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Home , Definition of planet, List of planet types, Rings of Saturn, Rings of Uranus, Rogue planet, Solar System model

Unit 7: Comparing

This material was developed by the Friends of the Dominion Astrophysical Observatory with the assistance of a Natural and Engineering Research Council PromoScience grant and the NRC. It is a part of a larger project to present grade-appropriate material that matches 2020 curriculum requirements to help students understand planets, with a focus on . This material is aimed at BC Grade 6 students. French versions are available.

Instructions for teachers ● For questions and to give feedback contact: Calvin Schmidt [email protected], ​ ● All units build towards the Big Idea in the curriculum showing our in the context of the and the , and provide background for understanding exoplanets. ● Look for Ideas for extending this section, Resources, and Review and discussion ​ ​ ​ ​ ​ questions at the end of each topic in this Unit. These should give more background on ​ each subject and spark further classroom ideas. We would be happy to help you ​ expand on each topic and develop your own ideas for your students. Contact us at ​ the [email protected]. ​ ​

Instructions for students ● If there are parts of this unit that you find confusing, please contact us at [email protected] for help. ​ ● We recommend you do a few sections at a time. We have provided links to learn more about each topic. ● You don’t have to do the sections in order, but we recommend that. Do sections you find interesting first and come back and do more at another time. ● It is helpful to try the activities rather than just read them. ● Explore the “Ideas for extending this section” and “Resources” sections at the end of each topic in this Unit - they aren’t just for teachers!

Learning Objectives ● The BC curriculum requires students to learn about: ○ the “Components of our solar system” ○ “Extreme environments” This unit covers part of that requirement by discussing and comparing properties of and other planets. ○ “First Peoples perspectives regarding borealis and other celestial phenomena”

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○ “How has the exploration of extreme environments on Earth and in space changed in the last decade?” and also “’s contribution to exploring extreme environments” (in the context of planets) ○ How to use ratios and percentages to compare quantities. Make measurements using conventional units. Use tables to interpret data.

Learning Outcomes ● Students will learn about different properties of planets and see the great variety that exist and how they differ. ● Students will not be limited to talking about planets in our solar system and will learn about planets around other in order to expand the range of environments and see our solar system as one of many.

Materials and tools needed for the activities ● Activity One ● Activity Two ● Activity Three ● Activity Four

Time Required ● Lesson time - 90 minutes ● Activity time ○ Activities One and Two : 10 to 15 minutes each ○ Activities Three and Four : 10 minutes each

Contents The activities are marked in yellow. ​

● Many environments, many worlds ● help us understand what planets are ○ Activity One: Resolving Power of your eye ● What a is and isn’t ● Basic Differences Between Planets ○ Activity Two: Comparing Sizes and Masses ○ Activity Three: How much do you weigh on different planets? ● Types ● ● Rings ● Volcanoes ● Planets, Magnets, and Aurora ○ Activity Four: Discover Earth’s ●

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● Oceans and lakes ● Changes in environment ● Spacecraft and their importance in planetary exploration

M any environments, many worlds

If you travel on planet Earth, in person or virtually, you soon learn that it is not the same everywhere. There are regions like Antarctica where there is and snow year round. There are places like the Sahara where there are huge waves of sand and where it barely ever rains. There are undersea environments where the are huge and it is always pitch black. There are mountain tops that poke high into our where the air is low and it is hard to breath.

Humans like to live in environments where it is not at the extremes of temperature or pressure on the Earth.

Our study of planets was limited to those in our solar system until the last thirty years, but we now know of 4,201 planets in 3,176 solar systems (as of August 15, 2020, according to ​ ​ Wikipedia). More are being discovered all the time, mostly using special telescopes in space designed just for that purpose. For comparison, all of the planets mentioned in all of the Trek series total less than 400. Planets have been found not just in our Milky Way but in distant galaxies. The word “exoplanet” refers to any planet outside of our solar system. In our galaxy alone may contain hundreds of billions of planets, according to current evidence and estimates.

Most of the planets found so far have been found using the Transit method - see our Exoplanet ​ Transit Activity on our ExoExplorations page to learn more about it. ​

T elescopes helped us understand what the planets are

The ancients knew very little about what planets were. To them, they were points of in the night sky that looked similar to stars but moved over weeks or months. We call them “planets”, Greek for “wanderer”, for that reason. Some ancient people, who had little else to go on, thought they were special for that ability and reasoned that they might be gods with special powers to move around the sky.

Some, like and , were at times brighter than the brightest stars. Some had distinct colours: was always red, Jupiter yellowish, and Venus a pure white. But other than the basic properties people saw with their eyes, people knew very little about them until the invention of the .

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Galileo heard about the telescope in 1609 and made one for himself, and turned it towards the planets. Even in his small telescope he could see that planets weren’t just points of light like stars. Jupiter was a circular disk and Venus showed phases like our , which proved it was ball-shaped (try holding a ball near a flashlight in the dark see similar phases). In Jupiter’s case he could see it had points of light slowly moving around it, and argued that they were moons, just as Earth has a moon. He and other proved that planets were worlds.

When looked at in 1610 he thought that Saturn had two large moons, one on either side that touched the planet. His telescope was then only about as good as a cheap pair of today’s binoculars. A few years later he looked again and described them variously as handles, ears, or arms. It wasn’t until about 50 years later that telescopes were good enough to reveal that they were rings not touching the planet. Progress was slow.

Figure 1: Galileo’s drawings of Saturn in 1610 and 1614

The planets known since ancient times - , Venus, Mars, Jupiter, Saturn - were all bright, so to learn more about them astronomers usually just required telescopes that could see more detail. We learned in Unit 3 - Seeing Stars, that bigger telescopes gather more light, but they also allow us to see more detail. The wider the telescope, the sharper the . If you have a telescope twice as wide, you should see twice as much detail, at least as far as the optics of the telescope are concerned. Activity One shows you how to measure how well your eye can see detail.

A ctivity One - Resolving Power of your Eye

The following activity shows that the detail you can see is dependent on your eye and distance. Your eye has a lens in it that is similar to Galileo’s telescopes, which also used a simple lens up front to collect and focus light.

R equired tools and materials: ● This link: Resolving Power of the Eye ​ ● A tape measure

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● A pen or pencil and some paper ● Optional: a printer and some tape, binoculars, glasses (if you wear glasses), some friends to compare results

Step 1 - The “Resolving Power of the Eye” link above contains an image. You can show the ​ image on your screen (use something bigger than a phone) or print it out and tape it to a wall. One figure shows a series of grey lines while the other is a solid grey. If you are looking at it close up, it is obvious that one is lines and the other is a continuous grey.

Step 2 - Write down how far your face is from the screen or paper when you look at it close up. ​ If you wear glasses, take them off. Can you still see the lines as being separate? If you are doing this with someone else you should each record what you see separately.

Step 3 - Move away in increments of half a , starting at half a metre, and record whether ​ or not you can still see the lines as being separate. If you have glasses, record how it looks with and without your glasses.

Step 4 - Keep going until the square with lines looks the same as the square without lines. ​ There will be different distances for this with and without your glasses, and if you are doing it with a partner they will likely have a different distance at which this happens. If you want, you can try to find the distance to the nearest centimetre at which the two squares look the same (or the nearest ten centimetres). When that happens your eye does not have enough “resolving power” to see the detail in the square. This is different from magnifying power, which is how big something looks.

