Which one of the following is NOT classified as a dwarf planet in the solar system?

Introduction / Context:
Modern astronomy distinguishes between full sized planets, dwarf planets, and many smaller objects in the solar system. This question checks whether you can identify which of the listed bodies is not a dwarf planet. It is a common general science topic, especially after Pluto was reclassified from a planet to a dwarf planet by the International Astronomical Union. Knowing a few key examples from each category helps you answer such questions quickly.

Given Data / Assumptions:
– The options listed are Pluto, Mars, Eris, and Makemake.
– Pluto, Eris, and Makemake are well known examples connected with dwarf planet discussions.
– Mars is one of the classical planets studied in school astronomy.
– We assume the current classification followed by the International Astronomical Union, where planets and dwarf planets have specific definitions based on orbit and ability to clear neighbourhood.

Concept / Approach:
A planet is a celestial body that orbits the Sun, has sufficient mass for its gravity to form a nearly round shape, and has cleared its orbital neighbourhood of other objects. A dwarf planet also orbits the Sun and is nearly round but has not cleared its neighbourhood. Pluto, Eris, and Makemake are all trans Neptunian objects in the Kuiper Belt and are officially classified as dwarf planets. Mars, on the other hand, is a terrestrial planet, similar to Earth and Venus, and has satisfied the criteria for a full sized planet. Therefore, among the listed options, Mars is not a dwarf planet; it is a regular planet in the solar system.

Step-by-Step Solution:
Step 1: Recall that Pluto was once called the ninth planet but is now classified as a dwarf planet. Step 2: Remember that Eris and Makemake are Kuiper Belt objects officially grouped as dwarf planets. Step 3: Recognise that Mars is one of the four inner rocky planets, orbiting between Earth and Jupiter. Step 4: Identify Mars as the only option that is a full planet rather than a dwarf planet, making it the correct answer to this question.

Verification / Alternative check:
Any recent school level astronomy chart or a reliable space agency website will list eight major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Dwarf planets are listed separately and typically include Pluto, Eris, Makemake, Haumea, and Ceres. Checking these lists confirms that Mars appears among the main planets and not among the dwarf planets, while Pluto, Eris, and Makemake are all in the dwarf planet category. This independent check from authoritative references supports the conclusion that Mars is not a dwarf planet.

Why Other Options Are Wrong:
Pluto: Officially classified as a dwarf planet since 2006 because it has not cleared its orbit of other Kuiper Belt objects, so it is not the answer to this question.
Eris: Another large Kuiper Belt object recognised as a dwarf planet, similar in status to Pluto, so it is not the correct choice here.
Makemake: Also a Kuiper Belt dwarf planet discovered in the early twenty first century, so this option is not correct for identifying a non dwarf planet.

Common Pitfalls:
Many people remember Pluto as the ninth planet from older textbooks and may mistakenly think that it is not a dwarf planet, especially if they are unaware of the reclassification. Others may simply pick a name that sounds unfamiliar, such as Makemake, assuming that the unfamiliar name must be the odd one out. To avoid these mistakes, it helps to memorise the current list of eight planets and remember that several objects beyond Neptune, including Pluto, Eris, Makemake, and Haumea, form the core group of known dwarf planets.

Final Answer:
The object that is not classified as a dwarf planet is the full sized planet Mars.