Stardust and eternity – 3.1.7

Minor bodies of the Solar System

Besides the nine major bodies – the Sun and the eight major “terrestrial” and “jovian” planets – our Solar System is composed by a plethora of smaller orbiting objects, including asteroids, comets, trans-neptunian objects, and small planetary satellites and rings. These “minor bodies” mostly represent original planetesimal pieces, i.e. remnants from the time of Solar System’s formation – approximately 4.6 billion years ago – which never accumulated enough material to form proper planets.

Most of the known minor bodies (about one million) lie in an orbit between Mars and Jupiter, the so-called “asteroid belt”, taking three to six years to make a complete revolution around the Sun. Moreover, tens of thousands of them are actually near-Earth objects that can potentially approach our planet more or less closely, posing a potential threat – if large enough – to human life. A well-known example is the 10 km asteroid that impacted the Yucatan Peninsula in Mexico about 66 million years ago, bringing the dinosaur era to an abrupt end. Astrophysicists suggest that some of the water and organic compounds that were first brought to Earth were actually delivered by asteroid impacts: this is why asteroids can be rightly considered as celestial bringers of both life and death! Asteroids are relatively small, rocky or metallic objects without an atmosphere, whose size ranges from a few meters up to hundreds of kilometers; their combined mass is less than that of our Moon. Ceres, the largest known asteroid – about 1,000 km in diameter – is now classified as the only “dwarf planet” within Neptune’s orbit.

Similarly to asteroids, “comets” are relatively small (up to a few kilometres) dirty snowballs that revolve around the Sun in highly elliptical orbits and represent remnants from the time of the solar system formation. They mostly originate from a doughnut-shaped region located at the edges of the solar system, composed by millions of so-called “trans-neptunian” – beyond Neptune – objects: when one of these frozen rocks is perturbed by the gravity of another object, it is thrown into the inner solar system and becomes a “comet”. Unlike asteroids, comets are characterized by the presence of an extended atmosphere of ice and dust which is loosely bound and forms their typical tails as they approach the Sun. This phenomenon occurs because solar radiation causes the volatile icy materials – mostly water – inside the comet to vaporize in a tenuous atmosphere, which is called “coma”. The particles expelled by the dirty snowball are pushed away from the Sun by the solar radiation pressure and wind, regardless of the direction in which the comet is moving. After several revolutions around the Sun, comets are doomed to increasingly vaporise and eventually break apart completely.

Although asteroids, comets and transneptunian objects numerically constitute the vast majority of the so-called minor bodies of the Solar System, hundreds of planetary satellites or moons around the major planets are known – besides Mercury and Venus – together with their systems of rings. Among these moons, only three of them – i.e. our Moon and Mars’ Phobos and Deimos – orbit terrestrial planets. Other satellites orbit outer gaseous “jovian” planets. Jupiter and Saturn each have around 80 known moons, with Jupiter’s Ganymede being the largest moon of the Solar System – it is slightly larger than Mercury – and Saturn’s Titan being the second largest natural satellite. Titan is the only moon in the solar system that has a dense atmosphere, as well as the only known celestial body with rivers, lakes, and seas (albeit of liquid ethane and methane) on its surface.

Finally, the four jovian planets display systems of rings, the most spectacular being that of Saturn, approximately 400,000 km wide and 100 m thick; they consist of countless small particles, up to meters, made almost entirely of water ice, with only a trace of rocky material.

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The Galilean satellites (Kenneth R. Lang, Tufts University)