Optional Step 5 - Take the ratio of distances with and without your glasses to get the ratio of ​ resolving powers. For example, if the distance is 5 meters without your glasses and 10 meters with, then you can see only half as much detail without your glasses. If you are doing it with a partner, find the ratio of your resolving powers (divide your partner’s distance by your distance).

Optional Step 6 - If you have a pair of binoculars, how far would you have to go until the two ​ squares look the same? The binoculars will make things look bigger as well, but because their lenses are wider than your eye they have greater resolving power. For a big pair of binoculars you may have to go across a field, but for a small pair it may just be across your yard or down a long hallway. What is the ratio of resolving power of the binoculars compared to your eyes? Now compare that to the ratio of the lens of your eye (about 5mm) to the diameter of the front lens of the binoculars. If your eye had a good lens, it should be the same. What do you think of the quality of the lens in your eye compared to the lens of the binoculars

Acknowledgements This activity is based on one at Brigham Young University (see below for the link).

Ideas for extending this section:

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● Compare the resolving power of the to your eye. How much better is it? (Find the ratio) Use the following figures: your eye has a width of 5mm indoors and the Hubble Space Telescope has a width of 2400 mm. Hint: Divide the Hubble Space Telescope’s width by the width of your eye. In truth, your eye has a poor lens so Hubble’s resolving power is much better than this ratio.

Resources and references: ● Galileo and the telescope ● Have telescopes changed our view of the Universe? (NASA - JPL) ​ ● Resolving Power of the Eye (Brigham Young University) ​

Review and discussion questions: ● What are some of the things people do to improve the resolving power of their eyes if they don’t want to wear glasses?

W hat a planet is and isn’t

Let’s go back to some basic definitions about what a planet is and isn’t. We already talked about stars, and that they are hot enough to make their own light. The other celestial bodies do not do that, and are divided into more categories.

To modern astronomers, planets are more than just wanderers; they made a specific definition for them as of the year 2006. To be a planet in our solar system, a celestial object must: 1. around the . (So moons don’t count as they orbit planets). ​ 2. Be heavy enough to have been made very nearly round by its own gravitational pull. (So nothing potato shaped). ​ 3. Be dominant, in that it has cleared all other objects from its orbital path, except any moons that might be orbiting it. (So no loose collections of in the way). ​

Figure 2: Planets are round(ish), asteroids aren’t - Earth and Ida

Earth from Apollo 17 (courtesy NASA) Ida and its moon Dactyl from the Galileo spacecraft (courtesy NASA)

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Point #3 is what disqualifies all of the dwarf planets from being called planets. Dwarf planets look like they should qualify as they orbit a star, and are mostly round. But in our solar system, is in the same orbital path as many asteroids in the belt. The others, like and , orbit where there are many asteroid or -like bodies.

Not all astronomers agree with point number 3, however, and this is not something we can determine for exoplanets. For exoplanets, the definition is broader: it simply has to be a detectable non-stellar, non- object that a star. Some astronomers think there should be a single definition for planets in our solar system and exoplanets.

Even point #1 isn’t true for all exoplanets. There are planets that don’t orbit a star and instead wander through their galaxy on their own. There is good evidence for these “Rogue Planets” both inside our Milky Way and in another galaxy.

And just to make things more complicated, there is also a type of object that is not a star and not a planet. We call these “Brown Dwarfs”. They are objects that spend part of their time as stars making new atoms and part of the time not doing that, when they are more like planets. If an object has a mass 13 to 75 times higher than Jupiter then they can be considered Brown Dwarfs. They can either be on their own, form systems (like pairs of brown dwarfs, just like binary stars) or orbit stars. Planets can also orbit Brown Dwarfs, just as they orbit stars. It is estimated that in the Milky Way Galaxy there are 25-100 billion Brown Dwarfs.

Ideas for extending this section: ● Explore what is known about the dwarf planets in our Solar System.

Resources and references: ● The International Astronomical Union’s ● What is an asteroid? (NASA Space Place) ​ ● Extragalactic Planets (Wikipedia) ​ ● Brown Dwarf (Wikipedia) ​ ● An organically grown planet definition ( magazine) ​ ● (Wikipedia) ​

Review and discussion questions: ● Is a Brown Dwarf a ? ● If you discovered a cube shaped object orbiting the Sun, would you have discovered a planet, or something else? ● What would it be like living in a solar system with a Brown Dwarf that sometimes shone like a mini Sun and sometimes didn’t? How might that affect life on Earth?

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B asic Differences Between Planets

Almost all of the planets known are outside of our solar system. They are certainly not all the same, but we can lump them into different categories. We’ll look at those categories and see where the planets in our solar system fit. It often helps scientists to categorize things, or organize things into different buckets, to better understand them. Just as in Galileo’s time, it is hard to get much information about most of these planets, but we can figure out some basic things, like their size and their mass. We also think we can figure out whether they are rocky or mostly gas. Let’s look at some of these properties and their extremes.

Mass

All planets have a mass much less than a star.

The planets in our solar system have a great range of mass. The most massive planet in our solar system, Jupiter, is 5,782 times the mass of the least massive planet, Mercury. The most massive exoplanet is 6.5 million times more massive than the least massive exoplanet!

Table 1: Range of planet masses (exoplanets included)

Planet (in order of mass) Fraction of the Earth’s mass (the ratio equivalents are approximate)

Least massive exoplanet 0.00067 = 1/1,500

Mercury 0.055 = 1/18

Mars 0.107 = 1/9

Venus 0.815 = 4/5

Earth 1

Uranus 14

Neptune 17

Saturn 95

Jupiter 318

Most massive exoplanets 4,300 = greater than 13 times Jupiter’s mass

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The least massive exoplanet found has a mass about 1/1,500 times that of the Earth, about one third the mass of Pluto. There isn’t an exact “most massive” exoplanet found as they go all the way up to Brown Dwarf range (13 x the mass of Jupiter).

Size (Diameter)

There is also a big range in the size of planets (by size here we mean diameter). In our solar system, the biggest planet, Jupiter, is about 30 times the size of the smallest planet, Mercury. For exoplanets, the range is about 100 times. The biggest known exoplanet is about three times the size of Jupiter.

Figure 3 shows there are roughly three sizes of planets in our solar system. There are little planets like Mercury, Mars, Venus and the Earth. Then there are medium sized planets like and . Finally, there are giant planets like Saturn and Jupiter.

This isn’t the case for every solar system we find. Planets can potentially be any size that fits the definition described earlier.

Figure 3: Comparison of the sizes of the solar system’s planets - the distances between planets are NOT to scale (courtesy NASA/Lunar Planetary Institute)

When it comes to exoplanets however, we find even more varied sizes. Some of the largest known exoplanets, such as WASP-17b, were found to have a radius nearly twice as big as Jupiter, while the smallest confirmed exoplanet, called Kepler-37b, is barely bigger than the Moon!

