Lesson 8 – What is the Solar System? An introduction to the Solar System and the planets

Lesson learning outcomes:

By the end of this lesson, you should be able to:

1.   Explain the current theory of how the Solar System formed.

2.   Describe the planets in the Solar System and how they likely formed.

3.   Describe non-planetary objects in the Solar System and how they likely formed.

4.   Outline the different Solar System mission types and their advantages and disadvantages.

8.1 Introduction to the Solar System formation

Taking a step back from looking at the Sun last lesson, we are now going to look at the Solar System, starting with how it formed, and moving onto each of the planets.

It is important to note that we do not know how the solar system formed. We can only guess how it formed based upon how we think physics works in the Universe, how we see other systems behave (that are in the process of forming or have formed), and what we observe in our own Solar System. We will look at the details of our own Solar System later this lesson, but for now, we summarise the four main points that a solar system formation theory must satisfy/explain:

1.   The motions of the planets .

2.   Why planets are either rocky (terrestrial) or gaseous (Jovian).

3.   The existence of asteroids and comets, and why they exist in specific locations in the Solar System (i.e., Kuiper belt, Oort cloud, asteroid belt).

4.   General patterns observed, such as rotation axis, while also allowing exceptions to the rule.

In the  mid-18th  century,  Immanuel  Kant, a German  philosopher,  proposed that the Solar System formed via the gravitational collapse of a gas cloud. A few decades later, Pierre-Simon Laplace, a French mathematician, put forward the same idea. This hypothesis, which has, through modifications, gained significant momentum and is now the leading theory as to how the Solar System formed. We call this theory the nebula theory (nebula is Lain for “cloud”). Let’s look at the nebula theory in more detail now.

8.2 The Nebula Theory: the formation of the Solar System

The first question we can ask ourselves is, “Where did the gas that formed the Solar System come from?” To answer this question, we need to remember our discussions about gas recycling in the Milky Way in Lesson 5. As stars “burn” fuel, that is, they convert hydrogen into heavier elements, the total amount of hydrogen in the galaxy (and in the Universe) slowly decreases over time. Once stars die and most of the gas is returned to the interstellar medium , new stars are formed from this gas; it is not only new stars that are formed, but planets as well.

The motion of the planets

We looked at how star’s form from the gravitational collapse of a gas cloud in Lesson 6, which you should go back and revise if you’ve forgotten the basics of it. When we were discussing star formation, we focussed mainly on how the star forms, and not the rest of the gas in the cloud (because not all the gas in the cloud makes up the star that is being formed). Now, let’s consider what happens to the gas that does not eventually become the new star.  In 6.8, we mentioned that as gas clouds collapse and a star forms, the material surrounding the star flattens into a disk, a bit like how a spiral galaxy forms.

It is this spinning disk that explains the orderly motion of the planets. What do we mean by orderly motion? We mean the fact that the planets orbit mostly in the same plane, they almost all spin in the same direction (with a few exceptions), and the orbits are almost circular. All these facts can be explained by the presence of a spinning disk; all planets orbiting in a plane would arise if all the planets had formed in a disk, computer modelling shows that planets will tend to rotate in the same direction that they orbit but exceptions can arise due to many factors, and a disk also helps to explain why the orbits are circular. Planets forming in disks clear a path through the material in the disk (this is one of the definitions of a planet). We’ve seen this path clearing in other systems we have observed.

Figure 8.3 from the Cosmic Perspective. These images show flattened, spinning disks of material around other stars. The presence of different types of planets: terrestrial (rocky) and Jovian (gaseous)

As was the gas with gas clouds that eventually went on to form galaxies and stars, the temperature must be low enough for particles to clump together to form the “seeds” of planets. We call this condensation. However, we said that before that even though the disk would be predominantly made of hydrogen and helium, some other, heavier elements (about 2% of the total mass) would be present. Different materials condense at different temperatures, and this is useful to know when we consider the types of planets that would form in different locations . Remember, near the Sun is going to be at a much higher temperature than further out in the Solar System.

Table 8.1 from the Cosmic Perspective. A summary of the four types of materials present in the solar nebula. The

squares represent the relative proportions of each type (by mass).

