Question

Assume three irregularly shaped planet-like objects, each smaller than our Moon, have just been discovered orbiting the Sun at a distance of 35 AU. One of your friends argues 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

Disagree with both friends; objects are neither planets nor dwarf planets due to their irregular shapes.

Step-by-step solution

Understanding the Classification Criteria

To classify a celestial body as a planet or a dwarf planet, we need to refer to the International Astronomical Union (IAU) definition. A celestial body must fulfill three criteria to be considered a planet: 1) it orbits the Sun, 2) it has sufficient mass to assume a nearly round shape, and 3) it has cleared its orbit of other debris. Dwarf planets meet the first two criteria but have not cleared their orbits.

Analyzing Characteristics of Discovered Objects

The objects orbit the Sun, fulfilling the first criterion for both planets and dwarf planets. Since the objects are described as irregularly shaped, they likely do not meet the second criterion for planets or dwarf planets. Their shape suggests they haven't achieved hydrostatic equilibrium.

Determining the Classification

Given the irregular shape of the objects, they do not meet the requirement of being nearly round, which is necessary for both planet and dwarf planet status. Moreover, there is no mention of these objects having cleared their orbits.

Conclusion: Agreeing or Disagreeing with the Friends

Based on the IAU definitions, the discovered objects likely do not qualify as either planets or dwarf planets due to their irregular shapes. Therefore, I disagree with both friends; these objects should not be classified as either planets or dwarf planets.

Key concepts

International Astronomical Union (IAU)

The International Astronomical Union (IAU) is an authority that sets guidelines for naming and classifying celestial bodies. They help scientists worldwide speak the same language when referring to galaxies, stars, and planets. In 2006, the IAU made headlines when they redefined what constitutes a planet, a move that led to Pluto being reclassified as a "dwarf planet." New definitions and criteria standardize how we understand celestial bodies.

• For a celestial body to be classified as a planet by the IAU, it must orbit the Sun.
• It should have enough gravitational force to achieve a nearly round shape.
• Finally, the body must have cleared its orbit of other debris.

The IAU's role is essential because it provides a framework that ensures consistency in space exploration and study.

dwarf planet

Dwarf planets are intriguing celestial objects that, although not considered full-fledged planets, still play a significant role in our understanding of the solar system. According to the IAU, a dwarf planet fulfills only the first two criteria for planets. They orbit the Sun and are massive enough for their gravity to pull them into a nearly spherical shape. However, they do not have the gravitational clout to clear their orbit of other debris, which is the crucial third criterion for being a planet.
Pluto is the most famous example of a dwarf planet. Despite its demotion from planet status, it has maintained its allure and sparked interest in how we define celestial objects. Understanding dwarf planets helps us appreciate the diverse characteristics possible among objects within the solar system.

hydrostatic equilibrium

Hydrostatic equilibrium is a term used to describe when an object's gravitational force is strong enough to mold it into a nearly spherical shape. This state is typically achieved when the internal pressure of an object balances with the gravitational pull trying to collapse the body inward. For both planets and dwarf planets, reaching hydrostatic equilibrium is crucial. However, for planets, it is not only a matter of shape but also about stability needed to clear orbits.

• Known celestial bodies achieving hydrostatic equilibrium are typically round or nearly round.
• This balancing act is key for objects to be considered spherical enough to fulfill one of the IAU criteria.

The fact that the newly discovered objects mentioned in the exercise are irregular in shape suggests they have not reached this state, making them unlikely candidates for planet or dwarf planet classification.

orbital debris clearance

Orbital debris clearance means that a celestial body has removed or "cleared" its orbit of other objects such as asteroids, dust, or debris. This is one of the main components that differentiates a planet from a dwarf planet. A planet is massive enough to exert gravitational dominance, thereby clearing its own path around the Sun.

• The ability to clear an orbit is vital for defining a full planet.
• It shows the celestial body's influence and stability within its own orbit.

On the other hand, dwarf planets, despite having similar characteristics to planets, lack the gravitational force to clear their orbits, which is part of why Pluto and similar bodies are not classified as full planets. Understanding these differences is key to categorizing and studying objects beyond Earth.