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Table 2: Range of planet sizes (exoplanets included)

Planet (in order of size) Fraction of the Earth’s diameter (the ratio equivalents are approximate)

Smallest (non-controversial) exoplanet 0.3 x. This is Kepler-37b (slightly bigger than our Moon)

Mercury 0.38 = 2/5 x

Mars 0.53 = 1/2 x

Venus 0.95 = 19/20 x

Earth 1

Neptune 3.88 x

Uranus 4.01 x

Saturn 9.45 x (not including the rings)

Jupiter 11.21 x

Biggest known exoplanet (with a certain 33.6 = 3 times the size of Jupiter radius value)

Rocky versus Gassy

The other big distinction between planets is the state of most of its matter. Remember, we think of state as being “solid, liquid, or gaseous”. In our solar system we don’t have worlds that are mostly liquid, which may make you go “Wait, what about Earth?” It’s true that the surface of the Earth is 70% , but it’s not very deep compared to the rest of the planet, as we’ll see later in the section on oceans. Earth is mostly solid rock (even ice contributes very little).

In our solar system there is a strong distinction between “mostly rocky” planets and “mostly gas” planets:

● The mostly rocky ones are: Mercury, Venus, Earth, and Mars. ● The mostly gas ones are: Uranus, Neptune, Saturn, and Jupiter.

In the next section we’ll look at the exoplanets to see what they are like. We don’t yet know if there are planets that are “mostly liquid”, the other common state of matter.

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A ctivity Two - Comparing Planet Sizes R equired tools and materials: ● 2 shooter marbles (or Canadian dimes), a golf ball, two big oranges and tennis balls, a round sewing pin head, a soccer ball, an inflatable balloon, bracelets beads and/or modelling clay. ● A ruler ● A calculator (optional)

Gather all the materials you need on a table in front of you. You are going to build a model of the solar system that shows the correct relative scale for each planet.

As you can guess from the list, the soccer ball is going to represent Jupiter since it's the largest object available. By taking the ratio of Saturn’s size divided by the size of Jupiter (using the values from the tables above) we find that Saturn is about 9.45/11.21=85% of the size of Jupiter, so to model it you will have to inflate the balloon until its size is about 85% that of the soccer ball. Hint: A size 5 soccer ball is about 220mm in diameter. Calculate what’s 85% of 220mm to find the diameter you need for the balloon. Try getting as close as you can to this size, but remember that there’s no need to be too precise.

Uranus and Neptune are about the same size, so we’ll use two of the same object to represent them. By using the previous table and taking a similar ratio to previously, can you figure out which object is closest to the scaled size of our ice giants, the orange or the tennis ball?

As Earth is 11 times smaller than Jupiter, it should be modeled by an object 20mm wide (divide the size of the soccer ball by 11). The closest object available is the shooter marble, which should be 18mm wide (you can use a Canadian dime as well). If you have some modeling clay and really want to be precise, you can add some clay to form a shell around the marble and slightly increase its size.

Venus is barely smaller than Earth, you can just use another marble for it. If you covered the Earth marble with modeling clay, you can leave the Venus one bare to show the small size difference.

Mercury and Mars should be modeled by small balls that are respectively 8mm and 11mm wide. Many bracelet beads can be found around those size ranges, if you don’t have any available just make correct sized-balls with modeling clay.

Bonus: Using this scale, the dwarf planet pluto should be 4mm wide, that’s the size of a round sewing pin head, you can include that in your if you want.

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Bonus 2: You might have noticed that we didn’t use the golf ball in this activity. That’s because its size would correspond to that of a Super-Earth, which isn’t found in our solar system. If we wanted to model all possible exoplanets with household objects, we would need round objects even smaller than the pinhead we used for Pluto and balls up to 3 times as big as our Jupiter soccer ball such as an exercise ball.

A ctivity Three - How much do you weigh on different planets? R equired tools and materials: ● A scale ● This link: https://www.exploratorium.edu/ronh/weight/ ​

In daily life we often use the words “mass” and “weight” interchangeably, but to a physicist, those are two different but related concepts. As you already know mass is the measure of how hard it is to move that object. Weight however, is the measure of how hard the planet you’re standing on is pulling you; it’s related to a force.

If you have a scale at home (a cooking scale works best), try laying your hand flat on it, and read the corresponding weight. Now use any part of your hand to push on the scale, you’ll see a higher weight displayed by the scaled. But the mass of your hand is the same! So what changed? In the second case, you were applying an additional force with your hand.

The scale can’t actually measure the mass of the object placed on top: it measures the downwards force, then does some math and displays the mass that would be pulled by the Earth with this much force. So when you were pushing on the scale with your hand, the scale displayed the value of the mass whose weight would be equal to the force you were applying. So what would happen if you were to take this scale with you on a trip to other planets and weigh yourself?

Thanks to Newton’s law of universal gravitation, we know that your weight on a planet’s surface depends on three things, your own mass, the planet’s mass, and your distance from the centre of that planet (which depends on the size of the planet). Assuming you don’t eat too much ice-cream during your trips between planets, you could consider your mass to be constant and your weight will only depend on the planet you’re standing on. The greater the planet’s mass and the smaller it is, the stronger the pull of , and the higher you’ll weigh.

Here is an online calculator that will tell you how much you would weigh on other planets of the ​ ​ solar system. Enter your Earth weight in there, and it will display your weight on each planet. On which planet would you weigh the most? Of all the planets, is the force greatest or least there?

If you remember the last activity from Unit 2: Properties of the Stars, you might have noticed that the strength of gravity on a planet’s surface depends on the exact same factors as density: mass and size! Therefore if you have two planets that are roughly the same size (like Uranus

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and Neptune), you’ll have a greater weight on the surface of the planet that’s the most dense because it has more mass (which is Neptune in this case).

Be aware that a greater planet density does not necessarily mean that it will also have a ​ ​ stronger surface gravitational attraction: for example, Earth is much more dense than Saturn, but the latter is so much more massive than our planet that you’ll still weigh more on Saturn than on Earth.

Ideas for extending this section: ● Watch the video comparing planet sizes in the Resources below ​ ​ ● In the first activity above, what would you have to use as a model if you wanted to add the Sun? How big would it have to be?

Resources and references: ● List of Exoplanet Extremes (Wikipedia) ​ ● Planet size comparison (The last shown planet might actually be a brown dwarf) ​

Review and discussion questions: ● If you add up the masses of all the planets in the solar system except Jupiter what does it come to, in Earth masses? Is it as much as Jupiter at 318 Earth masses? What is the ratio? (Divide the total by 318) ● The distance between the Earth and the Moon is equal to 30 times the diameter of Earth. Use table 2 to add up all the diameters of the solar system’s planets. Could you fit them all between the Earth and the Moon ?

E xoplanet types

It’s also helpful to create categories that relate to things we already know about. Here are the names of the main types of planets we know about. These categories may change and there may be more categories added in coming years as we learn more about exoplanets. These are types of planets classified by size, mass, and composition.

Terrestrial

Terrestrial planets are those that are believed to be mostly rocky, and are similar to Earth in mass. Planets that astronomers consider to be “rocky” will be the most dense, and contain mostly metals. Your “home world” is this type of planet. Rocky planets are also called terrestrial, in that they are similar to Earth.

Super- and mini-

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These are thought to be rocky but significantly more massive than Earth - that’s where the “Super” part comes from - but less than the planet Neptune. Some may be a mix of characteristics of Neptune and Uranus and terrestrial planets. These planets’ sizes are generally between 1.4 and 4 times that of Earth, and the most massive ones are often nicknamed mini-Neptunes because they tend to have a similar atmosphere to that of our solar system's outer planet.

