The first retrograde co-orbital asteroid: 2015 BZ509
— a Trojan in retreat —

Paul Wiegert
Dept. of Physics and Astronomy, Western University, London ON CANADA
Centre for Planetary Science and Exploration (CPSX), London ON CANADA

Martin Connors
Centre for Science, Athabasca University, Athabasca AB CANADA
Centre for Planetary Science and Exploration (CPSX), London ON CANADA

Christian Veillet
Large Binocular Telescope Observatory, Tucson AZ USA

May 30 2018 update:

Our animation of 2015 BZ509 was featured on NASA's Astronomy Picture of the Day (APOD)

January 3 2018 update:

Our paper was listed as story #47 in Discover magazine's Top 100 science stories of 2017

March 30 2017

The co-orbital asteroids of Jupiter, also known as the 'Trojan asteroids'. The prograde asteroids are shown in white, and 2015 BZ509 (with a trail, shown in green) appears later. The planets and asteroids have been enlarged for visibility. Click above for a larger version.

In the March 30 2017 issue of the journal Nature, astronomers reveal that an as-yet-unnamed rare asteroid with the provisional designation 2015 BZ509 (nicknamed 'BZ') travelling in the opposite direction to all the planets and 99.99% of the other asteroids in our Solar System — a state referred to as retrograde motion— is also safely sharing the orbital space of the giant planet Jupiter.

There are about 6000 other asteroids which share Jupiter's orbit space with it. Called the 'Trojan asteroids', they co-exist easily with the giant Jupiter because they go around in the same direction, called prograde motion. If the Solar System were a giant race track around the Sun, the planets would be like monster trucks, and the asteroids like ridiculously tiny clown cars. Though all this traffic is inherently dangerous, collisions are usually avoided because the planets and asteroids go around the track in the same direction.

What's strange about BZ is, first, that it goes around the track in the opposite or 'retrograde' direction to most all the others. But it is not the only asteroid going the opposite way. A few others do, so this makes BZ unusual but not unique. The second and stranger thing is that BZ is also playing a cosmic game of 'chicken' with the giant planet Jupiter. The other retrograde asteroids tend to remain away from the planets. This makes sense: if a clown car is going to survive going the wrong way around the track, best to stay away from the big trucks. But BeeZed actually shares the same lane as Jupiter, and it does so while going around the track in the opposite direction. This is not what one would expect to be a very long-lived situation, but this study shows that BZ has done so safely for at least tens of thousands of 'laps', avoiding the monster truck by weaving in and out of the planet's path every time they pass. Jupiter's gravity actually helps it maintain this state by nudging it a little every lap to help keep BZ synchronized. 2015 BZ509 is the first asteroid known to have this relationship with any of the planets. Calculations show it will continue to safely navigate its unusual path for the next million years at least.

2015 BZ509 maintains a stable relationship with respect to Jupiter not simply by luck but —perhaps surprisingly— through the effect of Jupiter's gravity. BZ passes once inside and once outside Jupiter each time they orbit the Sun, and the two gravitational tugs that Jupiter gives the asteroid cancel out, giving BZ opposing 'nudges' that keep it on track. The link between the asteroid and Jupiter is a distinct but gentle one: BZ nevers gets closer to Jupiter than the Earth does to the Sun: the closest approach between the two is 176 million kilometers (109 million miles) centre-to-centre. Ironically, BZ would be more likely to crash into Jupiter if that planet had no gravity at all, because without the gravitational nudges, it would gradually drift out of sync with that planet.

That asteroids could theoretically be in such a unusual state, called a retrograde co-orbital resonance was first noted by Dobrovolskis (2012), and an excellent theoretical understanding of such behaviour was soon worked out by Helena Morais and Fathi Namouni in a series of research papers in 2013a, 2013b, 2015, and 2016. But 2015BZ509 is the first real physical object in this unusual state. How it got there remains a mystery.
Images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its retrograde co-orbital nature. The LBTO has two 8.4 meter-wide main mirrors side-by-side, hence the two images. The bright stars and the asteroid (circled in yellow) appear black and the sky white in this negative image. What are those weird white dots, spots and stripes? They are imaging artifacts in these raw images. Click for a larger view.

This asteroid may in fact be an inactive icy comet nucleus, rather than a rocky asteroid. Comets are much more often on retrograde orbits to begin with: Halley's Comet is just one example of a comet on a retrograde orbit. However, observations of BZ have so far failed to show any indication of comet-like behaviour, such as a tail. Complicating the picture is that comets only develop tails when the icy comet nucleus gets close enough to the Sun for its ice to start to vaporize. Though BZ has been observed when it was at its closest to the Sun, it does not get particularly close to our star even at its closest, so it may never receive enough sunlight to form a tail even if it had any icy composition. The answer to the origin and nature of this object will have to await further study.

Asteroid 2015 BZ509 always remains in the vicinity of Jupiter's orbit, and does not approach the Earth. The asteroid is not considered hazardous to our planet.

More on Co-orbital Asteroids

"Co-orbital asteroids" are so-named because they can share a planet's orbit with it, and they come in a wide variety of strange relationships with their companion planet. For more information on co-orbital asteroids, see the links at the bottom of this page.

