Astronomy in-class activity: planet classification schemes

Astronomy 210 In-class activity 12 v.13.06.10, fall semester 2013
Cuesta College, San Luis Obispo, CA

Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to discuss the evolution of how planets are categorized, and how to implement the current International Astronomical Union classification scheme to categorize different solar system bodies.





Some samples of student responses (fall semester 2008) to question 2(h) ("Discuss a plausible motivation for why it was necessary to revise the definitions of planets in the 1860s and in 2006") are given below. Students in each group are referred to by a confidential four-digit identification number.

Many responses discussed the narrowing of the planet definition in order to exclude a "multitude" of planet candidates:

"We were getting too many planets, in 1860 there were 200+ large bodies and in 1992-present there were [approximately 1,300+] bodies."
--0013, 1327, 4245

"Because of the massive amounts of large bodies that would have been considered planets."
--0217, 0911, 1186

"Because there were too many."
--1208, 3248, 9387

"[A lot of] planets are a lot of planets to track. If the large bodies in the Kuiper belt count why don't the bodies in-between Mars and Jupiter of the same size count?"
--3273, 3903

"Because of new discoverys [sic] of 'bodies' so they had to limit the definition of what an actual planet could be."
--1990, 2580

"Without the definitions it would include [too many] large bodies and that would be way too many."
--4555, 9259

"There are too many 'planets' like Pluto in the outer solar system."
--3912, 5805

Some other groups also discussed other plausible motivations, in addition to limiting the number of potential planets:

"They would be hard to determine because they haven't cleared their orbit even though they do have a shape 'rounded out' by gravity. It would be hard to study because there a lot."
-3089, 4207, 7120

"Because it would be hard to have an accurate scale to compare any two 'planets.' So thus limiting the requirements makes comparisons easier."
--2639, 4127, 8187

"The definition was too wide including objects that were extremely different from each other."
--2020, 8808

"Too many bodies to classify. Easier to maintain."
--0805, 7776

"Too many variations between [planets], too many planets. Needed new scientific classifications."
--1988, 5389

Other groups discussed other reasons, but still received full credit for their responses:

"The advancement of technology gives us more accurate ways of classifying our solar system."
--1307

"We want to explore the universe and classify it."
--4018, 6586, 5997