Neptune-like and Ice Giants

Similar to the planets Uranus and Neptune, Ice Giants are another type of planet. It may be a little confusing at first, but these types are mostly in a gaseous state, not ice. It is thought that, since gravity is so strong deeper into the interior of the , that some portion of it is compressed down into a liquid state. In addition to and , “Ice giants” contain water, methane, and ammonia, which are all chemicals that have a low melting temperature. Astronomers call these chemicals “”, even though they are mostly a gas.

Gas Giants

Gas Giants are similar to Saturn and Jupiter, but bigger or more massive. These planets are mostly made out of hydrogen and helium gas. They are the largest of the planet types. Like ice giants, these giant planets are thought to have liquid throughout much of their interior.

The most common type of planet is …

Super-Earths and mini-Neptunes! Perhaps the most surprising fact is that even though this is the most common type of detected exoplanets, it's the only type not found in our solar system. This led many scientists to wonder why such planets don’t exist within our system. There are no definitive answers yet about why that is. There are a number of ideas: for example, the conditions may simply not have been right for them to form, or the gravity of Jupiter flung them out of our solar system. We hope to discover why in the coming years.

Ideas for extending this section: ● Look at the “Exoplanets: What we are learning” link below to see how common different types of planets are. The diagram they show works this way: if the bar in the graph is tall, there are more planets. ● Learn in more detail what little we know about the interiors of gas giant planets.

Resources and references: ● Exoplanets: What we are learning (Lumen Learning - Skip the first graph. The second ​ graph show the how common each planet size is) ● Super-Earths and Mini-Neptunes (Wikipedia) ​ ​ ​ ● Ice Giants (Wikipedia) ​ ● Gas Giant Facts for Kids (Kiddle) ​

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● (Wikipedia) ​

Review and discussion questions: ● What is the most common type of planet (including exoplanets)?

M oons

You probably know a lot about our planet’s already, the Moon. It shines because it reflects light from the Sun, its gravity contributes to tides on Earth, and its phases depend on its position relative to Earth and the Sun. If you read our previous unit, you’ll even know that scientists think it formed after a planet the size of Mars collided with Earth. But did you know that the other planets of the solar system also have moons?

In fact, Venus and Mercury are the only planets of our solar system which don’t have moons. Mercury is simply too close to the Sun to have a moon without it getting pulled away by the Sun’s gravity. For Venus however, the reason is more subtle. Astronomers think Venus had a similar collision to the one Earth had when our Moon formed, but it also had a second collision a few million years after the formation of its moon, which reversed the planet’s spin and made it lose any moon that had formed.

Moons in our solar system show great variety. Mars has two moons, and , which don’t look like Earth’s Moon. They’re among the smallest that you’ll find in our solar system, and look more like small lumpy asteroids than our round Moon. They are about the size of medium sized cities in Canada. Phobos is about 22 km across and Deimos about 13 km.

Jupiter has a total of 79 moons that have been identified so far, 53 of which are named at the time of writing. Perhaps its most famous moons are , , and , the four largest . These are called the as they were discovered by in January of 1610. These moons were the first known objects that clearly didn’t orbit the Earth, so their discovery cast doubt on the idea that the Earth was the centre of our solar system, which was the accepted idea at that time.

Ganymede is the largest moon in our solar system. It’s bigger than Mercury and nearly ¾ the diameter of Mars! Europa, the smallest of the Galilean moons, is a little smaller than our Moon.

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In addition to its beautiful rings, which we’ll talk about later in this unit, Saturn has 82 moons, making it the planet with the highest number of known natural satellites in our solar system. interests scientists the most and we’ll see why when we discuss atmospheres and oceans.

Uranus’ 27 moons aren’t as famous as some of the moons we’ve just listed, but interestingly, they’re all named after Shakespearean characters, while other moons are generally named after mythological people. You can read about them in the Resources section. ​ ​

Triton, the largest of Neptune’s 14 moons, was discovered by on October 10th 1846, 17 days after Neptune itself had been discovered. Lassell was an amateur , who used the fortune he made in business to buy quality telescopes. is a very icy moon, with a surface temperature of -240 degrees Celsius, and ice volcanoes spouting freezing nitrogen and methane that were revealed by the spacecraft.

Figure 4: Comparison of the size of some of the solar system’s Moons (courtesy Wikipedia) ​

An is the name we give to the natural satellite of an exoplanet. To this day, there are no confirmed exomoon detections, as observing or inferring the existence of an exomoon is

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trickier than detecting exoplanets. However, given how common moons are in our solar system, especially around giant planets, astronomers think there should be a lot of for us to discover. They even have a small list of candidates which could be exomoons but have yet to be confirmed. Use the link called “Exomoon candidates” in the Resources section to see this ​ ​ list.

Ideas for extending this section: ● There’s a lot more to read about the moons of the solar system. You can use the link in the Resources below to learn more about them if you’re curious. ​ ​ ● Learn how to sketch the Moon (Youtube) ​ ● Watch “Loony Moons”, a talk about the moons of our solar system (RASC) ​

Resources and references: ● The moons of our solar system (NASA - Solar System Exploration) ​ ● Exomoon candidates (Wikipedia) ​ ● The surface area of the terrestrial objects of the solar system (XKCD) ​

Review and discussion questions: ● How many moons are there in total in the solar system? What are some of the ways they differ from ours? (How many ways can you think of?)

R ings

When we talk about planets with rings, Saturn is the first one that comes to the mind of most people (Figure 5). These mysterious rings are made of an enormous number of ice and rock particles, whose sizes range from a few micrometers (millionth of a meter, the size of dust grains) to several tens of meters (some of them can reach the size of a house, or even a small mountain).

We think that Saturn’s rings were formed by , asteroids, or even moons that were torn apart by the planet’s gravity, and their remains kept orbiting the gas giant.

Saturn isn’t the only planet of the solar system to have rings. All the other non-terrestrial planets have rings, although these are much smaller than Saturn and are much harder to see. They’re more easily seen through telescopes. You can see an image of Uranus' rings in Figure 6 below.

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Figure 5: The Cassini image of Saturn and its rings (courtesy NASA/JPL-Caltech)

Figure 6: An infrared view of the (courtesy W.M. Keck Observatory)

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Some smaller objects can also have rings. For example, astronomers detected a small around , one of the solar system’s dwarf planets. In theory, even moons could have rings around them.

Using the transit method to detect exoplanets, which we explain in our Exoplanet Transit ​ Activity, astronomers are able to determine whether an exoplanet has rings, although it’s a bit ​ trickier than simply determining if there is a planet dimming a star, since the way the rings block the light would be harder to understand. Scientists are still figuring out ways to accurately determine whether an exoplanet is ringed.

The exoplanet J1407b is the first that was discovered to have a ring system, and is thought to ​ have rings about 20 times the size of Saturn’s! It’s been nicknamed Super-Saturn because of this. Some gaps in its rings indicate that it’s even possible that exomoons have formed in the gaps by accreting ice, rock and dust that make up the rings (in a similar fashion to how planets form in a solar system - see Unit 6), although that has yet to be confirmed. Saturn has small moons in the gaps of its rings that have formed this way. You can see a small moon of Saturn, , in Figure 7, that has made a gap this way. Daphnis’ gravity disturbs the surrounding material, setting up waves of disturbed material. You can see shadows of the moon and waves on the neighbouring rings.

