He was an Italian astronomer he provided a number of scientific insights that laid the foundation for future scientists. His investigation of the laws of motion and improvements on the telescope helped further the understanding of the world and universe around him. Both led him to question the current belief of the time, that all things revolved around the Earth.

 he's studies where all around Astronomy, modern physics,

cosmology, mathematics and philosophy. 

He invented an improved telescope that let him observe and describe the moons of Jupiter, the rings of Saturn, the phases of Venus, sunspots and the rugged lunar surface, the milky way was made up of stars and also a thermometer, compass, and the pendulum clock.

If Galileo Galilei never existed we would know so much less about outer space and our knowledge would not be nearly as complex however it would have prevented us from achieving many of our achievements today.

Around 1508, Copernicus had begun developing his own celestial model, a heliocentric planetary system. HE had invented a geometric planetary model with eccentric circular motions and epicycles, significantly from Aristotle's idea that celestial bodies moved in a fixed circular motion around the earth. In an attempt to reconcile such inconsistencies, Copernicus's heliocentric solar system named the sun, rather than the earth, as the center of the solar system.

Nicolaus Copernicus was a Polish astronomer who put forth the theory that the Sun is at rest near the center of the Universe, and that the Earth, spinning on its axis once daily, revolves annually around the Sun. This is called the heliocentric, or Sun-centered, system.

we hadn't discovered that we go around the sun as well as all the plant not the work d revolving around us.

German astronomer who discovered three major laws of planetary motion, conventionally designated as follows:  the planets move in elliptical orbits with the Sun at one focus, planetary orbit is proportional to the area of the sector between the central body, and lastly there is an exact relationship between the squares of the planets’ periodic times and the cubes of the radii of their orbits this is called the the “harmonic law”). 
If he never existed we would never know that the we evolved around the sun and we move with orbits around the sun, lastly we wouldn't have made the discoveries of Nasa and the new planets.