Planetary science

Reckless orbiting in the Solar System

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Planets and most asteroids revolve around the Sun in the same direction. But an asteroid that shares Jupiter's orbit has been revolving in the opposite direction for about a million years. See Letter p.687

The Solar System formed in a disk of gas and dust whose constituents revolved around the Sun in the same direction. In the final stages of planetary formation, trillions of bodies were expelled beyond the reach of the planets, forming a relatively thin disk of debris. The Galaxy's tidal forces then modified this structure into a spherical shell known as the Oort cloud that remains gravitationally bound to the Sun. This shell is located more than 10,000 times farther from the Sun than Jupiter is, and shelters objects that orbit the Sun in the direction opposite (retrograde) to that of planetary motion. On page 687, Wiegert et al.1 report the discovery of the first object that shares the orbit of a planet but revolves in the retrograde direction.

Thousands of objects called Trojan asteroids populate Jupiter's orbit, revolving around the Sun in the same direction as the planet (prograde). These objects sit near Lagrange points, which form an equilateral triangle with the Sun and Jupiter. Whereas Trojan asteroids have stable orbits, other prograde asteroids can enter a transient co-orbital state for several thousand years2, in which they have the same orbital period as a planet. Co-orbital states can take many shapes, depending on the asteroid's motion in space. These shapes include tadpoles, horseshoes, quasi-satellites — in which the asteroid stays close to the planet for many orbital periods — and combinations thereof (Fig. 1a).

Figure 1: Co-orbital states.

As small bodies travel through the Solar System, they can enter co-orbital states, in which they have the same orbital period as a planet. a, Co-orbital bodies that orbit the Sun in the same direction as a planet can follow trajectories (blue curves with arrows) that, from the perspective of the planet, look like tadpoles, horseshoes or 'quasi-satellites'. The white circle represents the orbit of the planet around the Sun. The tadpole and horseshoe shapes arise because the planet's gravitational attraction alters the body's orbital path — the body goes through a cycle of catching up with the planet and falling behind, seeming to change direction from the perspective of the planet. The quasi-satellite shape results from the two orbits having different eccentricities. When viewed from the planet, the body seems to loop around it. b, Wiegert et al.1 have discovered the first co-orbital body that orbits the Sun in the opposite direction to a planet. When viewed from the planet, the body's orbit takes the shape of a type of curve called a trisectrix.

The dynamics of most small bodies in the Solar System are complex, making it difficult to identify their source. Objects called Centaurs exhibit a 'random walk' in the outer Solar System because they have close encounters with the giant planets, and this limits their orbital stability to a few million years3,4. Centaurs can evolve into short-period comets, collide with the Sun or a planet, or be ejected from the Solar System. Most Centaurs orbit in the general direction of planetary motion, pointing to an origin in the Kuiper belt — a reservoir of remnants from the Solar System's formation that is situated beyond Neptune's orbit.

Other small bodies called Halley family comets (HFCs) typically have high orbital inclinations with respect to Earth's orbital plane. They are named after their most famous member, Halley's comet, which has a retrograde orbit and reaches perihelion — its closest approach to the Sun — roughly every 75 years. The source of HFCs is a matter of debate, however. Given their wide range of inclinations, HFCs could have evolved from long-period comets originating in the Oort cloud. Alternatively, they might have evolved from Centaurs5, or maybe they emerged from a hitherto undetected reservoir of high-inclination orbits beyond Neptune6. Small bodies that have orbits similar to HFCs but that lack cometary activity are known as Damocloids7.

Wiegert and colleagues study the object 2015 BZ509, which was detected8 in January 2015 using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii. Wiegert et al. use additional observations from the Large Binocular Telescope in Arizona to improve the accuracy of the parameters that describe the object's orbit. They show that 2015 BZ509 is in the 1:1 retrograde resonance with Jupiter. This means that the object is co-orbital (the ratio of its orbital period to that of Jupiter is 1:1) and that the two bodies exert regular, periodic gravitational influences on one another (orbital resonance).

When viewed from the perspective of Jupiter, the orbit of 2015 BZ509 takes the shape of a type of curve called a trisectrix (Fig. 1b). Such an orbit would be stable indefinitely if the asteroid's motion around the Sun were influenced only by Jupiter9. The authors confirm that 2015 BZ509 has long-term stability, having been in its current state for about a million years. This stability is a unique feature of retrograde resonances — even though 2015 BZ509 passes Jupiter every six years (twice per orbit), the duration of the encounters is much shorter than if the object were on a prograde orbit.

The authors also discuss the nature and origin of 2015 BZ509. Such small bodies in orbit around the Sun can be either rocky or icy, depending on whether they formed in the inner or outer Solar System. If icy bodies pass close to the Sun, they can become comets, but rocky or icy bodies that do not show signs of cometary activity are loosely said to be asteroids. Wiegert et al. observe 2015 BZ509 when it is near perihelion, about three times farther from the Sun than Earth is. They detect no signs of cometary activity, which suggests that the object would be classified as an asteroid. It is likely to be a Damocloid that was captured in the 1:1 (co-orbital) retrograde resonance with Jupiter.

However, 2015 BZ509 is not the only known retrograde asteroid in resonance: 2006 BZ8 and 2008 SO218 are currently in the 2:5 and 1:2 retrograde resonances with Jupiter, respectively, and 2009 QY6 is in the 2:3 retrograde resonance with Saturn10. Indeed, 2006 BZ8 could even enter the 1:1 retrograde resonance with Saturn in the future10. Simulations have shown that capture in resonance is more likely for objects that have retrograde orbits than for those that have prograde orbits11.

Resonances extend the lifetimes of objects on planet-crossing orbits by protecting them from disruptive close encounters with planets4,5,11. The particularly long lifetime of 2015 BZ509 on its retrograde orbit, in the same region of space as the largest planet in the Solar System, makes it arguably the most intriguing small body in this region. Further studies are needed to confirm how this mysterious object arrived at its present configuration. Footnote 1


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Correspondence to Helena Morais or Fathi Namouni.

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Morais, H., Namouni, F. Reckless orbiting in the Solar System. Nature 543, 635–636 (2017) doi:10.1038/543635a

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