Figure 7: The small moon Daphnis in the Keeler Gap of the (courtesy Cassini/NASA)

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Ideas for extending this section: ● Learn about the Cassini mission to Saturn and find pictures of Earth and the Moon taken by the Cassini . ● Look at images of the rings of Saturn taken by the Cassini space probe from behind Saturn. How do the rings look different?

Resources and references: ● Cassini (Wikipedia) ​ ● Ring system (Wikipedia) ​ ● Super-Saturn (Space.com) ​ ● Saturn: In depth (NASA) ​ ● Keck observatory photo gallery ● Rings of Saturn (Wikipedia) ​

Review and discussion questions: ● Are the rings of Saturn solid like a plate or are they made up of many small pieces?

V olcanoes

Our planet has a molten interior, meaning the rock is so hot it behaves like a liquid. There are three main reasons the inside of the Earth is so hot. ● First, there was leftover heat from when the planet formed. The collisions between matter as the Earth formed created heat, some of which was trapped inside of the planet as more matter was added to it. Even 4.55 billion years later, some of that heat is expected to be there. ● As the Earth formed some of the denser matter, like iron, sank to the centre, causing friction. ● In addition to that, some of the matter our planet formed out of was radioactive, and gives off heat.

It’s so hot in the middle of the Earth, in fact, that the temperature may be as hot as the surface of our Sun, about 5,500 degrees Celsius. Some of the liquid rock - lava - bursts through the crust of the Earth, usually near the edges of the 17 tectonic plates that the continents and oceans sit on. A volcano is the rock formation that is created from the lava bursting through the crust. It is considered an “active” volcano if it erupted less than 10,000 years ago. If a volcano has not erupted in around that time, and is not expected to again (because, perhaps the lava cooled down and solidified into rock), it is called an “extinct volcano”.

What about other worlds? Mars does not have active volcanoes at the moment, although some of its volcanoes are bigger than the volcanoes on Earth. The biggest one, Olympus Mons, counts as the second tallest mountain in our Solar System and would cover France if it were on

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Earth. Unlike Earth, Mars did not have drifting continents that would move the volcano off a source of heat. Olympus Mons and the other volcanoes on Mars sat over sources of lava from the centre of the planet and grew and grew. There is evidence that some of the lava flows on it date as recently as between 2 and 115 million years ago.

The moon Io that goes around Jupiter has an unusual source of heat. Most of its heat is thought to be due to tidal forces: gravity from Jupiter is stronger on the nearest edge of the moon compared to the furthest edge. That stretches the moon slightly. As Io spins, some parts relax while other parts get stretched. It’s a bit like squeezing a rubber ball. After squeezing it a bunch of times the ball starts to get hot due to all the friction inside. Despite being roughly the size of Earth’s Moon, Io has over 400 volcanoes, making it the most volcanically active moon in our solar system. What's even more surprising is how it looks. Io has a great deal of sulfur. When sulfur is at room temperatures it is a bright yellow, but as you heat it up it goes through dramatic colour changes. This is why images of Io make it look a bit like pizza, with bright yellows and reds showing sulfur at different colours (figure 8).

Figure 8: An enhanced colour view of Io (courtesy NASA)

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With the worlds mentioned previously, the heat melted the rock, turning it into a liquid. In an environment cold enough that water is usually found as a solid - ice - it may not get hot enough to create liquid rock, but it could be hot enough to make liquid water! There are some moons in the outer parts of the Solar System where this happens. Triton, a moon of Neptune, and , a moon of Saturn, have been known to spray water ice particles from cracks in their surface. This process is thought to be very similar to how volcanism works for liquid rock, but with liquid ice instead. It is called cryovolcanism: liquid water sprays out and turns to ice.

In the case of Enceladus, the ice can spray hard enough to get into . The space probe Cassini flew by Enceladus to take photos of it, which you can see in Figure 9.

Figure 9: Cryovolcanoes on Enceladus spewing ice into space (courtesy NASA)

There are some exoplanets that might be very much like Io, in that they are highly active with volcanoes. We would be detected by being -sized, and very hot. Volcanic particles would be thick in its atmosphere. While some exoplanets have some of this evidence, they are yet to be confirmed.

Ideas for extending this section: ● Read more about volcanoes in our solar system at the Volcano Discovery link below.

Resources and references: ● Volcanoes in the Solar System (Volcano Discovery) ​ ● Why is the core of the Earth so hot? (Scientific American) ​ ● Volcano (Wikipedia) ​ ● Olympus Mons (Wikipedia) ​

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Review and discussion questions: ● What are some of the ways volcanoes in our solar system are different from each other?

P lanets, magnets, and aurora

Earth’s magnetic field is related to the same hot core that causes volcanoes. Magma (what lava is called inside a planet) deep in the core is rich in metals, and that moving metal, combined with Earth’s rapid spinning around its axis, creates a magnetic field.

You can imagine that there is a giant bar magnet with a north and south pole that goes through the centre of our planet. The south end of this magnet is inside the northern hemisphere of the Earth. The floating arrow in a compass is also a bar magnet: the arrow end labelled “N” is the magnetic north pole of the magnet in the compass. But since opposites attract in magnetism, that means that it must be attracted to the south end of the Earth’s giant magnet! That means that the magnetic pole of the “bar magnet” inside of the Earth that is close to the geographic north pole of the Earth is actually the south magnetic pole. The Earth’s magnetic field goes out beyond our atmosphere, forming what we call the .

Figure 10: Earth’s magnetic field is similar to that of a bar magnet (credit: FDAO) ​ ​

What about other planets? Mercury has a weak magnetic field, about 1/100 the strength of Earth’s. It’s thought that its core has cooled and become less active.

Venus has approximately 1,600 volcanoes, but all of them appear to be extinct, so it appears that its core is not as active as the Earth’s. On top of that, Venus’ rotation around itself is

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extremely slow (one Venusian day corresponds to 243 earth days), because of that, it has no magnetic field.

Mars also has volcanoes, as we saw, but it doesn’t have circulation of hot magma inside it anymore. It does have areas of its surface crust that are still strongly magnetic, however, and some areas are more powerful than Earth’s magnetic field. You can think of these areas like bubbles of magnetism sitting over parts of the surface. Other areas of the planet have none.

Jupiter, Saturn, Uranus, and Neptune all have strong magnetic fields. Jupiter’s is the strongest of any planet in our solar system. Their magnetic fields also depend on metallic matter moving inside them, but it isn’t hot metallic lava like it is on Earth. You can read more about them in the link “The magnetic fields of our solar system” in the Resources section. ​ ​

What about exoplanets? Astronomers have measured the magnetic fields of some exoplanets using the Canada-France-Hawaii Telescope and found them to be much stronger than Jupiter’s, about 40 to 240 times the strength of Earth’s magnetic field. There is also a planet that has a strong magnetic field that is so close to its star that it creates sunspots on the surface of the star (that was also discovered with the Canada-France-Hawaii Telescope). Note that only very strong magnetic fields have been detectectable so far. Measuring the magnetic field strength of an exoplanet similar to the planets in our Solar System requires better instruments.

