Yes that’s right, it’s that time of the month for everybody’s least favourite astronomy blog: My Tour of the Solar System. This is the fourth amazing entry of the tour, so if you’re new, or you don’t know what to do, I promise it’s ok. The three previous entries are: Meet the Family, The Sun, and Planets vs. Dwarf planets.

When we last left the tour, I was waffling on about the differences between planets and dwarf planets, because you know, dwarf planets are a real thing. Seriously. So because of this, we move onto the first planet from the Sun, Mercury.

Named after the messenger of the gods in Roman mythology, Mercury is quite odd, considering a few of its siblings are overachievers. Mercury is believed to be 4.5 billion years old, so its quite similar to Earth in age.

Mercury is the smallest planet in our Solar System, with a mean diameter of 4,880 km; which puts it slightly larger than the Moon, at 3,474 km. Basically, if there was an arm wrestle between the two of them, your money should go on Mercury; but keep your cryptocurrency on Earth, because no other planet wants it.

Even though Mercury is the closest planet to the Sun, it is not the hottest; but it’s still worth swiping right. Mercury orbits the Sun at a range of 47 million km (29 million miles) to 70 million km (43 million miles); with an average distance of 58 million km (36 million miles). To put this into perspective, Earth orbits the Sun at an average distance of 150 million km (93 million miles).

Given its proximity to the Sun, Mercury is a hot mess. The length of time it takes for light to travel from the Sun to Mercury, is 192 light seconds or 3.2 light minutes, which is less time to boil an egg.

Temperatures can vary as much as Cookie Monster can eat all of the cookies in a single day. The surface temperatures on Mercury are insanely hot and cold. Day temperatures can reach 430°C (800°F), while night temperatures can plummet to -180°C (-290°F). Because it has no atmosphere, Mercury can’t hold onto the heat during the night, resulting in cold temperatures. Can you imagine the electricity bills?

And like the Flash, Mercury lives up to its namesake. Compared to all of the planets in the Solar System, Mercury is the fastest; even Usain Bolt would be moving like a statue in comparison. Not a Weeping Angel, though. To hell with that.

The further away a planet is from the Sun, the slower it travels; however the opposite is also true. Because Mercury is the closest planet to the Sun, it has the shortest distance to travel around it. This has resulted with Mercury travelling through space at 47 km (29 miles) per second, but it also has some counterintuitive discoveries.

• Mercury spins slowly on its axis, at a rate of once every 59 Earth days.
• It takes 88 Earth days for Mercury to orbit the Sun; which equals one Mercury year.
• It takes 176 Earth days for Mercury to complete one solar Mercury day. Mercury has an elliptical orbit, which means twice during its orbit, it gets quite close to the Sun and speeds up, so it appears the Sun would be going backward; if you were standing on Mercury’s surface.
• This means one day on Mercury, is longer than a Mercury year.

And just to add some more fun stuff; Mercury has no rings, no natural satellites (moons), plus it is tidally locked with the Sun.

Just like Red, Boober, Mokey, Gobo, and Wembley are all Fraggles; Venus, Earth, Mars, and Mercury are all rocky or terrestrial planets. Mercury looks like our Moon, with craters covering the surface, however with the crust and mantle being made of rock, the core is solid iron. This works out to be 75% of Mercury’s diameter. Because of Mercury’s smaller size, when the core cooled down, it become solid, which made Mercury shrink and caused the surface to wrinkle. This didn’t happen to Earth because it was larger, along with the fact our outer core was liquid, and the inner core was solid.

The majority of the craters on Mercury are actually named after famous artists, composers, and writers. The most famous crater is Caloris Planitia, which is an impact basin, stretching 1,550 km (960 miles) in diameter. For perspective, the length of the United Kingdom is about 1,000 km (600 miles). The Caloris Planitia is surrounded by a ring of mountains; and no, they are not the Ash Mountains, nor the Mountains of Shadow; but rather the Caloris Montes, which are 1-3 km high.

Another reason to swipe right, is the fact that Mercury’s mass is less than 6% of Earth’s. This means Mercury’s gravity is weaker, so if you weighed 70 kg on Earth, you would only weigh 26.6 kg on Mercury. Imagine how high you could jump?

Do you remember what I said about Mercury not having an atmosphere? Good job, you were paying attention. Well, Mercury does have a thin exosphere though. This exosphere is mostly made of oxygen, sodium, hydrogen, helium, and potassium. If you could stand on the surface of Mercury without a protective space suit, the cold, heat or just lack of air, would kill you in under 90 seconds. That’s something to put on the tourism brochure!

All and all, Mercury is a special and unique world, but just like visiting the dentist, you really wouldn’t like to go there. And with that, it’s time to wrap up another tour stop of the Solar System. Yah! This coming week, I’m going to be releasing two special short blogs, celebrating two important days on the Geek calendar. Do you know what they are? The first one will be on Thursday, while the second will be on Saturday. Three blogs in a week? I’ll need a lie down to do this.

