When was pluto formally downgraded from planet to dwarf planet? a. after the new york times published an article about pluto b. after a vote by the international astronomical union c. after dr. tyson received a letter from a concerned child d. after the hayden planetarium's solar system exhibit opened

B. After a vote by the International Astronomical Union.

Explanation:

Pluto was downgraded following a meeting of the International Astronomical Union (IAU) in August 2006. The Union revised what astronomy defined as "planet," and settled on three essential characteristics that a celestial body needs to have in order to be considered a planet: the object should be in orbit around the Sun, it should have sufficient mass to assume a nearly round shape and it should have cleared the neighbourhood around its orbit. Following a vote, it was established that Pluto did not fulfill the necessary requirements to be considered a planet.

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explanation:

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