Answer : 

Odd One Out: Gases

Gases are the odd one out among the given options. Let's discuss why.

Asteroids
- Asteroids are rocky objects that orbit the Sun, mainly found in the asteroid belt between Mars and Jupiter.
- They are composed of various materials, such as rock, metal, and sometimes even ice.
- Asteroids can vary in size, from small boulders to objects several hundred kilometers in diameter.

Planets
- Planets are celestial bodies that orbit the Sun, have a nearly spherical shape, and have cleared their orbits of other debris.
- There are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
- Planets are much larger than asteroids and have their own unique characteristics, such as atmospheres and moons.

Comets
- Comets are icy bodies that orbit the Sun and often have a tail that forms when the comet gets closer to the Sun.
- They are composed of a mixture of ice, dust, rocky particles, and organic compounds.
- Comets originate from two regions in the solar system: the Kuiper Belt and the Oort Cloud.

Gases
- Gases, in this context, refer to the various gases found in the atmosphere of planets and other celestial bodies.
- While gases are present on planets and other objects, they are not distinct entities like asteroids, planets, and comets.
- Gases are more of a characteristic or component of celestial bodies, rather than being separate objects themselves.

Conclusion
When considering the given options, gases stand out as the odd one out. Unlike asteroids, planets, and comets, gases are not individual objects but rather a characteristic or component of celestial bodies.