Astronomy of Planets

Overview

Where are we in the universe? When are we? How is it that human beings confined to the Earth are able to know anything about a Universe that we have no direct access to? We have only recently begun sending out probes around our Solar System, but realistically it looks like that may be the extent of what we can explore directly. How is it, then, that science can teach us anything about any of the rest of it?

If you’ve ever been curious about these things, then you’ve come to the right place. The astronomy of planets is our first step towards discovering the Universe. In fact, without the other planets in our Solar System there can be little doubt that humans would never have understood gravity, and would still believe the Earth lies fixed at the centre of the Universe.

This course is about the discovery of our Solar System and the subsequent exploration of it. We will look in detail at how the process of doing science developed as humans sought an explanation for the apparent motions of the planets with respect to background stars. We will then look in more detail at how modern astronomers are able to use observations from the confines of the Earth to learn all about objects we can only view through a telescope. We’ll see that much of our confidence in what we’ve learned comes from experimental science we have conducted on Earth, as we are able to apply the knowledge we obtain here to our astronomical observations. From there, we will embark on a systematic study of all that we now know about the objects in our Solar System—our best source of information that we can use to decode the workings of everything else we see.

This module will provide you with an introduction to our observational arena—the night sky—and an understanding of what it means to make our observations from here and now. You will be introduced to many of the conventions astronomers use when describing the night sky, and you’ll develop an understanding of how the daily and annual motions we observe as we look up translate to the motions of the Earth. In short, you’ll learn how to view the sky as an astronomer does.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Develop a sense of here and now in relation to astronomical observables
2. Identify why humans study astronomy, incorporating the scientific method in principle and in practice
3. Explore the celestial sphere, the naming of stars and constellations, and the magnitude scale
4. Explain the daily and annual cycles, and the cause of seasons on Earth

Key Terms and Concepts

• scientific method
• celestial sphere
• constellations
• magnitude scale
• equinox (vernal, autumnal)
• ecliptic
• solstice (summer, winter)

Overview

Sir Isaac Newton famously said, “If I have seen farther it is by standing on the shoulders of giants.” In this module, we begin our exploration of the main developments that led to our description of the Sun, Moon and planets as a Solar System. There are two main reasons why we study these historical developments. The first is that they provide the most direct path to understanding why we should believe that the Earth and all the planets orbit the Sun: through the rejection of alternative hypotheses in light of empirical data. That, in a nutshell, is the scientific method. The second reason why it is important to study the early history of Astronomy is that, as the quote from Newton indicates, if we want to see what he saw—and if we want to see what Einstein saw after him—we need to get up onto the shoulders of the giants who came before.

In module 3 we will explore Newton’s universal law of gravitation as well as Einstein’s theory of gravity—general relativity. Module 3, which covers the shift from an Earth-centred, or geocentric view of the cosmos, to a Sun-centred, or heliocentric one, provides a key element in the development of human understanding that led to Newton’s discovery. But in order to appreciate what was really achieved by Copernicus, Kepler and Galileo during the Scientific Revolution, and thus fully grasp the power of the scientific method, it is important to first see how nascent science—and not simply ignorance—did favour a geocentric theory, and led to a sophisticated description which, for more than a millennium, was thought to be correct.

In modules 2-3, we will come to our modern theory of Solar System mechanics by assuming nothing about the cosmos except what we can observe in the sky, the hypotheses we can make on the basis of those observations, and the consequences that can be deduced as a result. This is the process of scientific discovery, an overarching theme in this course which will be frequently highlighted throughout, while we consider the evidence that was gathered over time and the ways that people fought to interpret it. In the current module, we will explore the details of the scientific process through the earliest developments in astronomy—examining what makes for “good science,” as we learn how exceptionally good science initially pointed to geocentrism rather than heliocentrism. In the next module, we will see how continuation of the scientific pursuit of understanding eventually showed that this was wrong.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Relate how astronomy developed as a science up to the Ptolemaic model.
2. Discriminate between “observable phenomena” and “things that happen or exist,” and explain why multiple hypotheses, such as geocentrism and heliocentrism, can potentially explain a particular phenomenon.
3. Draw on empirical evidence in order to assess the validity of hypotheses.
4. Demonstrate the phenomenon of parallax, explain its importance in astronomy, and use it to illustrate the vastness of space.
5. Use empirical evidence to show that the Earth is spherical, and describe Eratosthenes’ method of measuring its radius/circumference.
6. Assess the difficulty in obtaining precise astronomical measurements, and examine subsequent steps that may be taken in formulating scientific models.
7. Describe the different elements of the Ptolemaic model and relate the particular function of each.
8. Critically examine the scientific method, evaluating its strengths and limitations.