The above table is telling us that if we start at the Sun and slowly work our way towards the edge of the Solar System, metals will have the ability to condense first, because they condense when temperatures are quite high, and this occurs relatively close to the Sun. As we move further out, we find rocks will condense. Remember, condensing means that the particles are solid and form clumps; this explains why rocky planets, Mercury, Venus, Earth, and Mars, are located closer towards the Sun than the gaseous planets of Jupiter, Saturn, Uranus, and Neptune.

The formal process of clumps growing into planets by accruing more clumps is called accretion. Particles start off sticking together like static electricity causes materials to stick together, but, eventually, the mass of the planet seed will be massive enough to start attracting more clumps due to gravity. Once the clump is big enough, we call it a planetesimal. Growing from a planetesimal to a planet sounds easy, but it is surprisingly difficult; collisions can easily disrupt the formation process, which takes millions of years. Meteorites, rocks that have fallen to Earth, support the accretion theory. We find that meteorites typically have metallic fragments embedded in rock if they formed in the inner solar system, whereas if they formed further out, they contain more carbon and water.

Isn’t metal and rock denser (i.e., heavier) than gas, so wouldn’t that be the reason that rocks were closer to the Sun and why rocky planets formed there? No, and this is because the rocky particles are so tiny. They were distributed relatively evenly throughout the disk; the temperature at various locations from the Sun dictated where materials condensed, hence the differing locations of the different planet types.

Beyond the frost line, there still existed metals and rock, but there were also substantial amounts of ice. Every object that formed in the outer solar system (comets, Jovian planets) show this icy composition. However, Jovian planets aren’t just ice, metal, and rock; when we look at the Jovian planets, we are seeing excessive amounts of hydrogen and helium. So how did they obtain all this gas?

The leading theory is that because there was so much ice beyond the frost line, the planetesimals became sufficiently massive to accrete hydrogen and helium. This created a snowball kind of effect, where the planetesimals continued to grow in mass by accreting gas to become the planets we see today .

This model of Jovian planet formation also explains the moons around the planets. As gas was accreted onto the planets, you can think of them as being their own little solar systems, with their own accretion disks, and the moons formed in the same way that the planets formed around the Sun!

What about asteroids and comets?

Analysis of meteorites, theoretical models, and spacecraft missions all support the idea that asteroids and comets are leftover planetesimals. More specifically, asteroids are the rocky remains of the inner planets, whereas comets are the remains of the Jovian planets.

Exceptions to the rule: moons orbiting in the wrong direction, large moons, etc.

If the moons around Jovian planets formed from a disk of gas, how is it that some moons orbit in a direction opposite to the others? The leading theory is that the moon was probably captured, but how? For this to happen, the moon to be captured must lose orbital energy and go from an unbound orbit (think of it like a flyby) to a bound orbit. The theory of how captured moons lose orbital energy is through friction with the gas surrounding the planet. This is the same process of why satellites in relatively “low” Earth orbits need constant boosts; friction between the satellite and the gas causes the satellite to lose orbital energy.

The problem with the captured moon theory is that it does not explain how our own Moon came to be. The Earth is simply not massive enough to capture an object the size of the Moon.

It would be natural to assume that the Moon and Earth formed at the same time. However, if that would have happened, they should have the same composition and density. The Moon, though, is significantly less dense than the Earth. The leading theory is that a large, Mars-sized object hit the Earth at a speed and angle that caused the outer layers of the young Earth to be ejected into space, to be captured in orbit and form the Moon. This theory is supported by the composition of the Moon; it is similar to the outer layers of the Earth but also lacking in water, which makes sense given that the water would have been turned into gas during the impact.

8.3 Dating the Solar System

How do we know how old the Solar System is? Well, we work out how old the rocks are that constitute the planets . For example, if we can work out how old the rocks are on Earth, we have a good approximation of the age of the Solar System. However, how do we work out how old the rocks are on Earth?

