THE MODERN SOLAR SYSTEM

Our knowledge of the Solar System has changed startlingly since I was first learning about it at school in the 1960s! Now that we have a much closer acquaintance with some of it, much more has been discovered, due mainly to the diverse stack of equipment on board the Internal Space Station (ISS) and the regular additions to an armada of telescope laden and probe bearing spacecraft which have sailed forth since the late 1970s to give us close-up planetary views and spectral assessment and even physical sample analysis.

As astronomers' knowledge increases in leaps and bounds with the advent of more and more advanced technology, the outlook for onlookers will be a fascinating journey that will unentrench many ideas that have reigned in comfort for decades. No doubt some of the information on this page is already out of date, as so much is being discovered recently.

I have outlined some of the relatively recent discoveries below. 

The Planets and their Moons

What is probably the most widely known change in our ideas about the configuration of the modern Solar System is in fact just a reclassification. I'm talking about the way that Pluto has been kicked out of the planet family, demoted from a planet to a `dwarf planet'. This was mainly due to the recent discovery of another Solar System body beyond Pluto, an object which is thought to be larger than Pluto. It was named Eris and is rather ironically classified as a `plutoid'!

Although the prefix dwarf only implies smallness, the International Astronomical Union (IAU) decided that it no longer qualifies as a `planet' at all. So, we now have only eight planets in the Solar System, four inner, rocky ones and four outer `gas giants', the outermost one being Neptune. Of course, Pluto still endures as it was, despite the impotently important reclassification.

The eight planets of the modern Solar System are shown, together with their very approximate orbits, in the blatantly not-to-scale diagram below:

Looking now at our immediate neighbours, let's start with Mars, a planet that is finally yielding some of its secrets. It has recently been discovered by the Curiosity Rover that the soil in the Martian Gale Crater contains about two percent water by weight. Evidence of ancient stream beds has also been found. Although we can now be confident of the lack of developed life on Mars, it is still hoped that Curiosity may find some microbial life that is producing the small amounts of methane in the planet's atmosphere.

Our other neighbour, Venus, is still relatively veiled in secrecy beneath its thick and cloudy atmosphere. This is the barbarian of atmospheres, comprising a cocktail of clouds, which we now know are composed of sulphuric acid and carbon dioxide, packaged with a slaughteringly powerful surface atmospheric pressure of over 90 times that of Earth's! Estimates now put the surface temperature of Venus at 875 Celcius, revised steeply upwards from what was surmised back in the 1960s. Thanks to the doomed entry of recent probes into that atmosphere, we know that the surface has many craters, mountains, volcanoes and lava fields.

Another interesting new discovery is the presence of a diffuse circumsolar dust ring near to the orbit of Venus. It is thought to have been created by dust from comet debris and asteroid collisions accreting under the influence of the planet’s gravity. There is a similar ring close to Earth's orbit. 

So far, I haven't mentioned our nearest neighbour by far, the moon. The moon has received much attention during the early years of the 20-teenies, much of that attention driven by the prospect of lunar mining now becoming a possibility. It is now suspected that there are large reserves of under-surface water at the poles of the moon. How convenient is that for establishing manned lunar bases for mining of its minerals? Wow!

Meanwhile, the first ever plume of water vapour has now been observed squirting out from Jupiter's moon Europa! It has been known for quite some time that this moon has an icy crust, but, of course, the uncertainty is whether liquid water exists beneath.

Moving now beyond the imaginable realms of possibilities for the existence of water, let's look at the planet closest to the insufferably fiery heat of the sun. Little Mercury has now, at last, yielded some interesting secrets, thanks to NASA's Messenger orbiter, established in 2011. 

An intriguing fact about this planet is that its rotation rate is the same as its rate of orbit, so one side of the planet is always baked to near-annihilation by the sun whilst the other side is in deep-chilled darkness. Unsurprisingly, it's the Mercurial dark side that has yielded the most exciting discoveries, specifically in the region of the permanently shaded north pole. Surprisingly, water ice sheets have been found in some of its craters and also organic molecules have been found! Also, Messenger has discovered water molecules high up in the planet's thin atmosphere. Isn't that amazing?

I can't help thinking how the Solar System seems incredibly endowed with what seems like credible convenience for the potential of life, with the way that its basic elements arrange themselves like this! I know that our nature compels us to keep a hopeful little background eye open for `design for life' at the core of physics, but to think that water exists not only on earth but in these other three Solar System bodies, in this most unlikely, Goldilocks Zone stretching way! Prior to these discoveries, many people would comment that there's no way water would be found in the Solar System anywhere other than Earth, but, lo and behold, that life-giving H2O substance exists in its own remarkable niches.

What else is new? Ah yes, of major interest, we now know of four planets with rings around them. Jupiter, Uranus and Neptune have joined Saturn in this category. We can collectively thank the Voyager 1 spacecraft, the Voyager 2 spacecraft and the Hubble Space Telescope for the discoveries of the rings of Jupiter and Uranus, although the rings of Neptune were first spotted and identified as `arcs' from the European Space Agency's La Silla Observatory in Chile, before being imaged clearly by Voyager 2. 

