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 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 agree with the second friend; the objects should be classified as dwarf planets.

Step-by-step solution

Understand Planet Classification

To classify celestial objects in our Solar System, we refer to the International Astronomical Union (IAU) definition. A planet must orbit the Sun, have sufficient mass for a nearly round shape, and have cleared its orbit of other debris. A dwarf planet meets the first two criteria but has not cleared its orbit.

Analyze the Orbit

The three objects are orbiting the Sun at a distance of 35 AU. This fulfills the first criterion for both planets and dwarf planets—they must orbit the Sun.

Assess the Shape

To classify as a planet or dwarf planet, the objects need sufficient mass to maintain a nearly spherical shape. Since they are described as planet-like, it may be implied that they have this shape, but 'irregularly shaped' suggests otherwise.

Evaluate the Orbital Zone

For an object to be a planet, it must have cleared its orbital zone of other debris. The description does not provide information on whether the objects cleared their orbits, implying they likely haven't, similar to Pluto.

Conclusion

Based on the information, the objects orbit the Sun and may not be spherical or have not cleared their orbits. Thus, they should be classified as dwarf planets, as they do not meet all the criteria for a planet set by the IAU.

Key concepts

International Astronomical Union (IAU) definition

The International Astronomical Union, commonly known as the IAU, plays a crucial role in categorizing celestial bodies within our universe. It provides us with the official definitions that help distinguish one type of object from another. For a celestial object to be classified as a planet, it must meet three specific criteria: it must orbit the Sun, it needs to have sufficient mass for its self-gravity to form a nearly round shape, and it must have cleared its orbital zone of other debris and celestial objects. These guidelines ensure consistency in how we discuss our Solar System and beyond. Regular updates and reviews by the IAU keep the definitions relevant to our advancing understanding of space.

dwarf planet criteria

A dwarf planet shares some common features with planets but diverges in a key area. According to the IAU, to be classified as a dwarf planet, a celestial object must fulfill two main criteria. First, like full planets, dwarf planets must independently orbit the Sun. Second, they must have enough mass for their self-gravity to overcome rigid body forces, allowing the object to take on a nearly round shape. However, unlike planets, dwarf planets do not have to clear the area around their orbit. This is the major distinction that sets them apart from regular planets. It allows celestial bodies like Pluto, once formerly classified as the ninth planet of our Solar System, to now fit snugly into the category of dwarf planets.

celestial objects

The term "celestial objects" refers to any natural bodies outside of the Earth's atmosphere. This includes stars, planets, moons, asteroids, comets, and even meteors. In the context of our Solar System, celestial objects can range from massive gaseous giants like Jupiter to small rocky bodies, such as asteroids or dwarf planets. These objects vary significantly in size, composition, and orbit. Their classification depends largely on these characteristics, as well as their relationship with other objects within their orbital path. Understanding these diverse elements helps in piecing together the dynamic and complex nature of our universe.

orbital zone clearing

Orbital zone clearing is an important concept in defining what makes a planet, according to the IAU. It refers to the ability of a celestial body to either incorporate or push away smaller objects within its orbital path around the Sun. Any celestial body that accomplishes this is considered to have a "cleared orbit." Essentially, it means that outside of its natural satellites, there should be no other significant objects sharing its orbital region. Planets like Earth and Jupiter have vast gravitational influences due to their massive sizes, enabling them to clear their orbits effectively. On the contrary, dwarf planets like Pluto lack this ability, which is a key reason behind their classification status.