Planet Nine discovery: What is Planet 9? Is Planet X in our solar system?

Scientific convention states there are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. On August 24, 2006, Pluto ceased to be recognised as the ninth planet, following a change in definition. But debate has reignited over Planet Nine, for a mysterious celestial body posited to be out there could claim the title for itself.