Solar System

A first introduction to our neighborhood

The solar system consists of a central star, the Sun and eight planets… plus several dwarf planets, dozens of moons or satellites, millions of asteroids and Trans-Neptunian Objects (TNOs), and myriads of comets and meteoroids. So yes: there is much more than only the Sun surrounded by the famous eight (well... eight?) planets!

In 2006 the International Astronomical Union (IAU) specified the differences between the objects in our System:

• A planet is a celestial body that:

  • is in orbit around the Sun;

  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium which makes it to have a (nearly) spherical shape;

  • has cleared the neighborhood around its orbit.

• A dwarf planet (or a planetoid) is a celestial body that:

  • is in orbit around the Sun;

  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium which makes it to have a (nearly) spherical shape;

  • has not cleared the neighborhood around its orbit;

  • is not a satellite.

• All other objects orbiting the Sun shall be referred to collectively as Small Solar System Bodies. These include most of the asteroids, Trans-Neptunian Objects, comets, and other small bodies.

So, to answer one of the most debated questions: according to the IAU’s 2006 definition, Pluto is a dwarf planet and the prototype of a new category of Trans-Neptunian objects.

This is the highest-resolution color departure shot of Pluto's receding crescent from NASA's New Horizons spacecraft, taken when the spacecraft was 120,000 miles (200,000 kilometers) away from Pluto. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Pluto lies in the Kuiper Belt, a donut-shaped region of icy bodies beyond the orbit of Neptune. In this distant region of our solar system there may be millions of these icy objects, collectively referred to as Kuiper Belt objects (KBOs) or trans-Neptunian objects (TNOs). But not too many details now: the whole story is available in the members area of our website. Being a member if completely FREE: just log-in or register here!

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