What is a Planet?

There has always been some talk about redefining what a planet is.  The link below gives a brief summary of the debate that erupted in 2017.

http://www.astronomy.com/news/2017/03/is-it-time-to-restore-plutos-status

The word planet comes from the Greek word planan which means 'wander'.  Ancient observations showed that these objects in the sky would wander around compared to the background stars.

According to the International Astronomical Union (IAU) Q&A section it states in plain terms:

A planet is an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round (or near-spherical) shape. In addition a planet orbits in a clear path around the Sun. If any object ventures near the orbit of a planet, it will either collide with the planet, and thereby be accreted, or be ejected into another orbit.'
The exact wording of the definition is given below:

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

 A dwarf planet is defined as the following:

'A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.'

This would put Pluto in the category of Dwarf Planet.  Four other recognized dwarf planets are Ceres,  Haumea, Makemake and Eris.

Pluto (Image Credit: NASA)

The planets as everyone knows are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.  Most of them have satellites which we call Moons.  Some Moons are larger than the dwarf planets but they do not fit the criterion is not a satellite and therefore remain categorized as such.  As an interesting fact, the largest Moon in the Solar System, Ganymede, is actually larger than Mercury.

All of the 8 planets are visible from your backyard.  Mercury, Venus, Earth, Mars, Jupiter, Saturn, and if you have really good eyesight, Uranus are visible with the naked eye.  Neptune will require optical assistance as it is fairly dim.  The image below contains some photographs I have taken of the planets and the Sun.  Uranus, Mars and the Earth are the only photographs below that I did not take.

Solar System

Ceres and Pluto can be viewed from your backyard.  Pluto requires a significant skill and effort as it is on the cusp of limiting magnitude for most amateur telescopes.

Ceres (Image Credit: NASA)

Keep an eye on our Facebook Page for updates as to when you can easily see each planet!