We use a process called radiometric dating, which uses radioactive decay of elements.  Radioactive decay is the spontaneous change of elements into other elements. Recall that atoms contain protons and neutrons in the nucleus, and surrounding electrons. Elements are characterised by the number of protons in the nucleus. Isotopes of an element will have the same number of protons in the nucleus but different neutrons. Certain isotopes are what we call radioactive, and either some of their neutrons will combine to form protons, or they will break apart entirely, either way, parent nuclei or parent isotopes will produce daughter nuclei or daughter isotopes via radioactive decay.

You can probably see that there are useful things we can measure here. For example, if we measure how much each of parent and daughter isotopes exist in a material, that is useful information, but the last thing we need to know is how quickly the decay happens. The time it takes for half the parent nuclei to decay into the daughter nuclei is called the half-life, which we can measure in a lab. Parent nuclei undergo successive half-lives as it decays.

Let’s take a closer look at half-life. Let’s assume that we have potassium-40, 40K, that decays into argon-40, 40Ar. The half-life of 40K is 1.25 billion years. If we have 1g of 40K, after 1.25 billion years, there will be 0 .5g of 40K and 0.5g of 40Ar. After another 1.25 billion years (2.5 billion years total), there will be 0.25g of 40K and 0.75g of 40Ar. After another 1.25 billion years (3.75 billion years), there will be 0.125g of 40K and 0.875g of 40Ar. You can see the trend.

Typically, rocks contain more than just one element or isotope. By performing this half-life method on all the different  isotopes, we can constrain the age of the rock. However, there is a slight issue: there are rocks on Earth that are        different ages. How? The Earth’s surface has changed remarkably over time. In fact, many Moon rocks are older than those on Earth, which suggests that the Moon’s surface has not changed much over time. To get an age of the Solar System, we need to date rocks that have not changed much over time since the beginning of the Solar System:          meteorites. They show that the Solar System is around 4.56 billion years old.

8.4 Introduction to the Solar System planets

We will now briefly introduce the Solar System planets before diving into each one with a bit more detail. The Solar System consists of planets, dwarf planets, asteroids, and comets. There are 8 planets, which are, in order of increasing distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto, which used to be a planet, is now considered a dwarf planet, and wasreclassified in 2006.

Some basic data about the planets are included in the table below, but we will go through each planet in more detail.

Table 7.1 The Planetary Data from the Cosmic Perspective

8.5 Mercury

Mercury is the closest planet to the Sun and is also the smallest. It has no moons, has a radius that is about 40% of the Earth’s, a mass that is around 5% of the Earth’s, its composition is mainly rocks and metals, and it has extreme temperatures. It’s natural to think that Mercury is the hottest planet, given that it is closest to the Sun, but given that it has no substantial atmosphere, daytime temperatures are 700 K (we leave you to do the conversion!) during the day and 100 K during the night. It is postulated that Mercury lost its atmosphere in a combination of ways: solar wind from the Sun stripped the atmosphere, which was possible thanks to the lack of a magnetic field surrounding Mercury to protect  it,  and, due to  its small  mass, the speed  required  by  particles to escape  Mercury was  low enough that atmospheric gases easily obtained that speed.

Mercury is in a 3/2 spin-orbit resonance, which means that it rotates three times on its axis for every 2 orbits around the Sun. This is a very stable state for the planet to be in,but how it occurred is still under debate.

The MESSENGER and Mariner spacecrafts captured incredible images of Mercury’s surface, from which we have learned that Mercury is very cratered. Comparing the surfaces and crater distribution of Mercury and the Moon, which are both small worlds now absent of volcanism, we can infer that Mercury probably had more molten lava cover some of the impact craters than on the Moon. Some of these craters are releasing vaporised gases that leave behind a light- coloured residue and are causing the rock to crumble.

Figure 9.21 Images of Mercury from the MESSENGER spacecraft.

It is also postulated that Mercury has shrunk over time, based on cliffs on its surface.

8.6 Venus

Venus is often considered a sister planet to Earth. It is the second planet from the Sun at ~70% the distance from the Sun to the Earth from the Sun, and 95% and 82% of Earth’s radius and mass respectively. Like the other terrestrial planets, it is composed of rock and metals, but like Mercury, Venus has no moons. It also rotates on its axis in the opposite direction to Earth. Another key feature of Venus, which is arguably its most well-known feature, is its incredibly

dense atmosphere and, consequently, surface temperatures.

