Fun stuff: Some facinating aspects of our Solar System !

In the series: Note 2: Some facinating aspects of our Solar System.

Version 0.5, 30 April, 2020.
Status: Ready.
By: Albert van der Sel.

If you would be an astronomer, who specializes on our Solar system (or even a single Planet
in our Solar system), you absolutely would not quickly run out of topics.

It's great to think about galaxies, clusters, dark matter, Cosmological models and stuff,
but really "close" to home (our Solar system), there is so much to see and to investigate.

This note will be a quick intro and "overview" of the Solar system.

1. Overview Solar System and Planets.

Figure 1. My own Jip and Janneke figure, illustrating the Solar System.

The Earth, the third Planet as counted from the Sun, is about 150 million kilometers (about 93 million miles),
away from the Sun (as an average distance, since the orbit is slightly elliptic).

Astronomers use this distance as a unit, the Astronomical Unit (AU), which helps to nicely
charcterize the distances of the other Planets (and other objects) to the Sun.

So, for example, we can say that the Planet Saturn, is about 9.5 AU away from the Sun,
which is somewhat easier, instead of expressing that distance in kilometers or miles.

The main Planets in our Solar System are (as counted from the Sun):

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Where is Pluto? Formerly, Pluto was typed as the 9th Planet. But modern insights made it clear that it
likely originated from the "Kuiper Belt", which contains many astroids, en dwarf planets as well.
Some of those dwarf Planets are likely to be quite similar to Pluto, and sadly, as a result,
Pluto lost it's status as a "real" Planet. But many folks still cannot get used to that idea.

The next (public domain) figure (from Wikipedia) gives a great impression of the relative sizes
of the Planets, as well as their main moons which they may have, and lots of other data.
Really a nice illustration!

Wikipedia commons: great picture of our Planets with lots of data.

Next, take a look at the table below, to get a good grip of the distances of the Planets to the Sun,
and their masses, expressed in Earth mass (how many times the mass of Earth).

Table 1: Some important parameters of the Planets:

Planet:	Distance to Sun (AU):	Mass:	No of Moons:	Type:	Avg. Orbital velocity (km/s):
Mercury	0.4	0.055	zero	Rocky	47.4
Venus	0.7	0.81	zero	Rocky	35.0
Earth	1	1	1	Rocky	29.8
Mars	1.5	0.107	2	Rocky	24.1
Jupiter	5.2	318	79 up to now	Gas giant: iron-nickel core, H, He outer layers	13.1
Saturn	9.5	95	82 up to now	Gas giant: iron-nickel core, H, He outer layers	9.7
Uranus	19.2	14	27 up to now	iron-nickel core, methane +other stuff outer layer	6.8
Neptune	30.1	17	14 up to now	iron-nickel core, methane + other stuff outer layer	5.4

Note that the 4 inner planets (Mercury, Venus, Earth, Mars) have a solid surface,
and therefore are often typed as the "rocky planets".
Also, often they are referred to as the "terrestrials".

The 4 outer planets are of typed as "gas giants", although they likely have a solid
core of mostly iron-nickel composition.
Anyway, the term "Gas giant" is most often applied on Jupiter and Saturn.

For Jupiter and Saturn, it's likely that the layers around the core, consists mainly
metallic Hydrogen (H), with lower amounts of Helium (He).

From the table above, note the enormous mass of Jupiter. It's about 318 times the mass of Earth.

Also note the average orbital velocity (or speed) of the Planet around the Sun.
The closer to the Sun, the higher the speed is. For example the orbital velocity of Mercury is 47.4 km/s,
while for Neptune, it is only 5.4 km/s.

2. The Belts and Clouds in the Solar system.

Note how far Neptune is. It's more than 30 times as far from the Sun, as Earth is.
An enormous distance!

⇒ The Kuiper Belt:

Still furher out, we have the "Kuiper Belt", which is illustrated in figure 1.

The "edges" are not extremely fixed, but astronomers say that it's a region between
30 au, up to about 50 au. So, the inner edge is close to Neptune.
It consists mainly of composite objects, resembling smaller astroids, but this time
mainly composed of methane, waterice, and some other composites.

