How do we know there are other planets out there? Is there any proof?
Over 3,000 exoplanets have been discovered.
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.