Key Terms and Concepts

• Phenomenon
• Scientific Method
• Hypothesis
• Geocentrism
• Heliocentrism
• Parallax
• Model

Overview

In the previous module, we explored how the scientific method developed as astronomers grappled with the problem of describing the motion of the planets—the “wandering stars”—through the celestial sphere. We learned that science begins and ends with empirical measurements; that scientists make assumptions about the nature of physical reality by inferring from the evidence, and those assumptions subsequently form the bases of mathematical descriptions that they take into the world and compare against more evidence. We learned that the assumptions scientists make, and the elaborate descriptions resulting from them, are supposed to explain the phenomena. And we learned that the first successful explanation of all the phenomena associated with the problem of planetary motion—from retrograde motion to the lack of observed stellar parallax—was finally given by Ptolemy in the second century.

In this module we will see how science eventually corrected its own mistaken result. We will see how a few people with an idea that they thought would better explain planetary motion sought a description based on that idea and worked to ensure that it agreed precisely with the available evidence. We will see how Ptolemy’s geocentric theory was replaced by a heliocentric theory which was more consistent with the discoveries Galileo made with his telescope, which fit precision measurements of planetary motion more accurately, and which revolutionised our understanding of Earth’s place among the stars. We will then see how Newton brought together the theories and insights of his predecessors and developed a theory of universal gravity that explained not only the apparent motions of the planets, but the tides as well. Finally, we will see how further scientific progress eventually led Einstein to yet another revolutionary proposal, which led to as great a revision of our scientific worldview as the heliocentric proposal achieved during the Scientific Revolution.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Explore the strengths and weaknesses of the Ptolemaic model, the Copernican model, and the Tychonic model prior to Kepler and Galileo.
2. Investigate Kepler’s theoretical accomplishments and Galileo’s empirical accomplishments, and interpret within the context of scientific explanation.
3. Examine Galileo’s investigation of motion and inertia, and its influence on Newton.
4. Explore Newton’s explanation of both orbital motion and tides as gravitational phenomena.
5. Compare Newton’s universal law of gravitation and Einstein’s general theory of relativity, as two explanations of the same phenomenon, and describe the tests that show general relativity describes gravitation more accurately.
6. Assess the historical revolutions that have occurred in astronomy, and identify common themes in the works of revolutionary scientists.

Key Terms and Concepts

• Inertia
• Kepler’s laws of planetary motion
• Newton’s laws of motion
• Newton’s law of universal gravitation
• Equivalence principle

Overview

In the Modules 1-3, you were introduced to the big picture of astronomical science. Module 1 began your orientation into the world of astronomy. You explored both what it means from a cosmological perspective for astronomers to be situated ‘here’ and ‘now’, and also the implications of our particular vantage point for the way astronomers must view and describe the night sky. In Modules 2 and 3, you were then taken back to the beginnings of astronomy where you explored the emergence of the scientific method as people sought to explain the motions of the planets—the wandering stars that move with respect to the rotating background celestial sphere. You learned that those motions were ultimately explained as a consequence of regular motions within a gravitational field in which planets move freely along elliptical paths through curved space. The apparent wandering motion of the planets was therefore explained as a consequence of viewing all of those regular motions from the perspective of a planet (namely, Earth) that follows one such elliptical path while simultaneously spinning on an axis once every twenty-four hours.

