While finding a missed ring in your home might give you enough headache, think about finding an unknown abstract body lost in a universes of universes. I say lost here, to signify the existence not being known.
One method scientists say they could possibly give a chance of finding a planet would be to identify any wobble effect of a star, and this should be significant enough to be detected. It seems that, it becomes extremely difficult rather impossible to identify the planets based on the millions of photographs captured for decades by the human plugged bodies in the space; I mean the satellites. The reason being; the fact that a planet is trillion times fade'r with the effect of star in its galaxy.
To experiment put a ~0 Watt bulb closer to a 150 Watt bulb, and move away (out of the scope) from the home and what you can see, all is a little piece of light. It is highly impossible to distinguish the sources. And as always known the stars are tremendously larger than the planets.
So having said the above, how could a new planet like "51 PEGASI b" be found and confirmed their presence? click here.
Theory: The objects that come closer to us will emit wavelength close to that of the blue spectrum and the ones going further generates red. As stars revolve in their respective galaxies, the ones that produces the wobble effect with varying patterns of blue and red wavelengths emissions, could possibly be neighboured by a planet (neighbor might be 1000's of times the distance between the Sun and the Earth).
This is the way the new planet "70 Virginis.
" was found. It is also known as the planetary heavy wieght (7-10 times bigger than Jupiter) which is in the Virgo galaxy.
1) Doppler technique - mass of the planet but no the size.