Thanks once again for reading, following, and subscribing to Some Geek Told Me. Remember to walk your dog; read a banned book; stop running with scissors; and I’ll see you next week.

Hey, it’s you! I’m so glad you could make it! This is the third chapter of our little tour of the Solar System and I was beginning to think you weren’t going to make it. For the obligatory recap of the tour, the first chapter was the introduction, while the second was all about the Sun.

Since the last stop was the Sun, we should be moving onto the next celestial object, which is Mercury; the first planet from the Sun. The problem is, I feel it could be important to discuss just what a planet is, and how they are different from a dwarf planet.

I briefly mentioned dwarf planets in the introduction, because let’s face it, they sound made up. It’s ok to admit it, you’re among friends. Dwarf planets are real, but they’re not as famous as their larger siblings; however they are no less important. Seriously.

Ok Scott, stop talking and actually say something! What is a dwarf planet? That is an insightful question, so here is a ridiculous answer. Let’s hop in our DeLorean time machine of the mind and travel back in time to visit three important dates: 1930, 2005 and 2006.

In 1930, a planet was discovered by Clyde Tombaugh, which was later named as Pluto.¹ Pluto become known as the ninth planet of the Solar System, and history and science textbooks would never be the same. Um….that’s not truly accurate is it? I remember learning about Pluto at primary school: we had nine planets in the Solar System and even though Pluto was the smallest, it was still identified as a planet.

For over 60 years, Pluto got to hang out with its older siblings, because it was still identified as a planet. Well, until 2005 it was. Over the years, technology has advanced with new developments, that has allowed humanity to view the universe in amazing new ways. Because of this, new objects were being discovered in our Solar System and beyond; but especially in and around the Kuiper belt, which is a disc like region of space, beyond Neptune.

And just so we are on the same page, Pluto resides in the Kuiper belt.

So back to 2005. Among the media released discoveries of Makemake and Haumea, another object was discovered: Eris. When the first measurements of Eris were released, it appears that it was actually larger than Pluto, even though it was further away. This development led a lot of scientists to call for the reclassification of Pluto. They also thought another object named Ceres, which was discovered in 1801, finally needed a permanent classification as well.

We can now move to 2006, which was Pluto’s least favourite year ever. Enter the International Astronomical Union (IAU). The IAU has been described as:

…a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and development through global co-operation.

Basically the IAU get to decide things about astronomy.

In 2006, the IAU held a general assembly, with one of the issues concerning Pluto and what to do about it; along with some other objects. This led to the definition of a planet, which consists of three criteria.

1.) Is in orbit around the Sun.

2.) Has sufficient mass to assume hydrostatic equilibrium.

3.) Has “cleared the neighbourhood” around its orbit.

The first criteria is obvious, the planet must orbit around the Sun.

The second criteria talks about the planet achieving hydrostatic equilibrium, which is just a shape that is nearly round.

The third criteria is about when a planet orbits the Sun, it must be the most dominant gravitational object, in that orbit. It means the planet needs to be able to sling or clear the neighbourhood of any other smaller objects in its path.

When Pluto was measured against these three criteria, it passed the first and second, but failed the third. And a long story short, Pluto was declassified as a planet and reclassified as a dwarf planet, along with others.

Textbooks were rewritten and suddenly Pluto no longer could sit with the cool kids, but needed to sit at a different table.

This being the case, a definition for a dwarf planet was established.

1.) It must orbit the Sun, and not be a moon.

2.) Has enough mass to be round.

3.) Has not cleared its orbit of debris.

To date, our Solar System has eight planets that meet all three planetary criteria. Starting with the closest to the Sun, we have Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

As for dwarf planets, they are planetoids that fail on one, two or even all three of the IAU planetary criteria; but pass all of the IAU dwarf planetary criteria. To the best of my understanding², there are five official dwarf planets in our Solar System. Starting with the closest to the Sun, we have some amazing names like Ceres, Pluto, Makemake, Haumea and Eris.

There are five other dwarf planets in the Solar System, which are known as Trans-Neptunian objects or TNOs, that are waiting to be officially recognised as dwarf planets.³ In order from the Sun, they are Orcus, Salacia, Quaoar, Gonggong and Sedna.

Basically that’s it. A planet is identified as a planet, because it passes all three IAU planetary criteria, whereas the dwarf planets, fail at least with one of the planetary criteria. Whether this is fair, is another story for another day. Clear as mud?

That’s it for another week, I’m sorry. I’ve got to go and bore my wife with rugby and football scores now. Thanks once again for reading, following and subscribing to Some Geek Told Me; it means a lot. Remember to walk your dog; watch a banned movie; stay away from Sour Cream and Chives; and I’ll see you next week.

¹ The story behind Pluto’s name, deserves its own tv movie.

² Let’s face it, that isn’t very much.

³ Again, to the best of my understanding.