In case you're wondering, Mercury is still substantially larger than the minor planet Pluto: Pluto's equatorial diameter is only 2,302 km, or almost half the breadth of Mercury.
However, Pluto has four times more mass than Mercury does. So, if you counted up all the particles that make up both planets, then probably Pluto would be bigger.
Mercury is also much fainter than Pluto. The solar eclipse in August 1999 was able to obscure Earth's companion planet from view for over a day - yet Pluto could be seen with the naked eye everywhere on Earth except in areas where it was blocked by clouds or dust. The fact that it can be seen with the unaided eye shows how bright it is.
Even though Pluto isn't as big as some people thought it might be, it's still worth studying because it's the only major planet that remains undiscovered. In addition, there are many other objects in our Solar System that aren't as large as Earth but are important to know about. For example, Ceres is larger than Pluto and was once considered to be a dwarf planet too, while Eris is one of the biggest planets and sits over 5400 km away from the Sun.
Scientists now know that Mercury is substantially larger than Pluto in terms of size. Mercury has a diameter of 4,879.4 km, whereas Pluto has a diameter of 2,360 km. As a result, Mercury is roughly twice the size of Pluto. And, because Pluto is smaller and denser than Mercury, it has a far lesser gravitational pull. The reason for this discrepancy in gravity is that both bodies are made up of rock and metal, but these materials are packed into different ratios on each planet. On Earth, for example, we have more water than land mass, while on Mars there is more rock than water.
Pluto was once considered to be a planet too, but it was removed from the list in 2006.
People used to think that Jupiter and Saturn were the only planets out there, but now we know that there are dozens of other large objects in our Solar System. Some of them even have satellites of their own! But despite the fact that there are so many others out there, it's still not enough to go around for all the planets.
In addition to Earth's eight known planets, there are also four major moons orbiting around Earth. These are called the Moon's primary satellites and they are: Mercury, Venus, Earth and Mars. There is also a small moon called Pluto that orbits around Earth's largest planet, Neptune.
So, as you can see, there are really no clear-cut differences between planets and moons.
Mercury is not a dwarf planet since its orbital path is devoid of objects... Mercury has a mass thirty times that of Pluto and a diameter that is 2.05 times that of Pluto (giving it a volume 8.65 times greater). Therefore, it cannot be considered a dwarf planet.
Furthermore, the definition of a dwarf planet requires that it be able to sustain itself over time without being destroyed. As mentioned above, Mercury is still orbiting the Sun and will do so for another 687 million years before it is engulfed by it.
However, as far as we know, Pluto may well have been destroyed during its last close approach to Neptune. If this is the case then it cannot be considered a dwarf planet anymore.
Overall, these are the two main reasons why Mercury is not a dwarf planet: it lacks the size requirement and it will be destroyed by the Sun in six hundred seventy-seven million years.
Pluto is one of a class of objects known as Kuiper Belt Objects. It, like Ceres, does not match the standard description of a planet. Pluto's status as a dwarf planet, incidentally, does not make it any less intriguing or significant. True, Mercury is far bigger and heavier than Pluto. But that's because it orbits much closer to the sun—about 60 million km compared to 48.5 million km for Pluto—and so it experiences much higher temperatures inside its atmosphere. The reason we don't see many dwarf planets like Mercury in our solar system is that they are usually too small to be retained by the gravitational pull of their parents over time.
Dwarf planets can still be important to the history of life on Earth and other worlds. For example, the most massive asteroid in our solar system, Vesta, was once thought to be a protoplanet that had grown large enough to become self-sufficient as a world. More recent research has shown that to be false; however, Vesta remains highly relevant to understanding how some planets form and evolve.
As another example, Ceres is important to astronomy because it is the largest object in orbit around the sun other than the earth. It plays a role in the evolution of other objects in space by acting as a cosmic lorry, transporting material from place to place within the solar system.
Mercury is the smallest and closest planet to the Sun in our solar system. Mercury is only slightly bigger than the moon of Earth. It circles the sun in 87 days.
Mercury is our solar system's tiniest planet, only slightly bigger than Earth's moon. But it is also the most important planet for life as we know it, because it is the closest planet to the Sun. Masses are estimated from orbitals around Mercury.
Venus is the second-closest planet to the Sun, but it resides in Venus' own sphere of influence, so it is not considered a good candidate for supporting life. Earth is the only planet in the Solar System that is large enough to be capable of supporting life as we know it.
Mars is the next-closest planet to the Sun, and although it used to be able to support life as we know it, recent findings suggest that it may not be able to sustain itself anymore. The Red Planet has been found to have limited resources for supporting life, including oxygen. Jupiter is our nearest planetary neighbor, but it is too big to be classified as a planet. It orbits our star every 10 years or so. Saturn is the fourth-closest planet to the Sun, and it takes 29 years to complete one rotation. Uranus is the seventh-closest planet, and it takes 84 years to complete one revolution around the Sun.