4.+Planets

Planets:

An artists expression of the extra solar planet 55 Cancri f orbiting its star 55 Cancri


 * __ Brief Description: __**

media type="youtube" key="V1wQPjFUTOw" height="344" width="425" Video talks about how extra solar planets can be found, the day and night on an alien planet, and Hot Jupiters media type="youtube" key="bHak2drJzeY" height="344" width="425" Planetary system discorved that may be very similar to our own
 * Planets** are defined as being a celestic body orbiting a star or the remains of one, it must also be large enough to be spherical in shape by its own gravity. There are two main types of planets, these include the **terrestrial planets** which are composed of mainly rock and metals and gas giants or **Jovian planets** which are mainly composed of hydrogen and helium gas. With these two types, there are a few classifications of planets, these include: silicon based planets, these are comprised mostly of silicon rocks, carbon based planets, comprised of carbon rocks, **ocean planets**, **hypothetical** planet covered in a deep ocean, **hot Jupiters**, which are large gas giants orbiting close to their stars and **eccentric Jupiters** which are gas giants which orbit there star in an eccentric way comparable to a comet.

**__Habitability:__**

The level of habitability on planets can be judged on various factors. Firstly the planet should be located at the **habitable zone** of a planetary system, in our solar system this zone is located 1 AU from our sun. In systems which stars are larger than our sun, this zone is located further away, while dwarf stars zones are closer. Secondly, planets with the best chance of life are terrestrial planets which are geologically active, this activity continuously refreshes the surface minerals necessary to keep a stable ecosystem. The planet also needs a steady gravity, able to keep a healthy atmosphere and a temperature capable to keep liquid water which is essential for life. The capability to host extra terrestrial life on Jovian planets is widely disputed, some scientists believe large flying creatures could survive on the atmospheres of these planets, these are called atmospheric beasts. However, there are many drawbacks to this theory, firstly the gravity on Jovian planets is immense and the amount of radiation that is emitted maybe be too much for any life form, gas giants also lack liquid water. The moons of Jovian planets are a better candidate

Earth takes up the habitable zone in our solar system


 * __Justification:__**

Planets are obviously the most suitable system to hold life, one very important point we must consider is the fact that earth is capable of hosting life and earth is infact a planet. This is the number one justification of why planets are suitable for life, for there is extremely strong evidence. Simply, Planets are most suitable for life because we live on a planet.


 * __Glossary__

Planet:** 'A celestic body orbiting a star or the remains of one, it must also be large enough to be spherical in shape by its own gravity.' (International Astronomical union) **hypothetical:** 'based primarily on assumption rather than adequate evidence' (Dictionary.com)
 * Terrestrial Planet:** 'A small planet of high density with a solid, rocky surface. ' (Encyclopedia of Science)
 * Jovian Planet: **A gas giant, planets which have no solid surface and are made up mostly of Hydrogen and Helium.
 * Habitable Zone:** 'An imaginary spherical shell surrounding a star throughout which the surface temperatures of any planets present might be conducive to the origin and development of life as we know it. Also referred to as the //ecosphere//.' (Encyclopedia of Science)
 * Hot Jupiter:** 'Extrasolar planets, similar in mass to Jupiter, with remarkably small, circular orbits around their host stars and orbital periods of less than 10 days. '(Encyclopedia of science)
 * Eccentric Jupiter:** 'Extrasolar planets, with a mass similar to or greater than that of Jupiter, which move around their host stars in highly elongated orbits'. (Encyclpedia of science)
 * Ocean Planet:** 'A hypothetical planet that is completely covered by ocean . ' (Encyclopedia of Science)
 * Extra-Terrestrial Life:** Life that originates from outside Earth