Unlike the other ringed planets, Uranus' rings seem to be unstable, changing their architecture over the years. Perhaps this is due to the planet's rotation; it is the only planet to be `tipped over' so that its axis is at about 90 degrees to the axes of the other planets. When Uranus was first observed more closely, the south pole was pointing directly at the sun; that has now changed as it progresses around its orbit. Voyager 2 discovered that the atmosphere of the planet contains varying and unstable clouds, which are now bubbling up as the planet is more `side on' to the warming influence of the distant sun. 

We also now know that Uranus has a strange magnetic field, with a wobble! This is due to the fact that the magnetic field is off center and off axis. In so many ways, Uranus shirks the expectations of laws of physics!

A great many more moons (natural satellites) have now been discovered orbiting some of the planets. This is especially true for Jupiter and Saturn, who are now known to have large herds of moons travelling with them, rather than the six or seven or so for each planet that was formerly thought to be the case. 

The statistics for each planet having moons are listed below:

And, I must mention, we even know now of 5 moons orbiting poor old relegated Pluto.

The largest moon of Neptune, Triton, is now known to be orbiting its planet in the opposite direction to every other sizeable moon in the Solar System. Its crust mainly comprises water ice, surrounding a dense core of metallic rock. It is thought that there is a possibility of a hidden ocean of water being between the ice and the rocky core. Similarly, Saturn's moon Dione is now thought to harbour a hidden ocean.

There have been some interesting discoveries to do with magnetic fields. It was previously thought that only Earth had a magnetic field, but now all four of the outer gas giants are known to have them. In addition, Mercury has one, discovered by Mariner 10, in 1974, and measured by the Messenger orbiter in 2011. Earth's magnetic field is the outcome of the motion of its churning and seething liquid-iron core. The fact that Mercury has such a magnetic field is strange, as its iron core was thought to have cooled long ago.

Asteroids

Between Mars and Jupiter is a region known as the Asteroid Belt, which...surprise, surprise...is the home of most Asteroids, which are rocky and metallic bodies up to a few hundred miles in length - also known as `minor planets', a term that is highly confusable with `dwarf planet'. 

The largest of the Asteroids has been known for a long time and is called Ceres (actually, it has been upgraded to a `dwarf planet', now, like Pluto, so the situation is fluid!); two more of the largest asteroids are Vesta and Pallas, shown in the diagram below. Ceres is about 2/3 the size of Pluto.

Not all of the asteroids in the Solar system inhabit this region; some of them have eccentric orbits which extend out as far as Uranus. Near-Earth asteroids have also been discovered, which are thought to have strayed from the asteroid belt due to gravitational interactions with Jupiter. It has been found that these usually have elliptical orbits which wrap around the earth's orbit for several months of the year.

Referring again to the diagram below, notice how the orbits of the planets are in approximately the same plane and also in similar ellipses, whereas the orbits of asteroids tend to be more erratic - often long and thin, typically with their orbital planes inclined by up to 29 degrees relative to the mean orbital plane of the planets.

As the orbits of the asteroids are so varied, some of them can be an impact hazard for Earth, as these two events that occurred in February 2013 highlight:

(1)   A meteor splintered off of a Near-Earth asteroid and exploded in the air just above the Russian city of Chelyabinsk, injuring more than 1,200 people.

(2)    We had a near miss with another Near-Earth asteroid (2012DA14 - I didn't see it; inevitably, it was cloudy here!) which passed us at a distance of only 17,000 miles. 

Famously, about a hundred years earlier, an object collided with Earth near the Tunguska river in Siberia and is reported to have wiped out an area of about 800 square miles. This is thought to have been either an asteroid collision, or a comet fragment. 

Meteor Crater, near Flagstaff, in Arizona, is thought to be the result of part of an asteroid fragmenting and hitting Earth. The result is a crater which is almost one mile across.

The smallest asteroid discovered so far is only about 20ft in length. Ceres, the largest, is just under 600 miles across its widest dimension. There are probably about a million asteroids. Several asteroids have even been discovered to have their own small moons orbiting around them.

Comets

When comets are distant from us, they are small, icy bodies containing rocky debris and dust. Some of them may have small rocky cores in the centre of their ice bodies, but this is not known. The ice they contain is not just water, but frozen gases like ammonia and methane. Typically, they are around 10 miles in diameter and accelerate as they approach the sun, hurtling through space at varying speeds averaging about 20,000 mph, but in excess of 1,000,000 mph (300 miles per second) near the sun.  

There are two classifications of comets - long period and short period (periodic). Long period comets may pass by us only once every few centuries, whereas short period comets are those that pass us every two hundred years or less.

Many long period comets are now thought to originate from the very outer reaches of the Solar System, now known to be up to one and a half light-years distant! Even the short period comets appear to have come from way beyond the furthest planet, Neptune. As they approach the sun, they heat up and display a visible coma (atmosphere) which usually blooms into an `ion tail', due to the Solar `wind' caused by the sun's magnetic field. For this reason, a comet's ion tail always points away from the sun. There is usually a second tail, known as a `dust tail', which trails behind the comet along its path. Sometimes, there is even a third tail, known as an `antitail', as was the case with Comet Holmes in 2013.

