article image

Retrograde motion – which bodies orbit in the opposite direction?

Kulvinder Singh, News reporter
May 30, 2015, 20:26 UTC

Sen—If you observe planets such as Mars, Jupiter and Saturn changing position in the night sky you may see something curious. Sometimes these planets appear to move backwards for a few nights before continuing as before. They are not actually moving backwards in their orbits. It is simply an illusion caused by the fact that Earth orbits faster around the Sun than these planets and overtakes them. The effect is called apparent retrograde motion. However, some objects in our Solar System—and in extra-solar systems—do in fact have true retrograde motion.

All eight major planets in our Solar System, as well as all known dwarf planets, orbit the Sun in the same direction that it rotates—known as prograde motion.

The entire Solar System formed from a hot disc of material some 4.56 billion years ago. All parts of this disc, though rotating at different rates depending on distance from the central protosun, would nonetheless have rotated in the same direction. When parts of the disc underwent gravitational collapse to form planets, these bodies continued orbiting the new Sun in that same direction. Viewed from above the Sun's north pole, the planets would orbit anti-clockwise.

A planet's solar orbit however—its year—is distinct from its rotation—its day. Alongside its orbit, each major planet—except for Venus and Uranus—has a prograde rotation about its axis. Venus's rotation axis is tilted by 177.36°. It has almost entirely flipped over, so looking from the Sun's north pole, Venus's rotation is clockwise. Uranus's rotation axis is tilted by 97.77°. It is lying on its side, with its axis almost within the plane of its orbit.

Small bodies such as asteroids, comets and moons may also have started out in prograde orbits—either around the Sun or their respective planet. But the effects of collisions, capture by large bodies and gravitational interactions mean that some of these objects now have true retrograde motion. One example is Triton, Neptune's largest moon. Triton's retrograde orbit with respect to Neptune's axial rotation tells scientists the the moon probably did not form in-situ—in Neptune's gravitational field—but rather was captured later on. Smaller moons of the other large planets also have retrograde orbits.

Uranus's 150-kilometer and 36 kilometer-sized moons Sycorax and Caliban are two examples. Other examples are moons belonging to Saturn's Norse group, and Jupiter's Carme, Ananke and Pasiphae groups. In total, as of May 2015 there are 62 known moons with retrograde orbits.

There are also 61 known asteroids with retrograde orbits around the Sun. Comets can also have retrograde orbits especially if they originate from the Öpik-Oort Cloud, a distant 'shell' of icy/rocky cometary bodies far beyond the orbit of Pluto.

Beyond our own Solar System there are examples of exoplanets (short for extra-solar planets) with retrograde orbits around their stars. The first two—WASP-17b and HAT-P-7b, in the constellations Scorpius and Cygnus respectively—were discovered in 2008. Since then there have been other examples. Retrograde orbits can arise in planetary systems due to collisions or near-collisions between bodies early in a planetary system's formation. Other reasons include gravitational interactions between bodies, or simply the star itself flipping over on its rotation axis.

In 2006 scientists discovered that a protoplanetary disc in the constellation Ophiuchus, IRAS 16293-2422—a disc of material still forming into a star and planets—had counter-rotating sections. This cannot arise due to collisions or gravitational interactions. Instead, protoplanetary discs sometimes stream-off gas and dust from large molecular clouds or other discs. Providing enough mass is taken, interactions with this new ring of material can tilt existing parts of the forming disc—and even flip it over. Any planets that formed in those sections would then be retrograde by default.