This picture we have of the Solar System and its relation to the apparent motions of other planets is the result of discovering what science is, and then of using science to explore and continually refine our understanding of the world we live in. As you learned in Module 3, the invention of the telescope in the early seventeenth century played a significant part in the transformation from a geocentric to a heliocentric view of the Solar System. However, little else was said about telescopes—e.g. how they work, and what purpose they serve for astronomers. Our concentration was on the big picture and how our understanding transformed as astronomers followed the scientific method. We therefore glossed over the means by which astronomers actually gather their evidence, since the evidence itself was all we needed for the purpose of that discussion.

In this module, our aim is to begin exploring both the nature of the evidence that astronomers collect (i.e. the nature of light) and the means by which we collect and record that evidence (i.e. with telescopes and cameras). We will begin by exploring the nature of light as it is understood by modern physics. Then we will move on to examine how telescopes work, investigating both how they are useful in astronomy and what their limitations are. Finally, we’ll explain how the light from astronomical objects that is gathered by telescopes eventually gets recorded so that it can be used in scientific analyses.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Explore the nature of light as electromagnetic radiation.
2. Examine how telescopes work and prepare a telescope for observation.
3. Investigate the powers and limitations of telescopes.
4. Explore instrumentation used to record and analyse light gathered by telescopes.
5. Compare ground-based and space telescopes in terms of their strengths and limitations.

Key Terms and Concepts

• electromagnetic radiation
• photon
• photoelectric effect
• electromagnetic spectrum
• light-gathering power
• refraction
• focus
• reflection
• resolving power
• magnifying power
• charge-coupled devices (CCDs)
• spectroscopy
• interferometry

Overview

Whenever we look at an astronomical object—in fact, whenever we look at anything—we are detecting light that interacted with matter at a distant location and travelled to us through space over a period of time. In Module 4, you were introduced to the concept of light as electromagnetic radiation, and you learned that visible light makes up only a small part of the whole electromagnetic spectrum. Aside from the data gathered by probes we have successfully landed on, or flown through the atmospheres of distant Solar System objects (viz. the Sun, Venus, Moon, Mars, Jupiter, Titan, a few comets and asteroids), our main source of information about astronomical objects comes in the form of light that has previously interacted with them.

In this module, you’re going to find out what matter actually is—at least, according to modern physics—and you’re going to see how it was by sorting out the peculiar effects matter has on light that physicists eventually discovered what it is. The module begins with a brief sketch of the components of atoms and the ways that they are differentiated. After that, we dive into the different features of spectra and the problem of figuring out their explanation. It’s an amazing story that led to a very surprising result: quantum physics. Then, having developed a basic understanding of why any signal of light comes to have the features it does, and what it can tell us about the last bit of matter it interacted with, we’ll discuss some of the different ways this theory is applied that help us sort out the properties of distant objects we can see. In particular, we’ll end by exploring various properties of the Sun that have been deduced, particularly the source of all the light it emits.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Describe the basic properties of atoms and molecules.
2. Explain how radiation is produced by all objects with temperatures above absolute zero.
3. Compare different types of spectra and explain how they are produced.
4. Explore what can be learned from spectra of celestial objects (temperature, chemical composition, radial velocity).
5. Apply theory of atoms and spectra to the Sun and analyse its physical properties such as its temperature and luminosity, and chemical composition, and explain the source of solar energy.

Key Terms and Concepts

• spectrum (absorption/emission/continuous)
• orbital
• element
• Kirchhoff’s three laws
• blackbody
• Planck’s law
• luminosity
• Stefan-Boltzmann law
• Wien’s law
• quantum mechanics
• solar constant
• nuclear fusion
• solar neutrino problem

Overview

Four hundred years ago, our Moon and Mercury would have seemed an odd pair to be studied together. The Moon is our constant companion, orbiting from the daytime to the night time sky once every month, its largest surface details visible to humans throughout history. Mercury, on the other hand, is the most difficult of the naked eye planets to observe. It remains always close to the Sun, alternating from evening star to morning star, poking above the horizon to a maximum altitude of anywhere from 28 to just 17 degrees before heading swiftly back towards the Sun. Before the telescope, this is all we knew.

