Pluto became part of the solar system on March 13, 1930. However, for the past 87 years, it still faces its planetary identity crisis.
The scientists at Lowell Observatory in Arizona searched for millions of stars in the solar system in hope to discover a new planet. In February 18, 1930, astronomer Clyde Tombaugh found Planet X and called it Pluto later on.
On March 13, 1930, the Planet X becomes an official part of the solar system. It was named Pluto to honor Percival Lowell, a mathematician, businessman, and astronomer; founder of Lowell Observatory. Clyde Tombaugh and his boss Vesto Slipher worked at the observatory where they began searching Planet X for 24 years before it was discovered.
Some astronomers started to question the identity of Pluto as Planet X; too small to create orbital perturbations. Fifty years later, the mass of Pluto was calculated, astronomers argued over, and then it was recalculated once again. When Charon - Pluto's moon - was discovered in 1978 they concluded that the mass of the planet is about 1/500th of the Planet Earth.
But then, in 1989 after receiving the data from Voyager 2 that crossed Neptune, the status of Pluto as Planet X was questioned once again. It turns out that some observations about the planet are insufficient; the mass of Neptune was overestimated too. There were also mess of bodies orbiting around the sun with the same area, size, and similarly skewed orbit as Pluto in early 1990s.
After all the gathered pieces of evidence, the International Astronomical Union (IAU) declared Pluto as a dwarf planet in 2006 and removed as part of the solar system. It now shares the same status with four other dwarf planets and six other candidates.
However, Pluto might be part of the solar system once again after the close-up photos of the dwarf planet from NASA's New Horizons in 2015. Some scientists spotted and believed that there might be some clouds that surround Pluto. The head of New Horizons, Alan Stern, said if a celestial body have clouds space travelers think it's a planet, and the clouds seen on Pluto was enough to believe that it is really a planet.