why is visual dectection of planets orbiting other stars close to impossile?

Answer 1

visual detection of those planets is not impossible per se because of any physics limitation, but it is not possible using our current instruments

Answer 2

Starlight swamps them out. For instance, the sun has a luminosity of 2E33 erg/s, while the earth reflects about 1 billionth of that, give or take an order of magnitude. In the thermal infrared the matchup is more even, maybe only a million times brighter. In a typical telescope, the spot size of the star even a few lightyears away would cover the Earth's orbit completely drowning the signal of the star. A technique called nulling interferometry, or more likely formation flying interferometers with a large baseline provides the best hope of imaging an earth like world (earth size and orbit) around another star, and it will probably be done in the IR to boot.

Read about Terrestrial Planet Finder to learn more about the U.S. proposed attempts to do this, http://planetquest.jpl.nasa.gov/TPF/tpf_sample.cfm

and the ESA's plan, Darwin: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31165

Answer 3

The size and luminosity of the star swamps out the size of the planet and very precise instruments only recently used have been able to detect planets around other stars. Even so, they are large planets, like Jupiter. I do not believe we have the capability to resolve planets the size of Earth.