However, since the telescope was invented notable similarities have slowly stacked up. In 1631, Mercury was seen to transit in front of the Sun, something like the Moon does during a solar eclipse—only blocking a lot less of its light. In 1639, Mercury was found to run through a similar series of phases as the Moon. As with Galileo’s observations in the case of Venus, this showed that Mercury orbits the Sun, adding even more weight to the heliocentric hypothesis.

While the Moon’s cratered, rocky surface was also revealed through the telescope’s invention, Mercury’s surface remained hidden for a long time due to its proximity to the Sun and the associated difficulty of observing it. In the meantime, however, other similarities were discovered. For instance, astronomers found that Mercury, much like the Moon, does not spin freely due to tidal forces that have locked its rotation into a resonant pattern with its orbit. Then in 1974, the first close-up pictures of Mercury showed an airless, rocky, cratered planet that indeed does closely resemble the Moon in overall appearance. Perhaps one of the most interesting coincidences is that both the Moon and Mercury contain enormous amounts of water ice that remains trapped in craters at their poles that never see sunlight.

Over the years, both the Moon and Mercury have revealed some big surprises, and these have shaped and reshaped our understanding of each of them. For example: discovering the Moon’s mare basalts, and discovering Mercury’s large iron core and high density. Later data have revealed Mercury’s magnetic field, volatile-rich surface composition, and evidence of volcanic plains extending much more recently than once thought. In this module, you will explore what we have learned about the Moon and Mercury through the observations that we make, beginning with an explanation of the phases and eclipses we observe, and then moving on to the discoveries that have come from sending spacecraft to study them.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Explain the Moon’s phases and the cause of solar and lunar eclipses.
2. Compare the Moon and Mercury in terms of orbits, observational aspects, atmosphere, temperature, impact craters, and evidence of water ice in their polar regions.
3. Summarise the results of past and ongoing missions that have explored the Moon and Mercury.

Key Terms and Concepts

• cycle of lunar phases (new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, new moon)
• sidereal period/synodic period
• solar eclipses (total, partial, annular)
• lunar eclipses (total, partial, penumbral)
• line of nodes
• eclipse season
• umbra/penumbra
• transit
• synchronous rotation
• maria (singular, mare)
• Apollo missions
• late heavy bombardment
• large-impact hypothesis
• Mariner 10 mission
• gravity assist
• MESSENGER mission
• BepiColombo mission (ESA–JAXA)

Overview

Compared to the Moon and Mercury, you will find Venus, Earth and Mars to be far more dynamic and complex. The Moon and Mercury are airless, lifeless worlds where there is little hope that life could ever have emerged. The Earth, on the other hand, is full of life. Earth is the standard to which we compare all other planets in our search for possibilities of life or potential habitability elsewhere in the Universe—and as you’ll see in this module, scientists now think that long ago both Venus and Mars were far more similar to Earth than they are today. Spacecraft and rover observations have confirmed that both planets once hosted flowing surface water and, likely, more temperate climates.

Life on Earth exists in hydrothermal vents deep in oceans where light is never seen. Organisms have been discovered which need no oxygen for growth. Studies tell us that there are just three essential ingredients for life: an energy source, organic material, and liquid water. Modern astrobiology also recognizes the importance of chemical energy gradients—sources of disequilibrium that can drive metabolism. Given the relative abundances of energy sources (e.g. stars, radioactive elements, hydrothermal vents) and the elements that form organic molecules (viz. carbon, though generally we’re interested in compounds containing hydrogen, nitrogen, and oxygen, elements which are now abundant due to fusion processes that happen in every star throughout the Universe), by far the rarest of these seems to be liquid water.

