Is there an exoplanet in the Solar system?

A.~J. Mustill, S.~N. Raymond, M.~B. Davies

Research output: Contribution to journalArticlepeer-review


We investigate the prospects for the capture of the proposed Planet 9 from other stars in the Sun's birth cluster. Any capture scenario must satisfy three conditions: the encounter must be more distant than ∼150 au to avoid perturbing the Kuiper belt; the other star must have a wide-orbit planet (a ≳ 100 au); the planet must be captured on to an appropriate orbit to sculpt the orbital distribution of wide-orbit Solar system bodies. Here we use N-body simulations to show that these criteria may be simultaneously satisfied. In a few per cent of slow close encounters in a cluster, bodies are captured on to heliocentric, Planet 9-like orbits. During the ∼100 Myr cluster phase, many stars are likely to host planets on highly eccentric orbits with apastron distances beyond 100 au if Neptune-sized planets are common and susceptible to planet–planet scattering. While the existence of Planet 9 remains unproven, we consider capture from one of the Sun's young brethren a plausible route to explain such an object's orbit. Capture appears to predict a large population of trans-Neptunian objects (TNOs) whose orbits are aligned with the captured planet, and we propose that different formation mechanisms will be distinguishable based on their imprint on the distribution of TNOs.
Original languageEnglish
Pages (from-to)L109-L113
JournalMonthly Notices of the Royal Astronomical Society: Letters
Publication statusPublished - 2016 Jul 21

Subject classification (UKÄ)

  • Astronomy, Astrophysics and Cosmology

Free keywords

  • Kuiper belt: general
  • planets and satellites: dynamical evolution and stability
  • planets and satellites: individual: Planet 9
  • planetary systems
  • open clusters and associations: general


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