Question

Assume that three irregularly shaped planet-like objects, each smaller than our Moon, have just been discovered orbiting the Sun at a distance of \(35 \mathrm{AU}\). One of your friends argues that the objects should be classified as planets because they are large and orbit the Sun. Another friend argues that the objects should be classified as dwarf planets, such as Pluto. State whether you agree or disagree with either or both of your friends. Explain your reasoning.

Short answer

I disagree with both friends; the objects don't meet the necessary criteria to be classified as either planets or dwarf planets because they are not spherical.

Step-by-step solution

Understanding the Definition of a Planet

A planet is defined by three criteria from the International Astronomical Union (IAU): (1) It must orbit the Sun, (2) it must be spherical in shape due to its gravity, and (3) it must have "cleared the neighborhood" around its orbit. We need to consider whether the objects meet all three criteria or not.

Applying the Criteria for a Dwarf Planet

According to the IAU, a dwarf planet must (1) orbit the Sun, (2) be spherical in shape due to its own gravity, and (3) have not cleared the neighborhood around its orbit. In contrast to a full planet, a dwarf planet does not fulfill the third condition.

Analyzing the Given Objects' Characteristics

The objects are described as irregularly shaped, indicating they do not meet the second criterion for being a planet or dwarf planet (spherical shape) according to IAU definitions. They are at 35 AU and orbit the Sun, which fulfills the first criterion for both classifications.

Arriving at a Conclusion

Since the objects are irregularly shaped, they do not meet the criteria for being classified as a planet or a dwarf planet. For dwarf planet status, they must be spherical due to their own gravity, which these objects are not.

Key concepts

International Astronomical Union

The International Astronomical Union (IAU) is the leading authority in the astronomical field recognized globally for naming and classifying celestial bodies. Established in 1919, the IAU plays a crucial role in ensuring consistent astronomical practices across the globe. It is responsible for setting the standards by which celestial bodies are defined and categorized. For people interested in astronomy, understanding IAU guidelines is essential for classifying cosmic objects, be it planets, moons, or other astronomical entities. This not only includes naming conventions but extends to descriptions on how these objects need to meet specific criteria to be considered within certain classifications—like planets or dwarf planets.

dwarf planet criteria

Dwarf planets have specific criteria set by the IAU that differentiate them from the planets in our solar system. To be classified as a dwarf planet, an object must meet the following three conditions:

• It must orbit the Sun, just like other planetary bodies.
• Its shape must be spherical due to its own gravitational forces.
• Unlike a planet, a dwarf planet has not cleared the neighborhood around its orbit.

If a celestial body meets all these conditions except the last one, i.e., not having cleared its orbital path of other debris, it can qualify as a dwarf planet. This is a significant distinction from full-fledge planets, which do manage to dominate their orbital zones.

orbital dynamics

Orbital dynamics refers to the study of the motions of celestial bodies as they travel through space under the influence of gravitational forces. It includes understanding how the shape of an object influences its path and stability as it orbits a star, like our Sun. Objects like planets, moons, asteroids, or newly discovered cosmic bodies exhibit specific orbital characteristics based on their mass, speed, and gravitational interactions with other bodies and their environments. These dynamics are a fundamental consideration when classifying an object as a planet or dwarf planet, as part of the criteria involves whether the object can "clear its neighborhood"—an indication of its influence over the gravitationally bound debris and smaller bodies near its path.

celestial bodies classification

The classification of celestial bodies entails systematic approaches to categorizing various objects in space according to their physical and orbital characteristics. Categories include planets, dwarf planets, asteroids, comets, and more. The criteria set by the IAU for these classifications help astronomers communicate and understand the universe more effectively. By following a formalized set of guidelines, astronomers can accurately place newly discovered objects into contextual categories, facilitating better research and knowledge sharing. This framework not only aids in organizing the celestial zoo that is our universe but also influences naming conventions, scientific discussions, and educational content disseminated to the public.