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This the solar system, our “neighborhood”. 

 

Our solar system continues to deliver surprises 400 years after Galileo Galilei used his telescope to get the basics sorted out.

Our solar system seemed a very different place when Galileo Galilei first turned his rudimentary telescope to the night sky 400 years ago.
Most people at the time believed Earth was the center of the universe. Five known planets - Mercury, Venus, Mars, Jupiter and Saturn - one Moon and the Sun orbited Earth.

Galileo was among a group of scholars who suspected the planets - including Earth - revolved around the Sun. This heliocentric theory was developed by Nicholas Copernicus half a century earlier, but had not gained widespread acceptance.
The telescope gave Galileo the perfect tool to put the theory to the test.
His earth-shaking discovery of four moons orbiting Jupiter - Io, Europa, Ganymede and Callisto - and his observations that Venus appeared in the sky in phases like our own Moon helped confirm the heliocentric theory - and eventually changed the view of our solar system forever.
Four hundred years later, we are still adjusting our definitions of the solar system.
Pluto, for example, was recently reclassified from planet to dwarf planet to plutoid after new discoveries suggested the variety of unique objects in our solar system didn't fit into the traditional definitions.
Of course, our view today is far better than Galileo's. Advanced telescopes and a fleet of robotic explorers are extending our vision deep into space and returning a flood of new information. Armchair astronomers can call up the latest images and data on their home computers