To find a planet, you have to be able to see it. Of the eight planets in our solar system, six are visible with the naked eye: Mercury, Venus, Earth (just look down!), Mars, Jupiter, and Saturn. To see the other two, Uranus and Neptune, a telescope was necessary, but astronomers were able to discover these planets over 150 years ago! (Pluto is no longer considered a planet, but it was discovered about 100 years ago.)
To find more planets, people have used increasingly powerful telescopes. However, a telescope on the earth is limited by the earth's atmosphere, which absorbs some of the light from space from reaching the telescope. To see even farther into space, scientists have launched telescopes into orbit around the earth or around the sun. This allows the telescopes to take pictures of space largely outside of the earth's atmosphere. And, NASA is not the only agency launching telescopes into space to find planets. India, Japan, Russia, and a coalition of European countries are all looking for new planets!
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