When
Some of the planets have been known to people for as long as they have been watching the stars. Mercury, Venus, Mars, Jupiter, and Saturn can all be seen with the naked eye, so it's impossible to see the outer planets without a telescope. In the 1700's, a more powerful telescope was invented to let more people see the outer planets that can not be seen with the naked eye. People such as William Herschel and Johann Gottfried Galle saw the outer planets with the telescope that was invented to let people see the outer planets and see the other things in space. Such as the stars, The other planets, asteroids, and the moon.