Final answer:
Pluto lost its planet status in 2006 when the IAU redefined what constitutes a planet and Pluto did not meet the third criterion because it has not cleared its orbital neighborhood. The case of Pluto and the early chaotic solar system has implications for understanding planetary formation, including Mercury's strange composition.
Step-by-step explanation:
The planet that recently lost its status as a planet is Pluto. In 2006, the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet.
The IAU established three criteria for a celestial body to be considered a planet: it must orbit the Sun, be spherical in shape due to its own gravity, and have cleared its orbital neighborhood of other debris. Pluto did not meet the third criterion because it shares its orbital space with objects in the Kuiper Belt. The decision to downgrade Pluto's status led to a significant discussion among astronomers and the general public, with varied reactions from support to disappointment.
Understanding the early solar system is crucial, as it was a chaotic place with potential impacts that could strip planets like Mercury of their rocky material. Early giant impacts are thought to explain why Mercury has a large metal core and a comparably smaller silicate mantle, showing evidence that it lost part of its original material.