A number of dwarf planets have been discovered. The most important ones are Pluto, Haumea and Makemake.
Ofcourse Pluto was already known for quite some time, but today it's reckognized as to be
a member of the Kuiper Belt.

Note how wide the Kuiper Belt is. Neptune is about 30 AU away from the Sun, and the Kuiper Belt,
runs at least to 50 AU.

⇒ The Oort cloud (far beyond the Kuiper Belt):

This is an enourmous, partly spherical shaped region, likely the home of millions
of icy objects and Comets.
There are still losts of unknowns, but the region may strech from 2000 AU to about 100000 AU.
Astronomers have found clues that we may divide the cloud into an inner region, and an outer one,
of which the latter one streches to about 50000 AU. Some astronomers say that's even much wider.

3. The scale, in comparison to the nearest star "Proxima Centauri".

Take a look at figure 1 again. Neptune has a distance to the Sun of about 30 au,
which is about 30 * 150 million kilometers (roughly 30 * 150000000 km).

Suppose we "compress" this part of the Solar system, that is the whole region up to
the orbit of Neptune, to 30 centimeters. Yes, that's almost "immoral" to think of, but let's consider
it to be a "thought experiment".
On that scale, you could not see the Planets anymore, and the Sun would resemble a glowing
grain of sand.

But how would that compressed region measure up against the nearest star, namely Proxima Centauri,
on the same scale?

Distance of Neptune to the Sun: about 30 AU = (about) 30 * 150 000 000 km = 30 * 15 * 10⁷ = 45 * 10⁸ km.
So the diameter of the orbit of Neptune, would then be close to 90 * 10⁸ km.
This is the region we compress to 30 cm = 0.30 meter = 0.0003 kilometer.

The distance of Proxima Centauri to the Sun, is about 4.2 lightyears.
This is about 4.2 * 9.4 * 10¹² km = 39.5 * 10¹² km.

It's all a rough calculation, but we only want to see the order of magnitude, of how the scale works out.

Now, apply the compression factor:

(0.0003 / 90 * 10⁸) * 39.5 * 10¹² = (about) 3.3 * 10^-14 * 39.5 * 10¹² = (about) 1.3 km.

Yes, so if the Solar system, up to the orbit of Neptune, is rescaled to 30 cm, then Proxima Centauri would be 1.3 km away.

4. More on the Orbits of the Planets.

⇒ Kepler's laws.

The orbits of the Planets, may look like circles, but they are in fact elliptical.
You know how an ellipse looks like. Some are quite close to a circle (low eccentricity),
while others look very oval (higher eccentricity).

In this section, we are going to discuss Kepler's laws. For a "2 body" situation,
(like the Sun and a Planet) the Laws of Kepler applies.
However, our Solar system has multiple Planets, of which Jupiter is the most massive one.
Indeed, planetary disturbances on any "2 body" situations, exist.

But Kepler's laws are very good approximations for any of our 8 main Planets.
Indeed. Kepler's law even work (as approximations) in Galactic systems.

As you know, Copernicus (1543) was the first (except from a few classical Greek philosophers), who placed
the Sun in the Center (of the Universe), instead that everything revolves around the Earth.

Not much later, Brahe performed highly accurate observations, over an extended period of time,
on positions of Planets. Unfortunately, he never arrived at the correct model of our Solar system.

Around 1600, Kepler became Brahe's assistent. Still, at that time, even Keppler believed
that "the beautiful harmonic symmetric" circle, simply had to be the solution for
the orbits of the Planets. However, the accuracy of the observations of Brahe, proved otherwise.
Between 1609 and 1619, Kepler formulated his three Laws, which astonishgly good, explains the
motion of the Planets around the Sun.

It seems highly likely, that Kepler's formulations were purely empirical, meaning that
his formulations were solely based on observations. Incredable work, indeed.

Figure 2. Ellips, and Kepler's law.

-Kepler's first law states, that the Planet has an elliptical orbit, with the Sun in one
of the two focal points (any ellips has two focal points).

In our Solar system, the eccentricity of the ellips is usually low, meaning that the orbit
is not very elliptic. However, it still is an ellips.

-Kepler's second law states, that in equal timeslots, the planet covers equal area's,
defined by the arc of the orbit, and the radius of the positions to the Sun.
In the figure above, the two blue area's must be equal in size.