As comets head away from the sun, their tails and comas dissipate, leaving just the icy core again. The coma of a comet can be up to a million miles in diameter, and the tail of a comet can approach 100 million miles in length.

There are several differences between comets and asteroids. 

Asteroids tend to have fairly round, elliptical orbits and travel in groups. Comets are loners. 

Comets have an atmosphere (coma) and often a tail, whereas asteroids never do. 

Comets are mainly made up of ice and dust/small rocky bits, whereas asteroids are larger and basically lumps of rock. 

Comets also have very different orbits to asteroids, having greatly extended and elongated orbits, usually extending to the outer reaches of the Solar System, or even beyond, maybe. Some of them pass very near to the sun and are known appropriately as `sungrazers'. The now famous Comet Ison is an example of one of these, and it has been a good example of how a comet can meet its doom in the sun's hot, gravitational hug.

Meteors (Shooting Stars)

These are now known to be formed from debris left by passing comets or, less commonly, passing asteroids. They usually burn up in the atmosphere, but if they survive this and hit the ground they are known as meteorites.

The Scale of the Solar System

The extent of the Solar System is not just a circle centered upon the sun with its circumference extending to Neptune's, or even Pluto's orbit. The dominion of the sun has now been found to be far greater than previously thought.

What determines the outer limit of the Solar System? The Solar System is classed as the region of space where the sun's gravitational influence will outshine all other gravitational force - the zone of Solar supremacy! Of course, our knowledge of its limit is still imprecise, subject to refinement as the years pass, like all scientific knowledge. However, it can be said with confidence that it is wa-a - a  -  a   -   a    -    a     -     ay beyond the wandering ground of the planets. Let's look at what's beyond the planets. 

Beyond the outermost planet, Neptune, we have the Kuiper Belt, containing many icy/rocky bodies, including Pluto and dwarf planets like the aforementioned Eris and Makemake (I do love that name!). And there is more beyond the Kuiper Belt!

Before we look at that, it's time to put the distances involved into some sort of perspective.

The distance from the earth to the moon is around 250,000 miles.

The minimum distance from the Earth to Venus is about 24 million miles.

The minimum distance from earth to Mars is about 33 million miles.

The minimum distance from Earth to Jupiter is about 265 million miles.

The distance from the sun to the earth is 93 million miles. This distance is known as the Astronomical Unit (AU).

The distance from the sun to Uranus is about 1,790 million miles, or 19 AU.

The Kuiper belt extends from a distance of about 30 AU to about 55 AU from the sun.

Mind boggling, yet? OK, onto the rest...... 

Beyond the Kuiper Belt, we have the Heliopause (see diagram below), which extends for another 100 AU.

The Heliopause is enveloped by the Oort cloud (shaped like a sphere and largely made up of objects made from frozen gases), which is thought to extend to a distance of about 100,000 AU from the sun. Long period comets are thought to originate in the Oort Cloud, whereas the shorter period comets are thought to originate from the Kuiper Belt.

Just to add even more perspective (and awe!), the distance from the sun to the nearest star (Proxima Centauri) is about 271,000 AU, or the distance that light travels in 4 years.......and light travels a distance equivalent to about 7 times around the earth every second!

This is still impossible to visualise, isn't it. The easiest way to attempt a visualisation of the distances involved is to create a scale model. Imagine that the sun is represented by a football, located somewhere in London (UK). If this is the case, the earth could be represented by a peppercorn, at a distance of about 70 feet from the model sun. So where would the nearest star be? 

Answer.....New York!

Interestingly, the Oort cloud extends about a third of the way to the nearest star, a humongous distance of about one and a third light years. The Solar System has grown since I was a lad!

Our Place in the Universe

OK - we've been looking at distances. Now to really make our minds boggle with utter awe!!!

Our Solar System is a tiny area within our galaxy, the Milky Way. Remember, the nearest star is about 271,000 AU distant (= 4 Light Years). This galaxy is about 100,000 LY across it's widest dimension, and we are located on one of the outer galactic arms, as shown in the diagram below:

Every isolated star, isolated star cluster, constellation and nebula that we see in the sky is within our galaxy. But our galaxy is relatively tiny in the scheme of things. Another galaxy, known as the Great Andromeda Galaxy, is our nearest neighbour galaxy and that is about 2.5 million LY distant from us.

Amateur astronomers with medium sized and larger telescopes can see many distant galaxies. Those owning larger telescopes (generally 12 inch or larger aperture) can see the distant galaxies of the Coma Cluster (quite near to the Great Bear) and these are around about 300 LY distant! And there's much more, as the Hubble Ultra-Deep Field image shows us....an estimated 10,000 galaxies revealed through the window of a tiny area of space that appeared to be almost empty before Hubble's photographic eyes were turned upon it and set to work gathering light with many, many long time exposures! 

It's astounding; it's all so vast! Any attempt to fathom it is a stepping stone to the initially unsettling experience of staring into the all-round bottomless pit of forever. We are starting to feel the eternal and unlimited.