Defining Pluto and a 'Planet'

What is the issue?

• In 2006, the International Astronomical Union (IAU) voted to remove Pluto’s planetary status.
• Some researchers are now challenging this decision.

What was the 2006 IAU's decision?

• The IAU, in 2006, designated Pluto a ‘dwarf planet’ along with Ceres in the asteroid belt and Xena.
• Xena is an object in the Kuiper belt which is an icy ring of frozen objects that circle the solar system beyond Neptune’s orbit.
• The designation was a bid to overcome sentiment and go by scientific rationale.
• The meeting accordingly defined three conditions for a celestial object to be called a 'planet' -

1. it must orbit the Sun
2. it should be massive enough to acquire an approximately spherical shape
3. it has to ‘clear its orbit’ i.e. be the object that exerts the maximum gravitational pull within its orbit

• 'Dwarf planets', on the other hand, need to only satisfy the first two conditions.

Why is Pluto not a Planet?

• As per the third condition, if an object ventures close to a planet’s orbit, it will either collide with it and be accreted, or be ejected out.
• But, in case of Pluto, it is affected by Neptune’s gravity.
• It also shares its orbit with the frozen objects in the Kuiper belt.
• Based on this, the IAU deemed that Pluto did not ‘clear its orbit’ (the third rule).
• Hence, it was designated a dwarf planet.

What is the contention here?

• Third rule - The above rationale has been questioned by some who put forth several exceptions to the third rule.
• They cite the manner in which scientific tradition has dealt with the taxonomy of planets.
• The only work in history that used the third rule to classify planets was an article by William Herschel in 1802.
• It is also argued that this work was based on reasoning and observations that have since been disproved.
• However, the argument is not a strong enough case to give up what is, in fact, a sensible rule.
• Evidently, physics has many examples where an idea once discarded for being incorrect, later emerged in a different form and gained acceptance.
• Planet - Given these, even if Pluto were to be re-designated a planet, many more complications would arise.

• Charon, Pluto’s moon, is much too large to be called a satellite.
• Judging by this, the Charon-Pluto system should then rightly be called a binary planet system.
• This would then lead to classifying several other sets of bodies as binary planets.
• Also, both the Kuiper Belt and the Oort cloud contain objects that can then be called planets, thereby complicating the issue.
• Oort cloud is a shell of icy objects that surrounds the entire solar system far beyond the Kuiper belt.
• Denying planetary status to Pluto is the easy way out of the debate at this stage.
• Hence, Pluto remains a dwarf planet, although an exceptional one.

Source: The Hindu

Quick Fact

International Astronomical Union (IAU)

• The International Astronomical Union (IAU) was founded in 1919.
• Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation.
• It serves as the internationally recognized authority for assigning designations to celestial bodies and surface features on them.