Table 3: Planetary magnetic field strength compared to Earth

Planet Magnetic field strength relative to Earth

Mercury 1/100 x

Earth 1 x

Jupiter 8.6 x

Saturn 0.42 x

Uranus 0.46 x

Neptune 0.28 x

Range of magnetic fields of four exoplanets 40 to 240 x

In science fiction they like to talk about “Force Fields”: a magnetic field does act something like that to stop electrically charged particles from the Sun (called solar ) from eroding a planet’s atmosphere. On Mars, the magnetic “bubbles” over some parts of the planet help protect Mars’ atmosphere.

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On Earth, electrically charged bits of atoms from the Sun can only crash into our atmosphere or get to the surface of the Earth where the magnetic field lines of Earth’s magnetic field come out of or go into the planet. When these fast moving particles smash into the air near the magnetic poles little flashes of light are given off, with colours that depend on the composition of air - different colours for and nitrogen, for example. Usually the northern , as the aurora is called in the northern hemisphere, can only be seen in the far north.

Figure 11: Northern Lights seen in the night sky of (courtesy NASA)

But if the wind of charge particles from the Sun is strong enough, it will push the Earth’s magnetic field down further south, kind of like pushing a paper crown further down around your head. When that happens people can see the northern lights in southern Canada, or even further south, assuming there is no light pollution.

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Figure 12: A map showing where the northern lights are over the Earth at one moment (courtesy Spaceweather.com)

Since other planets have their own magnetic field, they too can have appearing in their atmosphere! However these aurorae on other planets aren’t always visible to the human eye; in figure 13, you will see them on Jupiter’s North Pole, taken using Hubble’s instruments.

Some astronomers have found ways to find exoplanets by looking for the particular type of radio signals given off by their aurora. In 2020 astronomers announced that they had detected a planet going around a star by finding radio emissions from the aurora on the planet.

Figure 13: An ultraviolet aurora in Jupiter, the full-color image of the planet was added separately to show where the aurora is (courtesy HST/NASA)

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Seeing the Northern Lights (the “Aurora Borealis”) is a thrilling experience. There are also Southern Lights (“the Aurora Australis”) that are in the Earth’s Southern hemisphere. Their motion, colours, and their mysterious origin led to many interpretations of what they could be prior to a scientific understanding of their origins. See the link in the Resources section for ​ ​ more stories from northerners about the aurora.

There are still new things to be discovered about aurora. For example, in 2016 a new type of aurora-related phenomenon was discovered by skywatchers from Alberta. It looks like a purple and green streak across the sky. It was jokingly named STEVE in reference to the animated movie “Over the Hedge” where characters try to pick an ordinary name for something unknown to make it less scary.

A ctivity Four - Discover Earth’s magnetic field R equired tools and materials: ● A compass (don’t use a phone’s compass app in order to avoid damaging your device) ● Two magnets (these need to be bar magnets, with North and South markings preferably) ● A piece of string, or a bowl of water with a small floating surface

Have you ever wondered how a compass indicates where the North is? This is what you’ll discover in this activity.

First of all, take your two magnets and slowly bring them close together. You’ll notice that if you approach the North end of one magnet to the South end of the other, both magnets will attract and stick together. However, if you try to move two poles of the same type together (South and South or North near North), they will push each other away. But you probably know this already if you’ve played with a magnet before, how does it relate to how a compass works?

A compass’ needle is actually a small magnetized stick that’s allowed to rotate freely, and aligns itself with the strongest magnetic field nearby, in most cases that’s Earth’s magnetic field. You can see for yourself if you move one of your magnets close to a compass. Your compass starts pointing either straight towards or away from your magnet, depending on which end of your magnet you used (North or South), since that’s the strongest nearby field.

Let’s use one of your bar magnets to make a compass.

Tie a piece of string around the centre of your magnet, and let it hang from your hand. After spinning a bit, your magnetic bar’s North end will point roughly in the direction of Earth’s North pole. You can check whether the direction it is pointing is correct by the compass. Make sure it’s far enough to not be affected by your magnet.

Alternatively, you can fill a bowl with water and put your bar magnet on top of a floating platform such as a piece of styrofoam, or a plastic lid. It will also point North, as you can see in the figure below.

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Figure 14: Notice how the bar magnet and both compasses point towards the same general direction

Ideas for extending this section: ● Read about some legends concerning Northern Lights (Aurora Borealis) in the CBC article linked below.

Resources and references: ● Legends of the northern lights (CBC) ​ ● Exploring magnetism - classroom activities (NASA/Berkeley) ​ ● Exploring magnetic field lines PDF (NASA Kids) ​ ● NASA’s aurora image gallery ● The magnetic fields of our solar system ● Earth’s magnetic field (Wikipedia) ​ ● Magnetic fields of hot measured for the first time (Space.com) ​ ● Hunting aurorae: astronomers find an exoplanet using a new approach (Astronomy.com) ​ ● STEVE (Wikipedia) ​

Review and discussion questions: ● What geographic hemisphere (North or South) is the North end of Earth’s magnetic field? (North as in corresponding to north on a magnet).

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A tmospheres

In Unit 6: The Birth of Stars and Planets, we said that there was a stage in planet formation where the was heavy enough to use its gravity to accrete the surrounding gas onto it. This is how the giant planets have such large and dense atmospheres. The smaller terrestrial planets did not attract as much gas, and most of what they did got blasted away by impacts. However, the hot interiors of these planets caused gasses to form and emerge from the molten interior.

Not all terrestrial planets have the same amount or same types of gas in their atmospheres. Mercury, in fact, was not massive enough to hold onto its atmosphere. Earth’s atmosphere contains less carbon dioxide and more oxygen than it used to in its early history. The change happened when Earth’s earliest forms of life started using the process known as photosynthesis to convert carbon dioxide, water and light from the Sun into sugar and oxygen. The sugar helped the life grow, and the oxygen went into the atmosphere. This process still occurs today in plant life.

Scientists think that the moons around the giant planets form similar to how the terrestrial planets formed around the Sun, so could have some atmosphere, but they are all smaller and less massive than Mars, and would have less gravity to hang onto their atmosphere. Most moons have almost no atmosphere.

Saturn’s moon Titan, however, is an oddity: It has a thicker atmosphere than Earth! Titan is one of the larger moons, as can be seen in Figure 4. The fact that it is farther, and thus much colder, is likely one of the reasons it hung on to its atmosphere. There are many other factors that can contribute (such as chemical composition of Titan’s atmosphere, and magnetic effects related to Saturn), but scientists are still unsure as to which factors are the most important, and which ones don’t affect it much at all.

The gas giant planets don’t really have a surface, but at some point deep inside the pressure turns the gases into a liquid. In Jupiter this happens at pressures about two million times greater than our air pressure at the surface of the Earth.

Table 4: Surface pressures on different Worlds

Planet or moon Multiples of surface pressure on Earth

Venus 93 x (like being under 900 of water)

Earth 1

Mars 1/160

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Titan 1.45 x

Exoplanets are really far away from our telescopes, and as a result, it is very hard to get much information from them. Consider how thin an atmosphere can be compared to the planet. The Earth is 12,700 km in size. Most of the Earth’s atmosphere - ¾ - is in a layer called the Troposphere that is just 10 km thick, and 99 % of it is below the top of the stratosphere, just 50 km from the surface. Ten km is 1/1,270 of the diameter of the Earth, which is less than the thickness of the paint on a compared to the globe’s width. When we look up at a blue sky it seems to go on forever, but it is actually very thin. The total amount is quite small, which is why humans can affect it.