It also means that when the Planet is near the Sun, the speed is slightly higher.
Conversely, when the Planet is further away from the Sun, it's speed is somewhat lower.

For Earth, the eccentricity is "0.017", in which case the orbit still is elliptical ofcourse, but
deviates not too much from a circle. For Pluto, the eccentricity is "0.248", and the orbit
already looks quite elliptical.

⇒ The Ecliptic Plane, and "tilts".

The orbit of Earth, and the Sun, defines a plane. Sure it does. Just imagine the Earth rotating
around the Sun, and yes... a plane appears where Earth's orbits sits in.

The other 7 main Planets, are (more or less) in that plane too, but not perfectly.
You thus might say that the planets revolve around the Sun, (more or less) in the same "disk".
It's quite remarkable. This fact is attributed due to the processes involved at the birth
of our Solar system (later more on this).

Two of the main exceptions are Pluto and Mercury.

Pluto 's orbit is quite exceptional because its orbit makes an angle of 17 degrees with the plane describe above.
For Mercury, it's about 7 degrees.
In figure 1 above, I tried to draw that a bit. I am not an artist, but that picture is, as if you watch
it from 40 degrees of angle (or so), and the orbit of Pluto is a bit tilted with respect to
the Ecliptic Plane.

5. Location of our Solar System in the Milky way (our Spiral Galaxy).

Our own Spiral Galaxy (the Milky way), is about 100000 lightyears across.

The Solar System, is in the order of about 27000 lightyears away from the Center of our Galaxy.

Here are some nice "maps" illustrating the structure of our Milky way,
and also illustrating the location of our Sun, in that enormous "star island".

(Static) Map 1
(Static) Map 2
(Static) Map 3

And next we have a striking picture, which zooms in to the position of the Sun
in the local Spiral arms (yellow circle represents the Sun):

(Static) Zoomed in position of the Sun.

6. Comets and Astroids in our Solar system.

⇒ Comets.

General aspects:

These fantastic objects, when they get nearer and nearer to the Sun, then CO₂, H₂O,
dustparticles,and other substances start to vaporize or loosens, leading to a "coma" around the object,
and most often lengthy tails, in some cases even up to hundreds of millions kilometers.

These icy objects usually are in the size range of about 1 kilometer, up to several kilometers in diameter.
Only a minority been observed with a size in the order of 30 kilometers, or thereabout.

They are often described as "dirty snowballs" which likely characterize them rather accurately.
A comet is probably some sort of looser structure of rock, dust, water-ice, frozen CO₂, and methane.
These are all components which easily start to vaporize, when Solar radiation and the Solar wind
gets more and more relevant, as the comet gets nearer and nearer to the Sun.

Any passage to and from the Sun, can be viewed as a "slimming cure", and indeed, there is a limit
to the number of passages of a periodic comet.
It's true, that as material vaporizes, small rock particles get freed too, which may lead to swarms
of such particles. Earth may sweep through such cloud, which is then often displayed as a "meteor shower".

Most comets originate from the Kuiber Belt or Oort cloud, which is home to millions of such objects.

For "short period" comets, it seems rather certain they originate from the Kuiper Belt.
They usually have highly elliptic orbits, and each periodic passage may take a couple of tens of years,
or up to a few hundreds of years.

It's very possible, that for example, gravitational interactions with Jupiter once in a while, slings a comet
from the Kuiper Belt, further into the Solar system.

There are even clues that in rare cases, a comet (or astroid) may have originated from another Solar system,
and has a hyperbolic, one time passage, around our Sun, only to dissappear in deep space again.

Orbits:

Figure 3. Some simple illustrations of Comet orbits (Red: Short, Blue: Long, Green: Hyperbolic).

-"Short period" comets have highly elliptic orbits, often stretching beyond the orbit of Neptune.
A classical example is Halley's comet, which period is about 76 years.

It's Perihelion (shortest distance to the Sun) is about 0,59 AU.
It's Aphelion (furtest distance from the Sun) is about 35 AU.

For such comets, they loose mass at every period, which will affect their future orbit too.

