What is a planet, after all?

It's been a few months since I posted, actually almost exactly 7 months... Let's see if I can keep it going now. Meanwhile, my science life has changed quite a while, with a much greater drive to astrobiology than for cosmology. And this, particularly the discussion about planets, is what brought me back.

You all may have heard that Pluto is risking to be disconsidered being a planet after the IAU Meeting that is taking place in Praga right now. In August 22 there will be a voting session, conducted by an IAU Commission, called the Planet Definition Committee, to decide, among other things, the statuts of Pluto as a planet, after the discovery of some other cousins with comparable dimensions and as large a period as Pluto.

Naturally, such discussions involve not only scientific but also social, historical and even philosophical aspects. For now, we accept the following criteria to define a planet:

1. Planets must have masses above 5E20 kg
2. Planets must have diameters above 800 km
3. Planets must be spheroids (coming as a consequence of hidrostatic equilibrium in the presence of self-gravity)
4. Planets orbit stars, being neither another star or a satellite of another object

What about binary or multiple planets and objects? Are they different from a planet and its satellites? If the primary (most massive) object satisfies the above criteria, it will be a planet. For the secondary or others, if the baricenter of the considered objects reside outside the primary, the secondary will be also considered as a planet and it will be a binary (or multiple) system. Otherwise, the secondary (less massive) will be a satellite.

We recognize now the eight classical planets, which move in almost circular orbits close to the ecliptic plane, and other planetary objects, such as asteroids, comets and the so-called TransNetunian Objects (TNO). From the above 4 definitions, the asteroid Ceres, for instance, can be considered a planet but, for historical reasons, it may be referred to as a "dwarf planet", to distinguish it from the classical ones. So, calling Ceres an asteroid is not technically correct any longer...

The TNOs are now being called "plutons" and include, besides Pluto and Charon, candidates such as Sedna, Quaoar, Orcus, Varuna, 2003 EL61 and 2003 UB313, all of them with dimensions similar (0.5 to 2 times) to Pluto, round and orbiting the Sun.

To close this post, all non-planet objects orbiting the Sun are now called "Small Solar System Bodies", as opposed to the old nomenclature: "Minor Planets".

Good to be back to this place.