Figure 15: Earth’s thin atmosphere seen from space (courtesy NASA)

So, if seeing an Earth-sized exoplanet is already challenging, imagine how hard it is to detect an atmosphere like ours. For this reason, we know very little about exoplanet atmospheres. However, large exoplanets, gas giants and ice giants, are mostly atmosphere, so some information is known about them. For example, molecules like water, carbon monoxide, carbon dioxide, and methane, have been detected on ice giants. Finding these kinds of molecules is not unsurprising if you recall what we know about ice giants and gas giants, but the ability to find them is an important first step in finding these or other molecules on planets that are more

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Earth-like. A Neptune-sized exoplanet called HAT-P-11b is the smallest planet for which its atmosphere has been detected. The next generation of telescopes are planned to be able to detect the atmospheres of smaller exoplanets. That is a key project of the Thirty Metre Telescope, which Canada is a partner in.

Ideas for extending this section: ● To get an idea of how thin the Troposphere is (which contains ¾ of our atmosphere), mark out 10 km on google maps in the area where you live: it’s not a large distance. The top of the Stratosphere is at 50 km and the very top of the atmosphere is considered to be at around 100 km. Calculate in minutes long would it take to drive 10 km, 50 km, and 100 km going at 100 km per hour. ● Learn about the atmosphere of Earth, the planet of which we know the most about. ● Use your imagination and draw a creature that could live on Venus or Mars. How would it have to be different from us to survive? Or imagine what your space suit would have to be like so you could survive.

Resources and references: ● Diagram of the Earth’s atmospheric layers (University Corporation for Atmospheric ​ Research)

Review and discussion questions: ● Mars is a terrestrial planet, so why does it not have the same amount of atmosphere as Earth? ● What would it be like to try to breathe on Mars? ● Earth is a terrestrial planet, so why does it have so much more oxygen than the others, which have mostly carbon dioxide?

O ceans and lakes

It is common for worlds to be partly liquid, whether it be molten rock, or liquid ices (water, methane, etc.). As has been noted in the descriptions of the different types of planets, the interiors are often liquid in one form or another. Smaller bodies, like asteroids, won’t have interiors compressed and hot enough to melt ices or make lava, so they are usually solid throughout.

There is some possibility of water under the surface of other worlds in our solar system, where it is warmer and not likely to evaporate. Since water is important to life on Earth, that gives us hope that there may be life on other worlds. There is some evidence that Mars and Pluto might have underground liquid water.

Some of the moons of our solar system also appear to have water oceans and lakes under their surface. Jupiter’s moons Europa and Ganymede appear to have salty subsurface water. The

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same may be true for the other two big moons of Jupiter, Callisto and Enceladus, Saturn’s moon Titan, and Neptune’s moon Triton. There are no other worlds in our solar system with lakes or oceans of water on their surface like the Earth.

Some people like to call our planet the “Water World” because about 70% of the Earth’s surface is covered in water, and we’re the only planet that currently has liquid water on the surface in our solar system (Mars used to). The actual volume of water is small. Figure 16 shows what the Earth looks like normally, then without water. If you look closely at the image without water you’ll see two drops of water. The bigger one shows all oceans if you were to gather them together in a ball. The small one to the right of it shows all the freshwater -rivers, lakes, and glaciers - if you were to gather it together in a ball: it’s much smaller. Freshwater is what we depend on for drinking water and there isn’t very much of it, as you can see.

Figure 16: The water on planet Earth (courtesy USGS/WHOI, Howard Perlman and Jack Cook)

Titan, in addition to potentially having a watery interior, has lakes of liquid ethane and methane on its surface. The temperature and atmosphere are such that there are entire weather patterns of lakes, rivers, clouds, and rain. It is very similar to Earth in that way, except for replacing life-giving water with methane.

There haven’t been any exoplanets identified as having surface oceans or lakes. As we saw in the section on atmospheres, HAT-P-11b has water vapour in its atmosphere, which could condense to liquid form as rain. There are several candidate exoplanets of the right mass and temperature to have liquid water and we expect astronomers will discover more in the next few years.

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Ideas for extending this section: ● Learn about the water cycle (the oceans, clouds, lakes, etc.) of Earth. While doing so, consider what Titan’s version of it might be like. If you’re really eager, you can research what is known about Titan’s methane cycle at the link below, but just keep in mind that it gets technical.

Resources and references: ● What percent of Earth is water? (US Geological Survey) ​ ● Water Cycle on Earth (US Geological Survey) ​ ● Methane Cycle on Titan ( Magazine) ​ ● Ocean Worlds (NASA) ​ ● How to find exoplanet oceans (American Astronomical Society) ​

Review and discussion questions: ● Knowing what you do now about how little of the Earth is water, would you still call it a water world? Why or why not? (There isn’t a single answer). Think about the area it covers, how much there is, and its importance in life.

C hanges in environment

The wildly different environments of the worlds we mentioned don’t stay like that forever, although on the scale of human lifetimes it can appear that way sometimes. Throughout ExoExplorations, we’ve talked about beginnings and ends, as well as what happens in between. The many worlds and their properties are no different.

Still, there are things that can change in our lifetime.

The average surface temperature of our Earth is 15 degrees Celsius, as can be seen in table 5. Without something called the Greenhouse Effect it would average -18 degrees Celsius. The Greenhouse Effect is named after what goes on with a greenhouse in sunshine: the light from the Sun comes in and is absorbed by the plants and other things, then is reradiated as infrared light. Most of that light can’t get out through the glass and is reabsorbed by the contents of the greenhouse. The air temperature goes up. You’re probably more familiar with what happens in the summer if you sit in a car on a sunny day with the windows up. It quickly gets dangerously warm.

The Earth’s surface and environment work in a similar way. Our atmosphere has gases in it that also block infrared light well, just like the glass in a greenhouse or car window. Those gases include water vapour, carbon dioxide, and methane. The concentration of carbon dioxide is changing, in large part due to people. In 1970, fifty years ago, the concentration was 326 parts

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per million; that is for every million molecules in the atmosphere, 326 of them are carbon dioxide. In 2020 it reached 417 parts per million, a 28% increase.

Table 5: Surface temperatures on planets and moons

Planet or moon Average surface temperatures

Venus 465 degrees Celsius

Earth 15 degrees Celsius

Mars -63 degrees Celsius

Titan -179 degrees Celsius

Now consider Venus. It is often referred to as Earth’s twin. It is similar in size and the kind of rock that it is made out of. The obvious difference is its temperature and its crushing atmosphere (remember Table 4). Because of this, Venus is what is known as a “pressure cooker planet”. The most common molecule in the thick Venusian atmosphere is carbon dioxide, giving it an extreme greenhouse effect. It is not 465 degrees Celsius just because it is closer to the Sun than the Earth: it is mostly because of the Greenhouse Effect. It’s thought that Venus might have had oceans like the Earth but changed dramatically, in part because stronger ultraviolet light from being closer to the Sun - the same kind of light that gives you a sunburn - broke the water molecules down, and the Greenhouse Effect took over. Venus also has many volcanoes, as you will recall, and those volcanoes likely accelerated Venus’ Greenhouse Effect.

