Moreira Morais,quoted in  [1])


  1. José Tadeu Arantes, A population of asteroids of interstellar origin inhabits the Solar System, in Agência FAPESP – news, July 12, 2020.
  2. F. Namouni and M. H. M. Morais, An interstellar origin for high-inclination Centaurs. Monthly Notices of the Royal Astronomical Society, 494 (2):2191-2199, 2020.

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