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 Past Perfect
Pluto had been considered the ninth planet until 2006 when it was renamed a dwarf planet.
 Perfect Aspect
 Present Perfect
If the result of the past action continues now, we use present perfect.
 Present Perfect
Scientists say they have found evidence of a giant planet in the solar system. There have only been two true planets discovered since ancient times. Planets have been predicted before and then were not found.
 Present Perfect or Past Simple (Part 1)
 Present Perfect or Past Simple (Part 2)