We recently looked at JAXA’s planned solar sail mission to Jupiter (see JAXA Sail to Jupiter’s Trojan Asteroids), but I want to come back around to the Trojans this morning in light of a discovery announced today. The more we learn about the Trojans, the better. Most appear to be class D asteroids, dark with reddish hues and probably covered in tholins, organic polymers that result from the solar irradiation of organic compounds. Tholins show up all over the place in the outer system’s icy objects, adding to the view that the Jupiter Trojans were probably captured into their present orbits during the early days of Solar System formation.

Asteroid 2015 BZ509, discovered by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in 2015, turns out to be a Trojan with a difference. Revealed in the current issue of Nature by discoverers Paul Wiegert (Western University, London), Martin Connors (Athabasca University, CA) and Christian Veillet (Large Binocular Telescope Observatory, Tucson), the object is a Trojan moving in a retrograde orbit. [Addendum: See the comments below, where some writers question my use of the term ‘trojan’ to describe this object].

We think of Trojans as moving in the same direction as Jupiter, though offset from the giant planet by 60° ahead or behind (at the L₄ or L₅ Lagrangian points), so this object is a surprise. While a few non-Trojan asteroids do orbit the Sun in retrograde orbits, 2015 BZ509 is in a tight relationship with Jupiter, tracking its orbit in reverse in a way that allows it to weave out of the planet’s path each time they approach each other.

Image: 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. Credit: Wiegert et al.

The situation is stable as 2015 BZ509 passes once inside and once outside Jupiter each time they orbit the Sun, with Jupiter’s gravitational tugs on the planet canceling out to maintain the relationship. The condition is called a retrograde co-orbital resonance, a topic explored by Anthony Dobrovolskis (NASA Ames) in 2012, and followed up on by Helena Morais (Universidade de Estadual Paulista, Brazil) and Fathi Namouni (Université de Nice-Sophia Antipolis, France) in the years following. Their work established the theoretical existence of such objects, but 2015 BZ509 marks the first time an actual asteroid has been found in this kind of resonance. How it got there remains a mystery.

Image: 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. The white dots, spots and stripes are imaging artifacts in these raw images. Credit: Wiegert et al.

Although 2015 BZ509 never gets closer to Jupiter than 176 million kilometers, it is Jupiter’s gravity that controls its movements and keeps it from what would otherwise be a collision with that planet. We may be looking at an inactive comet nucleus here, although there is no sign as yet of cometary outgassing or the formation of a tail, which in any case might not form because of the asteroid’s distance from the Sun.

While its retrograde motion sets it apart, 2015 BZ509 joins the ranks of co-orbital asteroids that share a planet’s orbit, a list that includes Earth’s co-orbital 3753 Cruithne, 2010 SO16 and 2002 AA29. All of these are in a 1:1 mean motion resonance with our planet, meaning that they orbit the Sun in the same average amount of time that the Earth does.

The paper is Wiegert et al., “A retrograde co-orbital asteroid of Jupiter,” Nature 543 (30 March 2017), 687-689 (abstract).