Why do astronomers find it difficult to locate extrasolar plantes with telescope

Ground based telescopes can not detect a planet in the brightness of Star.

Explanation:

The central star is several million times brighter than the planet..So to detect a planet from ground based telescope is very difficult.
Out of 3000 exoplantes now found 95% were found by Kepler space telescope which is specially designed to detect planet by transit method..Another method is to measure the wobbling of star when the planet comes near the star

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.