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FAQ - Dwarf Planets

1. Why is Pluto classified as a "dwarf" planet rather than some other label? It’s confusing.

The International Astronomical Union (IAU) decided several years ago that they needed to come up with a definition for a planet (for use with the objects in our Solar System). The IAU has various committees that approve the names of comets, asteroids, planetary satellites, and features on these objects. There was no committee that was empowered to name new planets. With the discovery of objects as large as or larger than Pluto, there needed to be a definition so that it could be used for determining what these newly-discovered objects would be called and thus how they were to be named. A definition for a planet was presented to the entire IAU, but a number of people objected to the definition that would leave Pluto as a planet but left the door open for dozens of other planets to qualify as planets. It is the opinion of this scientist that it came down to dynamics vs. geology and the dynamical definition won out, which left us with eight planets. Why dwarf planet? It followed the convention used for stars. Our Sun is actually a dwarf star (a little smaller than average). However, while a dwarf star is still a star, the dynamics people felt that it was better that a dwarf planet should not be a planet—less than a planet, but more than an asteroid. Many in the planetary community still do not agree with the new definition.

2. So is Pluto really a planet, a dwarf planet, or what is it?

People make definitions classify/group/categorize things in order to help us understand the world around us and to form a picture in our mind of what we are talking about (plant vs. animal, mammal vs. bird, dog vs. cat, etc). Thanks to the IAU (International Astronomical Union), whether we like it or not, we now have a definition for objects such as Pluto—they are dwarf planets. So now, we have a real teaching moment. Let kids decide whether or not Pluto (and the other dwarf planets) are planets or not. Have students think like a planetary scientist and ask what makes a planet a planet. Then look at what we know about Pluto and decide if Pluto is more like a planet than like an asteroid or comet.

3. If Pluto isnt a planet, then what are the characteristics of a planet?

The International Astronomical Union "voted" on several definitions of a planet. What they finally agreed upon was a definition that relies solely on the dynamical properties of objects that orbit the Sun (extra-solar planets are excluded). A planet is an object that has cleared its space (i.e., there is nothing comparable in size around it). Therefore anything in the asteroid belt or the Kuiper belt (where Pluto is) can be a planet. They came up with a new term: dwarf planet, but decided that dwarf planets are not planets, just big asteroids! This goes against how stars are classified since most stars (including our Sun) are dwarf stars, yet they are still stars.

1. From an educational perspective, humans (including scientists) classify objects in order to understand the world around us; we classify things from very early in life: shape, size, color, etc. Good science is done the same way. It helps us understand the nature of our world, look for differences (which may be significant), and helps us communicate our ideas to others.

2. There may be more than one way to classify Pluto and other objects, but to most planetary scientists, Pluto has many of the characteristics of what we picture as a planet (please note that no planet may have all of these characteristics): enough gravity to make it round, an atmosphere, warm enough interior so that in the past (and maybe the present) its surface is "active" with volcanoes, etc. Pluto and perhaps some of the other dwarf planets have many of these planet-like characteristics and as such, should be considered planets.

3. So, the bottom line is that Pluto has been reclassified as a dwarf planet. While the IAU would say that it is not a planet, the scientists who study planets would disagree and call Pluto the first of a new class of planets called dwarf planets that have many of the characteristics of the eight big planets (what we call the terrestrial planets and gas giants).

It is all how you classify and you cannot look at just one property in order to classify something.

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