The Sun
No, except mars
If they do we have not found them.
The meaning of the term "alien world" is somewhat ambiguous. No evidence of life on any planet in the solar system except earth, or on any planet around any other star, has yet been found.
It used to be. Scientists have found frozen water on Mars.
In the Solar System scientist found a new planet right now they have nicknamed it to Super Earth. They found water, plants, and another sun. They have a filling that there is life on that planet.
No, there's no such thing as Planet-X. It was a theoretical planet thought to be far out on the edge of the solar system, but was never found.
Every planet in our solar system except earth is uninhabitable to humans without proper precautions. On Mercury you would be simultaneously baked, fried, boiled and crushed due to it's atmosphere. However planets outside our solar system have been found which might have similar conditions to earth i.e. liquid water and oxygen.
The earth, along with the rest of the solar system, was formed approximately 4.5 billion years ago.
Yes! Life could be found on planet earth!
Earth is one of many planets. There are 8 planets in our own solar system and over 400 exoplanets found so far.
A big ball of gas and helium. Not if your planet is the Earth. Earth has iron at its center.
A planet found outside of our solar system is called an "exoplanet" or "extrasolar planet"