A Few Curiosities About The Solar System

I did not know all of this myself until I saw a couple of documentaries in which these curious things were explained. One never stops learning.

The first curious thing I want to explain today is that the size of the Solar System is quite a tricky topic. At school I just learned that the Solar System is composed by the Sun and the planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Later I learned that between Mars and Jupiter there is an asteroid belt. And then, in 2006, Pluto lost its category as a planet. Therefore, Neptune is the outermost planet of the Solar System. But the Solar System does not end in Neptune! It is much bigger than that!

If we were in a spaceship sent out towards the edge of the Solar System and continued travelling past the orbit of Neptune, we would eventually reach Pluto, or at least its orbit, and then other objects composed primarily of rock and ice. Pluto and the other objects form the Kuiper belt, a region extending from Neptune’s orbit, at 30 AU, to approximately 50 AU from the Sun.

To clarify the distances, 1 AU (astronomical unit) is the average distance from the Earth to the Sun: 150,000,000 (150 million) km, or 1.5 x 10⁸ km. If we keep in mind that the orbit of the Earth is elliptical, when it is closer to the Sun our home world is at around 147 million km, and when our planet is farther it is at around 152 million km. So if we multiply the average, 150 million km, by 30 we get to the average distance between Neptune and the Sun: 30 AU, i.e., 4.5 x 10⁹ km (a 45 followed by 8 zeros). And the Kuiper belt would end at 50 AU, i.e., at 7.5 x 10⁹ km.

But still, if we had been able to travel with our spaceship that far, we would not be exiting the Solar System yet!

The point at which the Solar System ends and interstellar space begins is not precisely defined since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun, which is considered the beginning of the interstellar medium. However, the Sun's effective range of gravitational dominance is believed to extend up to a thousand times farther.

Then there is also the Oort cloud, which is a hypothetical spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year -ly-), and possibly to as far as 100,000 AU (1.87 ly). It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.

So, from our planet, at 1 AU from the Sun, and Neptune, at 30 AU from the Sun, we have gone as far as at least 10⁵ AU!!! Isn’t it amazing? Our galactic district is much bigger than it seemed.

There are three other curious points I would like to mention. But now I would do a U-turn with our spaceship to go back home and have a closer look at the two closest neighbours of the good Earth, and to our planet as well.

If an observer could be placed above the Sun’s north pole, and had the capacity to see all the planets until Neptune, he would observe that the eight companions of the Sun orbit in an anti-clockwise direction from the viewing point. And he could also see that almost all the eight planets also rotate on their axis in an anti-clockwise direction (just like the Sun itself), but Venus and Uranus rotate clockwise. This is called “retrograde rotation”.

While I haven’t found a convincing and/or comprehensive explanation for these two exceptional retrograde rotations (OK, I have not done a deep research, but I guess that this is still being studied and the real explanation is yet to be found out), I saw in a documentary a plausible theory explaining the case of Venus: a huge mass impacted with Venus some millions of years ago, and the impact was so strong that it caused the planet to rotate in the other sense. Being the rotation of Venus the slowest in the system (it takes 243 Earth days, while its orbit around the Sun is faster, taking 224.7 Earth days), it looks quite a reasonable explanation to me. In any case, I find of significance the fact that out of 8 planets only 2 rotate in the different sense.

From Venus we move back to Earth for a while, and I would like to make a stop on the Moon as well. From Earth we always see the same face of the Moon. The explanation is quite simple: the same gravitational attraction between the Earth and the Moon which causes the tides on Earth does also cause that the Moon’s rotation period is the same as the time it takes to orbit the Earth. The Moon may have also dramatically affected the development of life on Earth by moderating our planet’s climate. And here we come to an interesting information regarding the Moon: how was it formed? Well, the most widely accepted theory of the Moon’s origin, the “giant impact theory”, states that it formed from the collision of a Mars-sized protoplanet with the early Earth. The Earth would have been smaller than it is now until that moment, and after the impact part of the protoplanet melted with Earth and the rest was expelled to become, first, a ring of rocks orbiting Earth and then, the Moon (gravity playing once again here). This hypothesis explains, among other things, the Moon’s relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth’s crust.

Our next excursion within the Solar System shall take us to Mars. It has always been a fascinating world for mankind, inspiring us and making us dream of the impossible. For now, I would just like to point it out that its mountains are so big and so high that if we stood in front of any of them we could not see it completely, so huge they are. Olympus Mons, the tallest mountain of the Solar System, is 22 km high, far above the 8.8 km height of Mount Everest on Earth. Similarly, the canyon Valles Marineris on Mars is much more impressive than the Grand Canyon of Colorado (but it is much easier for us to enjoy the latter!). It comes of no surprise Valles Marineris can be seen from space and is one distinctive characteristics of the Red Planet. It seems that Valles Marineris was formed from a combination of volcanic activity and asteroid impacts.

So with a size much bigger than one would initially think and many cosmic impacts along its history which gave shape to its planets and their corresponding satellites, the Solar System has a lot of mysteries for us yet to unveil. Who knows what other amazing stories are waiting for us out there...