What is an asteroid? What is a trans-Neptunian object? In what ways are these minor members of the solar system like or unlike the planets?

When we talk about the solar system, the term 'asteroid' often comes up. An asteroid is a relatively small celestial entity that orbits the sun. Imagine them as the cosmos's rocky leftovers from the solar system's formation. They are primarily found in an area known as the asteroid belt, a region teeming with these rocky bodies located between the orbits of Mars and Jupiter.

Though they vary in size, asteroids generally do not have enough mass to assume a spherical shape, unlike planets. Their compositions include mostly rock and metal, which differentiates them from comets that are icy in nature. The largest known asteroid in our solar system is Ceres, which is unique enough to be classified as a dwarf planet as well.

Beyond the orbit of Neptune lies a frontier sparsely populated with icy bodies called Trans-Neptunian Objects (TNOs). These objects reside in the outer part of our solar system and have orbits that lie greater than Neptune's, which is the eighth planet from the sun. TNOs come in various forms, some of which are classified as dwarf planets, like the famous Pluto, while others remain as small solar system bodies.

Their composition is distinct from the rocky asteroids, as they often contain a mix of water, ammonia, and methane ices alongside rock. Due to their distance from the sun, they are much cooler and less well-known than the inner celestial bodies. The region most known for TNOs is the Kuiper Belt, which is home to many of these icy wanderers.

Asteroids and planets share the trait of orbiting our sun, but their similarities largely end there. One of the stark differences is their size. Planets are much more massive than asteroids, a fact allowing them to have sufficient gravity to shape themselves into spheres. This spherical shape is a defining character of a planet.

In contrast, asteroids, with their smaller mass, often have irregular, jagged shapes. Further differing from planets, asteroids do not have atmospheres, and while planets have cleared their orbital paths of debris, asteroids exist in belts with numerous other similar bodies. Planets also exhibit a wide variety of compositions, from rocky terrestrial planets like Earth to gas giants like Jupiter, while asteroids are more homogeneous, mostly made of rock and metal.

Celestial bodies within our solar system come in a dazzling array of forms, each with unique characteristics. The primary bodies include stars, such as our sun; planets, which are sizeable spherical objects that have cleared their orbits; moons that orbit planets; asteroids; comets; and trans-Neptunian objects.

Each type of celestial body has distinct traits. Stars are luminous and generate their own light through nuclear fusion. Planets are large enough to be rounded by their own gravity and orbit stars. Moons are smaller and orbit planets, showing a vast range of terrains and geological activity.

Asteroids and comets are smaller and less rounded, with asteroids being rocky and comets icy. Comets also have characteristic tails of gas and dust when they approach the sun. Trans-Neptunian objects, while sharing similarities with asteroids and comets, are specifically found in the outer reaches of our solar system and have diverse compositions, reflecting their frigid and distant nature.