What is the Smallest Planet in our Solar system?

23rd Mar 2023
What is the Smallest Planet in our Solar system?

Over the past half-century, humankind has stepped far ahead in space exploration. We have interplanetary research probes and powerful orbiting telescopes that are able to research celestial bodies in detail at a distance from Earth that is difficult to comprehend. All this might lead us to believe that we have studied our solar system quite well and know about it, if not everything, then almost. But discoveries go on, and sometimes they force us to rewrite seemingly common facts.

In particular, until recently, scientists were sure that there were nine planets in the Solar system. But after 2006 there were eight of them. What happened to tiny Pluto, and what is the smallest planet in our Solar system now? Let’s find out the planets’ order from smallest to biggest!

What is the first smallest planet in our Solar system?

Until 2006, the answer to this question was unequivocal. Pluto was considered the ninth and smallest planet in the solar system. The object was discovered in 1930 by 23-year-old Clyde Tombaugh, an astronomer from Kansas, and named after the mythical ancient Roman underworld ruler (same as Hades for the ancient Greeks).

Pluto consists of ice and stone, has an atmosphere and even five satellites, but it is very small. It is not only smaller than all the other planets in the solar system; it is smaller than most of the other planets’ satellites, including the Moon (about three times smaller and six times lighter). Besides, it has the characteristic features of a comet — the solar wind blows gases from its atmosphere, as it does with comets. If Pluto was as close to the Sun as the Earth, it would develop a comet tail.

Until the 21st century, almost no scientist doubted that Pluto was a planet, but in 2006, after a heated debate, it was suddenly renamed a trans-Neptunian object in the Kuiper belt. As a result, the solar system suddenly ‘lost’ one planet, so the answer to the question, what planet is the smallest in our solar system changed. This title was officially transferred to Mercury.    

Why is Pluto no longer a planet

The scientists decided to deprive Pluto of the planet’s status after they began to discover more and more exoplanets, and they needed to give a clear definition of this celestial body. As a result, they formulated five conditions that a celestial body must meet to qualify for the status of a planet:

  • orbit around the Sun and be a satellite of the Sun, not one of the planets,
  • be massive enough to acquire a shape that is close to spherical,
  • be able to clear its surroundings from other objects, that is, to be gravitational dominant and not have objects of comparable size in its environment.

Pluto does not meet the last criterion — its mass is only 7% of other objects in the Kuiper belt, where it is located. Simply put, it can only qualify as a dwarf planet as long as the mass is concerned. However, some scholars firmly believe that Pluto needs to be reclassified back to planet status.

Why is Mercury the smallest planet in the Solar system?

This miniature planet has a diameter of about 3,030 miles, making it about the same width as the continental United States and only slightly larger than Earth’s moon. Saturn’s moon Titan and Jupiter’s moon Ganymede are both larger than Mercury. After Pluto’s disqualification, it is unarguably the smallest planet of the total eight.

There is a theory that Mercury is a former satellite of Venus. There is no scientific confirmation for this, but separate studies indirectly confirm this assumption. Mercury does not have its own satellites, has an iron core inside and a rarefied atmosphere outside.

Even though Mercury is located closest to the Sun, it is, surprisingly, not the hottest planet. But even more surprising is that despite its tiny size, it can be observed in the sky even with the naked eye or with field glasses with a 50 mm aperture. But, of course, only in general and for very short periods. To observe surface details, you will need a telescope with an aperture of 100–150 mm and a minimum magnification of 200.

What is the second-smallest planet in our Solar system?

So, we already found out that this is no longer Mercury. It is now the smallest, while the second place goes to Mars. Its diameter is 6792 km, which is three times the diameter of Pluto, twice the size of Mercury, and half the size of the Earth. Mars has very weak gravity and almost no atmosphere (a hundred times thinner than on Earth). If on Earth you weigh 80 kg, then on Mars, you will weigh about 30, and without a spacesuit, you will simply explode due to the low atmospheric pressure.

And yet, the red planet is the only potentially suitable place for human habitation in the solar system, where it will be possible to move in the event of the Earth’s death. If we prepare properly, we might be able to survive there. In the coming decades, when the first colonists go to Mars, we will find out how just realistic this theory is.

What is the 3rd smallest planet in our Solar system?

This is Venus. The second planet from the Sun and clearly visible in the sky with the naked eye, played a significant role in the mythology of many ancient peoples, including the Mayans and Greeks. Venus has long been considered similar to our Earth. Indeed, they are almost the same size (the diameter of Venus is only 640 km smaller than that of the Earth), but the similarity ends there.

Venus is so inhospitable that it is quite literally deadly. Its atmosphere consists of sulphuric acid, and the surface heats up to a record 470 degrees Celsius — it is hotter than Mercury, even though the latter is closer to the Sun. Due to very dense clouds, sunlight does not penetrate Venus, there is no water in any form, and a typical Venusian landscape is mountains and rocky deserts shrouded in eternal darkness.

What is the smallest gas planet in our Solar system?

Everyone knows that Jupiter is the biggest gas planet in the Solar system, but what about the smallest? This is Neptune —  the 5th smallest planet, the smallest of four gas giants, and the farthest planet from the Sun. Officially, Neptune was discovered in 1846, although Galileo Galilei observed this planet through a telescope back in 1612.

The diameter of the planet is 49,000 km, four times larger than the Earth, and its mass is 17 times the mass of the Earth. Neptune is dubbed the ice giant and also the planet on which diamond rains fall. Do you want to know why? Read our 12 Interesting Facts About Neptune!

What place is the Earth in?

Our planet ranks 4th in size among all the solar system planets. It is larger than Mars and Venus but smaller than Neptune. Earth is the third planet from the Sun, and the presence of a dense atmosphere saturated with oxygen allows it to maintain an average temperature of + 7 degrees Celsius. Planets located closer to our star are much hotter,  while those further away are significantly colder. This makes the Earth the only planet suitable for biological life as we currently know it.

And so it will remain at least for several million more years until the Sun, increasing in size, begins to absorb the planets one by one. But by then, we will probably cease to exist and will no longer be able to answer the question — what is the smallest planet in our Solar system?

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