Jean Bourke answered on 12 Nov 2012:
That’s a good question. There are many planets out there but we don’t know what they are like. I don’t really know how we find them, I assume it involves massive telescopes!
We don’t really know how many are out there or what they are made of, planets are still being formed in other solar systems and planets are being destroyed by the star they orbit dying. A lot of good (and a lot of really bad) science fiction has been written by people trying to imagine what these planets are like. Arthur C. Clarke has some great short stories!
Sorry for not being much use, maybe someone else knows more about how we work this out. You could also try asking someone in the space zone.
Best of luck.
Enda O'Connell answered on 13 Nov 2012:
Astronomers and humans in general have been looking at the night sky for thousands of years and lots was known about our solar system before modern times. The Earth, Sun, Moon, Mercury, Venus, Mars, Juptier and Saturn were observed prehistory, while Uranus and Neptune were discovered in the past 250 years. Pluto was discovered in 1930, and was called a planet until 2006, when it was renamed a “Dwarf Planet”. For the past 150 years then, no new planets have been found in our Solar System and it is unlikely any more will be. Astronomers have searched the sky using telescopes, radio telescopes, satellites and using calculations which say that it is unlikely that any other planets are there as their size and gravitational pull would have an effect on the orbit of existing planets, which just isn’t seen.
However, if you are talking about exoplanets found outside our solar system, this is where it gets really interesting as a whopping 843 have been found, as of last week! In astronomy, one of the latest projects is the search for planets which might support human life, and a space observatory called Kepler was launched by NASA in 2009 to do just this http://kepler.nasa.gov/. As you’d imagine, the area where most people are looking is similar to where the Earth is found, orbiting a star like our Sun. The area where a habitable planet is most likely to be found is called the “Goldilocks Zone” and is not too far from the star to be too cold or too near which would be too hot, but is just right for life!
Tim Downing answered on 13 Nov 2012:
If the universe is continually expanding then how will we every be sure? Astronomers can measure the time it takes for light to reach the telescopes here on earth to measure how far away different planets. This means we can get an idea of how many new planets we discover. One of Ireland’s biggest telescopes is in Birr in Offaly where we can explore space and maybe find new planets.