How do we know there are other planets out there? Is there any proof?

Hint :Telescope is an optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified.

Complete Step By Step Answer:
With modern satellites it is possible to detect planets around other stars. The Kepler telescope was specially designed for this task and has detected over 3,000 planets.
If a large planet orbits a star, the planet and star orbit around their centre of mass. The centre of mass of Jupiter and the Sun is above the surface of the Sun. From a distance this makes the star appear to wobble. This is one way of detecting planets.
If a planet passes between a star and Earth, it causes the star's brightness to dim during the transit. The spectrum of the star can also change due to the planet's atmosphere absorbing light at certain wavelengths.
The nearest star to our solar system in Proxima Centauri. It is now known that Proxima Centauri has a planet orbiting around it.

Note :
While it can be easily concluded about other planets existing in our solar system and elsewhere. The main question still exists whether there is any evidence of life existing at other planets. Only time will tell how scientists prove it.