The surface of Venus has not been imaged due to its thick atmosphere.

Figure 10.32 This colour-coded image shows Venus at ultraviolet wavelengths as seen by the Japanese Space

Agency’s Akatsuki spacecraft. At most wavelengths, clouds completely prevent any view of the surface.

It’s natural to think that Venus has extremely high surface temperatures (740 K) due to its proximity to the Sun, but these temperatures are comparable to Mercury, which is much closer to the Sun than Venus. It is not just proximity to the Sun that influences temperatures (night-time temperatures on Mercury are very low!). The primary reason for Venus’ temperatures is the greenhouse effect, which you are probably familiar with as a process occurring on Earth.

For the greenhouse effect to occur, gases known as greenhouse gases need to exist in the atmosphere of the planet. Greenhouse gases are gases that are excellent at absorbing infrared light due to their molecular structures. As light and energy enters the atmosphere from the Sun, infrared light is absorbed and re-emitted constantly by greenhouse gases. This causes an overall effect where more energy is trapped in the atmosphere than escapes, causing warming of the surface. If the atmosphere is dense and contains a high proportion of greenhouse gases, this can cause a runaway greenhouse effect, which is what happened on Venus.

The question is, however, how did a runaway greenhouse effect occur on Venus and not Earth? Presumably they should have had similar atmospheric compositions, are similar distances from the Sun, and are similar sizes.

For an atmosphere to exist around a terrestrial planet, some can be accreted during formation, but most of it comes from events called outgassing, where gas trapped within the planet is released via volcanic eruptions. Therefore, we know that Earth and Venus both underwent some type of significant volcanic outgassing event, where large amounts of carbon dioxide and water vapour were released into their atmospheres. Venus’ atmosphere today has large amounts of carbon dioxide (200,000 times that of Earth’s) and no water, whereas Earth’s atmosphere has very little of either. Where did the carbon dioxide and water end up?

On Earth, water in the atmosphere condensed into rain and formed the oceans. Carbon dioxide can be dissolved in water (you know this thanks to carbonated beverages such as soda water or soft drink), and when this occurs, it can be further absorbed into carbonate rocks or can undergo reactions to form carbonate rocks. Venus, on the other hand, does not have any liquid water on its surface available to absorb carbon dioxide from its atmosphere , so where did the water go? Venus does not have a significant magnetic field like Earth, and it is hypothesised that UV light interacted with water molecules in its atmosphere, breaking apart the hydrogen and oxygen atoms. The hydrogen atoms could escape to space.Observations have shown this to be extremely likely. Deuterium is a hydrogen atom with one proton and one neutron. When combined with oxygen, deuterium atoms create “heavy water” . This heavy water should have existed in both the atmospheres of Earth and Venus in relatively even amounts during their early histories. However, when UV light broke apart the heavy water, the deuterium atoms would not have escaped , and the ratio of deuterium to hydrogen atoms should be much higher on Venus than on Earth, which is the case.

8.7 Mars

Mars is a hot topic; it is perhaps the most famous of all the planets after Earth. Mars is 1.5 times the distance from the Sun that Earth is, its radius is 50% and its mass 10% that of Earth’s. It is the fourth planet from the Sun and the final terrestrial planet, and is, therefore, made of rocks and metals. Its surface temperature is only 220 K and it has two very small moons, Phobos and Deimos.

There have beenover 20 missionsthat have captured information and imagery of Mars, via flybys, orbiting, or landing on the surface.

NASA’s Perseverance Mars rover took a selfie with the Ingenuity helicopter, seen here about 13 feet (4 m) from the

rover in this image from April 6, 2021. It was taken by the WATSON camera, located at the end of the rover’s robotic

arm. Image viaNASA.

Mars has been well-studied, and we know a lot of information about geological processes that occurred on Mars.

Figure 9.25 This image showing the full surface of Mars is a composite made by combining more than 1000 images

with more than 200 million altitude measurements from the Mars Global Surveyor mission.

