You may have heard about the famous statement in Einstein's General Relativity that light bends around a gravitational field. Imagine there is a distant star A, and sometimes later a dimmer star B approaches and locates right between star A and us. The gravitation field of star B bends the light from star A as a convex lens does and we can observe a sudden brightening of star
A¡°. (Much brighter than the total brightness of star A and B.) This phenomenon is called microlensing.
Microlensing is not only an interesting and rare sight, it also provides us an essential tool to locate exoplanet (planets in other solar system). If star B has a planet, there will be a brief but prominent fluctuation in the light curve of star A. The mass of the planet can be as small as the Earth! Microlensing is more sensitive than other methods, which can only detect Jupiter-sized planets. Perhaps the only disadvantage of Microlensing method is the rarity.
Although our knowledge about exo-solar system is very limited, Earth-sized planets may be logical starting point for future search of extraterrestrial life. Mankind may visit these planets somedays!
¡°The image of star A is actually a tiny ring rather than a point. However, this ring is too small to be resolved even with the largest telescope.