How was the nature of 2015 BZ509 discovered?

Asteroid 2015 BZ509 was discovered by the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS) in 2015. At the time, its orbit was not sufficiently well-known to determine its behaviour. But it appeared to be suspiciously close to Jupiter's co-orbital zone, so the authors of this paper tracked it down and obtained further observational data using the Large Binocular Telescope Observatory in Arizona. From the analysis of these measurements, the retrograde co-orbital nature of this strange asteroid was determined.

Additional Video clips

Note: these and the previous video clips are only illustrative. The motions are correct but the sizes of the Sun, Jupiter and the asteroids have been exaggerated for easier visibility.

The green ellipse is the orbit of 2015 BZ509 around the Sun, the blue orbit is Jupiter's, and the other planets are shown in grey. The green dots represent the path of the asteroid as seen from the reference frame of Jupiter. The Sun is at the center of the frame. Jupiter and the asteroid are labelled. Click on the image to the left to see the video.

The path of asteroid 2015 BZ509 relative to Jupiter is shown in green for several orbits. The Sun is the yellow sphere, Jupiter is the large planet in the foreground centre, and the light blue circle traces out Jupiter's orbit around the Sun. Jupiter is moving counter-clockwise (to the right in this view). Note however, that the camera moves along with it to track its motion so while it is moving around the Sun, it appears stationary with respect to the camera. Click on the image to the left to see the video.

A view from across Jupiter's orbit, looking toward that giant planet. The prograde Trojan asteroids (white) of Jupiter along with asteroid 2015 BZ509 (green). The Trojans asteroids are white spheres which mill about ahead of and behind Jupiter along its orbit. The light blue circle traces out Jupiter's orbit around the Sun. The other planets and their orbits are shown in white. Jupiter is moving counterclockwise (to the left in this view), but once again the camera moves along with it to track its motion so it remains in the centre of the field of view. A few stray non-Trojan asteroids are also included. Click on the image to the left to see the video.

Want to know more?

The Nature journal article can be accessed at this link. Springer, the publisher of Nature, also kindly provides a SharedIt link: if you don't have access to a subscription to Nature, you can still view the article here. A commentary by Morais and Namouni is also available.
Interactive 3D visualizations of the orbit. These files are Wolfram CDF (Computable Document Format) objects generated with Mathematica 10 on a Raspberry Pi 3 computer. Ephemeris computations for them were done with the JPL Horizons system accessed through telnet 6775 . Wolfram CDF player (or equivalent) is needed to view them. The authors of these files take no responsibility for their playability, nor for the results of any interaction of them or of the player with user’s computers. When first displayed, some details may be missing. Click on the image to make these clear and to change the point of view by dragging. Magnification may help in seeing details.
  • XYZorbit.cdf The orbits of Jupiter and 2015 BZ 509 in the current epoch. The Sun is at the center. One orbit of each is shown: they have roughly the same period of 11.86 years.
  • XYZlibration.cdf The orbits of Jupiter and 2015 BZ 509 are presented over one half libration cycle of the asteroid (2017-2342), during which the orbit of Jupiter does not change significantly.

    Relative orbits: the orbit is shown in the frame that rotates with Jupiter. In this frame the asteroid describes a complex three-dimensional pattern but the view is scaled so that Jupiter is fixed at position (1,0,0). The Sun is at the center. The asteroid’s relative orbit moves through time due to libration. The basketweave pattern arises from the changes taking place mostly on the close passes to Jupiter.

  • relOrbit.cdf The scaled orbit of 2015 BZ 509 relative to Jupiter is shown, similar to that in Figure 1 in the paper.
  • relLibration.cdf The scaled orbit of 2015 BZ 509 relative to Jupiter is shown over one half libration cycle. The first orbit starts in 2017 and is shown in orange; the last ends in 2342 and is in green. Intervening orbits are shown in blue.
  • relLibrationColor.cdf The scaled orbit of 2015 BZ 509 relative to Jupiter is shown, over one half libration cycle. Z R values are color coded from the most negative value possible in blue, to the highest value possible in red. Z R is the perpendicular distance to 2015 BZ 509 from Jupiter’s orbital plane.
  • Related links

  • 3753 Cruithne, the Earth's first co-orbital asteroid
  • Another Earth co-orbital asteroid recently discovered is designated 2010 SO16, and is on a "horseshoe" class orbit.
  • A few other interesting co-orbital asteroids are discussed here.
  • Paul Wiegert's home page
  • Martin Connors' home page
  • Christian Veillet's home page
  • The Department of Physics and Astronomy at Western University in Canada
  • The Centre for Planetary Science and Exploration (CPSX) at Western University in Canada
  • the Centre for Science at Athabasca University in Canada
  • The Large Binocular Telescope Observatory in Arizona
  • The Panoramic Survey Telescope And Rapid Response System (Pan-STARRS) that first observed 2015 BZ509.
  • The Minor Planet Center (MPC), the international clearinghouse for scientific observations of solar system bodies.
    Have a question or comment? Contact me (Paul Wiegert at pwiegert[the @ sign]
    Retrograde co-orbital asteroid 2015 BZ509 page
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