Circle the odd one out:

a) Asteroids
b) Planets
c) Comets
d) Gases

Explanation:

Asteroids
- Asteroids are rocky objects that orbit the Sun.
- They are smaller than planets and are made up of different materials, such as metals and rocks.
- Asteroids are often irregular in shape and can vary in size from a few meters to hundreds of kilometers.

Planets
- Planets are celestial bodies that orbit around a star, such as the Sun.
- They are larger than asteroids and have a spherical shape due to their own gravity.
- Planets are composed of various materials, including gases, liquids, and solids.
- There are eight known planets in our solar system, including Earth, Mars, Jupiter, and Saturn.

Comets
- Comets are icy bodies that orbit the Sun.
- They are composed of a solid nucleus made up of ice, dust, and small rocky particles.
- As comets approach the Sun, the heat causes the ice to vaporize, creating a glowing coma and sometimes a tail.
- Comets are known for their long, bright tails that can extend for millions of kilometers.

Gases
- Gases are a state of matter and do not have a specific shape or volume.
- They are composed of individual atoms or molecules that are not bound together.
- Gases can be found in various forms, such as oxygen, nitrogen, and carbon dioxide.
- Unlike asteroids, planets, and comets, gases do not refer to specific celestial objects but rather to a state of matter.

Conclusion:
The odd one out in this list is option 'D' - Gases. Unlike asteroids, planets, and comets, gases do not refer to specific celestial objects but rather to a state of matter.