A new study allows Pluto to be embraced again as a planet in the Solar System. Today's definition of a planet has its roots in folklore and astrology and had to be dropped because it did not meet the needs of modern astronomy. Pluto is found in a ring of objects beyond Neptune's orbit known as the Kuiper belt. Then, in 1930, Pluto was declared the ninth planet in the Solar System. However, its status was questioned after several other objects of similar size were discovered in the Kuiper Belt. Then, the International Astronomical Union (IAU) downgraded Pluto to a dwarf planet in 2006. This is in line with the new definition of a planet adopted by the IAU, in which a celestial body must orbit the Sun, be nearly spherical in shape and have dominant gravity and have completed its own orbit. Pluto was eventually disqualified because its orbit intersects with Neptune's orbit and shares an orbital environment with other objects in the Kuiper Belt. To prove this, Philip Metzger of the Florida Space Institute at the University of Central Florida, United States (USA) studied most of the planetary literature over the last 400 years. According to a paper entitled 'Moons are planets', the definition introduced by Galileo in 1600- an is a planet only needs to be a geologically active object in space.
This definition has been used by scientists, but was eroded in the 20th century due to the proliferation of almanacs. "In the UK and the US enough almanacs are sold that each household can get one copy each year," explains Metzger.
Almanacs provide readers with a wide variety of information. From calendars of astronomical events to recipes and fiction. However, there is also a heavy emphasis on astrology, including astrological weather forecasts that can only be made if the number of planets is limited.
"This is a key period in history when the public accepted that the Earth orbits the Sun, not the other way around. They combined this great scientific insight with the definition of a planet derived from astrology," said Metzger.
The view that the Moon and its satellites should not be considered planets later became scientific literature. But this definition no longer works because astronomy began to rely on advanced technology that would allow it to study space more thoroughly.
"There's been a boom in the number of exoplanets we've found over the last 10 years. That will improve as we put better telescopes in space. We need to refine the definition of a planet before we get too far."
Metzger and his colleagues are pushing for a return to Galileo's definition and if their call is heard, Pluto will become a planet again with many other celestial bodies joined.