-"Long Period" comets have even more highly elliptic orbits, sometimes stretching over
hundreds of AU, or even thousends of AU.

-"Hyperbolic/Parabolic" orbits. Due to Jupiter, or the Sun, the comet has a one time passage
along the Sun, and possibly it will leave the Solar system, after passage with the Sun.

In general, for all comets, the tilt with respect to the Ecliptic Plane, usually can be rather high.

Here are a few figures from the internet, illustrating comet orbits.

source: deepimpact.umd.edu
source: space.com
source: newsroom.ucla.edu

A few words on ESA's Rosetta Probe:

Launched in 2004, the Rosetta probe was on it's way to the comet "67P-Churyumov–Gerasimenko".
Rosetta was the command module, while Philae was the actual lander module.
Indeed, the bold plan was to execute a landing on this comet.

After a complicated yourney, the probe finally reached the comet in 2014.
Philae initiated it's landing, but due to the extremely low gravity and some other
complications (using harpoons to hook into the surface), the lander bounced off
the original landing site, and came down somewhere near, unfortunately in the shadow
of a rock. That meant that the batteries could not get charched anymore.
Although important scientific achievements were certainly done, the main mission was severely hindered.

Among the achievements were the discovery that the water isotopes were significantly different
from water on Earth. This seriously placed questionmarks on the Theory that water on Earth,
originated from bombarments of comets, in the early times of Earth (when also comets were plenty around).

But not withstanding all the bad luck, the soft landing, and navigating to the comet, was an
excellent achievement by itself. A true milestone in Space navigation.

⇒ Astroids.

In figure 1 above, you may have noticed the "Astroid Belt", which is located between the orbits
of Mars and Jupiter. In reality, it is a rather "wide" belt.
Also, we have the Trojan clouds which are situated in the orbit of Jupiter.

If you see any picture of the "Astroid Belt" (or Trojan clouds), you may think it's a dense concentration
of rocky debris, and stuff like that. But it's not dense at all. Ofcourse, at such special places
the concentration is more than elsewhere, but Space is so big.... Just imaging the amount of total space
between Mars and Jupiter, albeit in some disk-like structure (Ecliptic Plane).

The total mass of all astroids in the "Astroid Belt", is even less than the mass of Earth.

If you would happen to have a spacecraft, you could fly right through it, and have no worries at all.

Actually, astroids are almost everywhere in the Solar system, and once in a while, a smaller one
passes Earth, on save distance, and occasionally, on a rather close trajectory (see the link below).

Astroids are pieces of rock of usually tens of meters, up to hundreds of meters, and ofcourse, there is a minority
of really big ones (but still a large number), of a few kilometers in diameter.
Astroids are "rocky" (minerals), and some are more metal rich, than others.

About sizes: What exactly is typed as a "minority" is a point of discussion: a study of one
of the Trojan clouds, returned an estimation of > 500000 astroids, having a size > 1 km.

A few (really a few), are more like dwarf planets, of sizes of hundreds of kilometers.
These are: Vesta, Pallas and Hygiea (around 400km), and Ceres, which is even bigger, at around 950 km diameter.

Astroids are composed of all sorts of minerals, metals, and rock-like stuff, not unlike rocks on Earth.
However, some large astroids (those of a dozens of kilometers in size), might actually be a rather loose
collection of all sorts of debri.

-The large "Astroid Belt", are likely to be remnants of the early phases of the Solar system,
which never "made it to become a Planet". This is reasonable hypothesis, but this is not fully
certain as of yet.

- The Trojan clouds in Jupiter's orbit, are located at the socalled "Lagrange" points.
The "Lagrange" points, are related to two massive objects (here the Sun and Jupiter), where the gravitational
forces, and centrepital forces, work in such a special way, that a sort of semi-stable location exists.

A few nice links:

Database of "Near Earth Objects" (you can select distance and dates and more).
Animation of Astroids (jpl.nasa.gov).

A nice Map from Wikipedia Commons:

Map of Astroids (Wikipedia)

Originally, I planned to put an overview of Space Missions to the various Planets, in this note too.
However, since those topics are so vast, I place that in a seperate note. That will be note 4.
This way, I keep this text dedicated to a simple overview of the Solar system itself.