The surface of Mars shows signs of extensive impact cratering and volcanism. By looking at the above composite image, we can see that the northern hemisphere of Mars has fewer large impact craters than the southern hemisphere, indicating that the northern hemisphere’s impact craters must have been erased by other geological processes on the surface. Although the level of volcanic activity is much less on Mars than on Earth (probably thanks to a cooler Martian interior due to its small mass), Mars is home to the tallest known mountain in the Solar System, Olympus Mons, located in the Tharsis Montes region.

It is unclear as to whether volcanic activity is completely over on Mars.A new studyhas determined that there are young lava deposits around a segment of a fissure system in the Elysium Planitia region on Mars. The researchers counted the number of impact craters and have determined that a volcanic eruption in the region could have erupted 53,000 years ago, which is very recent given the age of Mars is around 4.5 billion years.

More evidence for some geological activity comes from NASA’s InSight lander, which touched down on the Martian surface in November 2018. InSight has recorded hundreds of Marsquakes. It’s likely that Marsquakes are the result of active magma sources deep within the planet .

Mars doesn’t have tectonic plates that are on a global scale like Earth’s, but there are some tectonic plate-like features. The most prominent is a valley called Valles Marineris (named after Mariner 9, the spacecraft that first imaged it).

We also think that water once flowed on the Martian surface. While it is known that liquid water cannot exist on the surface of Mars, the debate about the current existence, origin, and stability of liquid water somewhere on Mars (i .e., under the surface) isstill ongoing. Surface and orbital spacecraft observations provide substantial evidence that water flowed on Mars in the past. These observations include that of erosion and riverbeds . The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) detected liquid water in the southern pole, as didESA’s Mars Express, andMars Express also imaged the Martian north pole.

A new mosaic from ESA’s Mars Express shows off the Red Planet’s north polar ice cap and its distinctive dark spiralling troughs.

8.8 Jupiter

Jupiter is the next furthest from the Sun beyond Mars and is more than 5x the distance from the Sun the Earth is. Its radius and mass are 11 and 320 times that of the Earth’s respectively. Jupiter is a gas giant/Jovian planet, meaning that its main constituents are hydrogen and helium. Its temperature is only just over 100 K, and it has, to date, 79 moons . However, not all these moons are well-studied, though some are of particular interest to astronomers.

Before discussing Jupiter’s interesting moons, we will discuss Jupiter itself. The process of determining anything in astronomy is to make observations, compare those observations to theoretical models, modify the models to better match data, and continually update when as new data comes in. This is the same for the interior structure of the Jovian planets. All the Jovian planets (Jupiter, Saturn, Uranus, Neptune) have rock and metal cores. For the larger of the Jovian planets (Jupiter and Saturn), the core is surrounded by metallic and liquid hydrogen. Beyond that (and above the core for Uranus and Neptune) are gaseous hydrogen and visible clouds. The clouds are at very low temperatures and densities, but as we progress further into the planets, the density and temperature increases.

Observations of Jupiter, including data gathered by theGalileo probe that plunged into Jupiter’s atmosphere, have given us a wealth of information about its atmosphere and moons. Jupiter has distinct cloud layers, like Earth. It is in these cloud/atmosphere layers that certain wavelengths of light are preferentially reflected, giving Jupiter its famous colours. Specifically, moving up through the lower layers of the clouds/atmosphere, we find a decreasing temperature with increasing altitude (this isn’t the case for the uppermost layers). Lower in the atmosphere, temperatures are low enough for water to condense. At higher altitudes, ammonium  hydrosulfides can condense to form clouds. Higher again, ammonia can condense. It is in these layers that the distinctive bands in Jupiter’s atmosphere come about: water and ammonia reflect sunlight (white light), and ammonium hydrosulfides reflect red light. Gas moves between the layers as it warms and cools, a bit like convective currents.

You might be wondering, how do the bands in Jupiter’s atmosphere come about? This is thanks to the Coriolis effect. Jupiter is rotating so rapidly that any wind in a direction that is not east-west will become an east-west wind, and the direction it moves depends on whether it was rising or falling. The different atmospheric layers (and their corresponding “colours”) are separated into dark belts and bright zones that circumnavigate the planet in opposite directions .