Earth’s environment has also changed over time. We’ve already mentioned that it used to have much more Carbon Dioxide in the era before plants, giving it a much higher temperature. There are some natural changes in our orbit and tilt of the Earth’s axis that lead to ice ages, for example, but these changes in our orbit and tilt take tens of thousands of years. The cycle for the change in the tilt of the Earth’s axis takes 26,000 years, for example, and the change in our orbit takes about 100,000 years. These effects are far too slow by a factor of a thousand to account for the recent and unprecedented changes we are seeing to Earth’s climate.

There is good evidence that Mars used to be very different. There are canyons on Mars much bigger than the Grand Canyon in the USA that were cut by water, and we know there is still frozen water under the surface. Mars has a very thin atmosphere compared to the Earth and even if it were warm enough for liquid water to be on the surface the low pressures would cause water to quickly boil off.

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The big question for scientists now is how Mars lost so much of its atmosphere. Recent results show that it was likely strong ultraviolet light from the Sun and the , the same stream of particles from the Sun that gives us the Northern Lights, that eroded Mars’ atmosphere over time, just as it broke apart the water vapour in Venus’ atmosphere. But why did that happen to Mars and not the Earth, which is closer to the Sun? Earth’s magnetic field helped keep the solar wind from stripping away our atmosphere, and the ozone layer, which comes from oxygen (most of which is produced by plant life) helps protect our atmosphere. Earth also has active volcanoes that continue to put gases into our atmosphere, but Mars does not.

Ideas for extending this section: ● Visit the NASA website and learn more about how our climate is being affected.

Resources and references: ● The Greenhouse effect (Wikipedia) ​ ● Climate Change (NASA) ​ ● What’s the hottest Earth’s ever been? (Climate.gov) ​ ● Water on Mars (Space.com) ​ ● NASA’s Maven reveals most of Mars’ atmosphere lost to space (NASA) ​ ● Carbon dioxide levels in the 20th and 21st centuries (NASA) ​ ● Greenhouse effects on other planets (European Space Agency) ​ ● The Greenhouse Effect on Earth (Australian Government) ​ ● NASA Climate Modeling suggests Venus might have been habitable (NASA) ​

Review and discussion questions: ● Oven temperatures are often given in Fahrenheit rather than Celsius. A decent baking temperature is 350 degrees Fahrenheit which equals 180 degrees Celsius. How does Venus compare to this? Could there be any liquid water on Venus now? ● Do you get warmest right away when you put a blanket on or does it take a while? What does this imply about the temperature of the Earth and current greenhouse gas concentrations? ● What is the average temperature difference between the Earth with the Greenhouse Effect and without it? (Subtract the lower figure from the higher figure). Is it important to have *some* Greenhouse effect?

S pacecraft and their importance in planetary studies

In Unit 3 - Seeing Stars, we talked about how telescopes help us see celestial objects more clearly by having “bigger eyes” to collect more of the light emitted or reflected by an . In addition to the object’s brightness, you should remember that an

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important factor in observing a star or a planet is how far away we are from it. If we can get closer to something, we can see it a lot better; it will appear much brighter and show more detail, often in much better detail than you can see from the Earth. This is why the best images of our solar system’s planets are taken by sending spacecraft near planets.

Using a - a space probe - is much cheaper and safer than sending people. We also don’t need to worry about getting a probe back to Earth in most cases, as we do with a crewed mission. They can visit places long before our is ready to send a person there. The first space probe visited another planet in 1962, Venus, almost 60 years ago, just a year after the first person orbited the Earth in a spaceship, and we still haven’t sent a person to anyplace other than our Moon. Now that artificial intelligence is getting better these spacecraft can also do more and make some decisions without having to get directions from someone on Earth

Being able to analyze a sample in a laboratory, even a portable laboratory on a space probe, tells us more than we can learn from telescopes alone. Space probes can measure things like magnetic fields and what soil is made of directly, things that would be difficult or impossible to do from Earth, and certainly not as well. There is only so much you can do from a distance.

Some space probes perform “flybys”: the probe zips by a planet in a few hours or days and quickly gathers data while up close. For example, between 1979 and 1989, NASA’s Voyager 2 space probe performed flybys near all the outer planets of the solar system as well as some of their moons. The Voyager 2 probe used the gravitational pull and momentum of each planet to fling it to the next world.

Other probes will orbit their target planet to study it for an extended period of time, like the Juno spacecraft, launched towards Jupiter in 2011, which is currently there gathering data. Some probes land on planets or moons, like the Mars Opportunity Rover that officially retired in 2019. The first space probe to make a soft landing did so on Venus in 1970, which measured the surface temperature and composition before being destroyed by Venus’ atmosphere after only 22 minutes. You can find a list in the Resources section with details on all the Solar System ​ ​ Probes so far.

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Figure 17: An artist’s conception of The Voyager 2 spacecraft (courtesy NASA)

Will we always be limited in what we can learn about exoplanets compared to what we can do with space probes in our solar system? Probably, unless we figure out how to get to exoplanets quickly. There is a proposal to send very small probes to visit the exoplanets around Proxima and Centauri. Called the Breakthrough Starshot project, the idea is to send about 1,000 space probes, each the size of a dime, on a trip that would take twenty to thirty years. Each will just do a flyby and send information back by radio signals, which will take 4.3 years to reach us. Each space probe would be attached to a sail one kilometer square, pushed by a laser from Earth. That’s a lot of effort, and a long time to wait, but it’s the best we can hope to do now, and that’s just for the closest exoplanets. Faster-than-light vehicles like you see in science fiction like Star Trek or Star Wars aren’t possible, as far as we know. Check out NASA’s Exoplanet Travel Bureau to at least take a trip using your imagination.

So instead of sending spacecraft towards other solar systems, we’re back to using telescopes to detect and observe them. We certainly don’t get images as good as with a spacecraft near a planet, but we can still gather lots of data on exoplanets this way.

That being said, most of the telescopes that survey exoplanets are in space: Kepler, TESS, Hubble and Spitzer are all space telescopes that have recorded or are recording data on exoplanets. That’s because space telescopes don’t have to worry about several challenges imposed by Earth’s atmosphere, which we talk about in detail in Unit 3: Seeing Stars. These challenges include light pollution, the twinkling of stars that ground-level observers have to deal with, but also more subtle things such as the atmosphere filtering out ultraviolet light that could contain useful information about exoplanets or their host stars.

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Ideas for extending this section: ● Check out the List of Solar System Probes, especially the different types that are defined: Flyby, Orbiter, Lander, Rover, Penetrator, Sample Return. Discuss the merits and challenges of each one, and what situations you would choose one type over another.

Resources and references: ● Space Probes (Star Child, NASA) ​ ● List of Solar System Probes (Wikipedia) ​ ● Exoplanet Travel Bureau (NASA) ​ ● Exoplanet projects and instruments (NASA) ​ ● The Breakthrough Starshot (Wikipedia) ​ ● landing on Titan animation (Youtube, NASA JPL) ​

Review and discussion questions: ● Why can’t we send spacecraft to every exoplanet we know of to gather data? ● If you could send a space probe to somewhere in our Solar System, where would you send it, and why?

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