Recent theoretical work in celestial mechanics has revealed that an asteroid may orbit stably in the same region as a planet, despite revolving around the Sun in the sense opposite to that of the planet itself1,2,3,4,5. Asteroid 2015 BZ509 was discovered6 in 2015, but with too much uncertainty in its measured orbit to establish whether it was such a retrograde co-orbital body. Here we report observations and analysis that demonstrates that asteroid 2015 BZ509 is indeed a retrograde co-orbital asteroid of the planet Jupiter. We find that 2015 BZ509 has long-term stability, having been in its current, resonant state for around a million years. This is long enough to preclude precise calculation of the time or mechanism of its injection to its present state, but it may be a Halley-family comet that entered the resonance through an interaction with Saturn. Retrograde co-orbital asteroids of Jupiter and other planets may be more common than previously expected.
We acknowledge the use of the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the USA, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona Board of Regents; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, The Leibniz Institute for Astrophysics Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. This work made use of data and/or services provided by the International Astronomical Union’s Minor Planet Center. This work was supported in part by the Natural Sciences and Engineering Council (NSERC) of Canada.
The sizes of Jupiter and the asteroid are exaggerated for clarity but the relative motion around the Sun is correct. The dotted path indicates the trajectory of the asteroid as seen in a frame which co-rotates with the planet (e.g. as in Figure 1).