Figure 11.9 Jupiter’s bands of colour represent alternating regions of rising and falling air:

At the division between belts and zones exist storms, the most famous being the Great Red Spot. The Great Red Spot could consume 2 Earths, though the reason for its bright colour remains a mystery.Hubble has observed the Great Red Spot changing over time. NASA’s Juno mission has discovered that the storms on Jupiter are “opposite cyclones” , which means that the cyclones are warmer at higher altitudes with lower atmospheric densities and colder at lower altitudes with higher densities. They also rotate in the opposite direction. There also exists smaller lightning storms on Jupiter, which have been confirmed to bemore like storms on Earth than previously thought.

This new perspective of Jupiter from the south makes the Great Red Spot appear as though it is in northern territory. This view is unique to Juno. NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstäd/Seán Doran © CC NC SA

Jupiter also has a very strong magnetic field, the strongest of the Jovian planets. Its magnetic field is 20,000 times stronger than Earth’s. As we said above, Jupiter has a layer of metallic hydrogen and rotates very rapidly; both properties create the strong magnetic field that we observe (Jupiter’s magnetic field is so prominent that it would appear larger than the full Moon in the sky if we were to observe it).

While Jupiter has many moons, we will only concentrate on the largest and most famous, being Io, Europa, Ganymede, and Callisto.

Io

Io, which is a little larger than Earth’s Moon, is the most volcanically active moon in the Solar System. There is so much volcanic activity (activity that is remarkably like that on Earth!) that Io’s surface does not have any impact craters; the frequent eruptions constantly refresh the surface.

Figure 11.17 Io is the most volcanically active body in the solar system.

The question is, what causes this volcanic activity? Firstly, Io orbits very close to Jupiter and is tidally locked (like the Moon to the Earth). This means that the same side of Io always faces Jupiter. Jupiter is not the only object that exerts large gravitational forces on Io: the other prominent moons (Europa, Ganymede, and Callisto) also exert forces on Io such that Io’s surface can contort and bulge by up to 100 m. Remind yourself that this is the surface of Io, and compare this to 18 m, which is how much water moves on Earth due to similar forces. Io has a thin atmosphere that is comprised primary of sulphur and sulphur dioxide. These gases are released from the volcanoes on the surface.

Europa

Europa is one of the most exciting of all the moons in the Solar System. This is because there is increasing evidence that not only might water exist on Europa, but subsurface oceans of liquid water may be present . This gives a lot of substance to the search for life on planets or Moons beyond Earth, because scientists use water as a proxy for (i .e., possible indicator of) life.

Interestingly, one of the first pieces of evidence for water on Europa was in the form of magnetic field detections by NASA’s Galileo spacecraft(no, it was not detections of water itself!). You might be wondering what this means and how magnetic fields can tell us about water. Europa is one of the few moons in the Solar System to have a magnetic field, and this magnetic field changes as Jupiter rotates. Scientists explain this change as such: Europa’s magnetic field is induced (or caused) by Jupiter’s magnetic field, and this can only happen if, somewhere within Europa, there exists liquid electrically conducting material. This material could be in the form of liquid salty water.

Further evidence for water on Europa came in the discovery of water vapourabove its surface, including inplume-like ejections. The surface of Europa is covered in red-brown cracks and has very few impact craters.

On the left is a view of Europa taken from 2.9 million kilometres (1.8 million miles) away on March 2, 1979 by the

Voyager 1 spacecraft. Next is a colour image of Europa taken by the Voyager 2 spacecraft during its close encounter

on July 9, 1979. On the right is a view of Europa made from images taken by the Galileo spacecraft in the late 1990s.

Credits: NASA/JPL

Scientists at the NASA Goddard Space Flight Centerdetected an amount of outgassed water vapourfrom Europa that could fill an Olympic sized swimming pool in minutes, but outgassing events of this magnitude are not particularly frequent. Water vapour was detected not by direct observations of water itself, but by the way light interacts with the water molecules: molecules absorb and re-emit the light as infrared radiation, which telescopes on Earth can detect and measure. TheEuropa Clipper mission, set for launch in October 2024, aims to determine whether there are places on Europa, particularly subsurface, that could support life. It aims to do this by performing around 50 fly-bys to making various  measurements of Europa, such as thermal  imaging, composition via spectrographic  measurements,  and Europa’s magnetic field.