It is difficult to see a planet close to a star. An artist's view.
Paris Observatory / UFE
As of today, we usually cannot directly observe exoplanets : we only know that some exoplanets are in orbit around stars because we detect the specific effects that they produce on these stars.
Why is it difficult to « see » an exoplanet?
First, exoplanets are distant, and therefore, very faint.
Indeed, a planet like Venus is easily visible to the naked eye because it is quite close to the Earth. But at a distance of 4 light-years from the Sun (the distance of the nearest star), its brightness would be divided by one hundred billion. Nevertheless, this faint light is still detectable with present-day large telescopes.
Second, an exoplanet orbits a star which is typically ten billion times brighter. Because of this, we are dazzled by the light of the star, and the planet appears to practically merge with it.
One can thus imagine that it is very difficult to detect among ten billion tiny light packets (photons) that reach us from a star a single one that comes from its planet.
Nevertheless, one has recently been able to observe a few exoplanets (4 observations up to July 2007) with direct imaging methods under very specific conditions. This was possible because the planets were of very large sizes and were orbiting faint stars (e.g. brown dwarfs) at very large distances. These extreme conditions can enable direct observations, but the systems one finds are necessarily very different from our own Solar System.