Why is it difficult to detect planets orbiting other stars?
It is difficult to detect planets orbiting other stars because they are distant, small and not very bright.
Planets are quite small objects and do not emit a lot of light like a star does. As the nearest star is over 4 light years away, any exoplanets are not going to be visible with even the most powerful telescopes.
Exoplanets are detected indirectly. If a large planet is in orbit around a star, the planet and the star orbit around their centre of mass. This causes the star to wobble. So, if a star wobbles it either has a companion star, a planet or both.
Another way of detecting an exoplanet is if it passes between its star and Earth. This is called a transit. During a transit the light from the star will dim slightly. Its spectrum may also change if the planet has an atmosphere and absorbs certain wavelengths of light.
Now that we have specialised space telescopes it is much easier to detect exoplanets. The Kepler telescope was designed for this purpose and has detected over 3,000 exoplanets.