A comparison of Venus, Earth and Mars, provides a useful basis from which to determine the factors that are required for a planet to sustain liquid water, since it was thought to be present on all three planets early in the Solar System’s history. Comparing the physical characteristics of these three planets brings to mind the story of Goldilocks and the Three Bears. Venus, with a surface temperature of about 737 K (464 °C) and a pressure ≈ 92 times Earth’s, is much too hot and dense to support life as we know it. Mars has a very thin atmosphere, and its average temperature is too cold—well below the freezing point of water. Earth is just right. If all three planets were once similar, what happened to Venus and Mars?

In essence, the thinking is that Venus is too close to the Sun and fell victim to a runaway greenhouse effect early in its history which boiled off all its water and carbon dioxide into a thick cloud layer that maintains a very high temperature by trapping what little light and heat it actually lets through (Venus reflects about 75–80 % of incoming sunlight). Recent observations from orbiters such as Venus Express and Magellan suggest ongoing or geologically recent volcanic activity, helping maintain its thick CO₂ atmosphere. If Venus had been further from the Sun, in a region known as the Sun’s Goldilocks—or, Habitable—Zone, scientists believe its atmosphere could have rained out sufficient amounts of greenhouse gases (e.g. H₂O, CO₂) to maintain lower temperatures and a healthy hydrologic cycle, and its evolutionary history would have been similar to Earth’s. Mars lies within the Sun’s habitable zone, but with a mean global temperature near 210 K (−63 °C) and a thin CO₂ atmosphere, liquid water can now exist only transiently or underground. Therefore, the fact that Mars has mostly lost its atmosphere, so that water no longer flows on its surface, indicates that there are other important factors required to sustain an Earth-like planet.

In this module, you will explore what is currently known about the factors that come together to sustain our planet’s hydrologic cycle. You will begin by looking at just the Earth on its own, exploring how it maintains its fine balance of enough, but not too much atmosphere. You will then move on to investigate the ways in which the mechanism that supports things on Earth is thought to have failed on both Venus and Mars, resulting in their loss of permanent liquid water and, we believe, their potential to support life as we know it.

Ongoing missions such as NASA’s Perseverance rover, ESA’s ExoMars Trace Gas Orbiter, and JAXA’s Akatsuki orbiter at Venus continue to refine our understanding of planetary habitability.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Examine the observational histories of Mars and Venus.
2. Investigate the conditions necessary for sustained liquid water on a planet’s surface.
3. Contrast Earth’s atmosphere with the atmospheres of Venus and Mars, and evaluate the evidence that Venus and Mars once had atmospheres similar to Earth, including evidence from recent orbital and surface missions.
4. Explore the geological and atmospheric evolution of Venus, Earth, and Mars, and assess the roles of volcanism, magnetic fields, and plate tectonics in planetary habitability.
5. Explain how the concept of the habitable zone defines the limits for surface liquid water around stars.

Key Terms and Concepts

• albedo
• global magnetic field
• plate tectonics
• volcanism
• greenhouse effect
• habitable zone
• runaway greenhouse effect
• carbon cycle
• dynamo effect
• atmospheric escape
• hydrologic cycle
• Perseverance rover
• ExoMars Trace Gas Orbiter
• Akatsuki orbiter
• Venus Express mission

Overview

In this module, you’ll leave behind the inner planets of our Solar System and the stability of their solid surfaces — that one common feature that meant that, no matter how hot or airless a planet may be, you could at least imagine what it would be like if you were there. In contrast, if you tried to land a spacecraft on Jupiter or Saturn, you’d plunge into their atmospheres and encounter rapidly increasing pressures and temperatures. Deeper down, the gases behave like fluids, and still deeper the hydrogen is squeezed into a metallic state where it can conduct electricity. (You would not make it that far — you’d just become part of that fluid.)

Because the giant planets lack solid surfaces, and because several of their moons and ring particles may be habitable or even prebiotic, missions to Jupiter and Saturn are deliberately ended by sending the spacecraft into the planet, to avoid contaminating nearby moons such as Europa, Enceladus, or Titan. In some ways, Jupiter and Saturn are more like failed stars — massive, mostly hydrogen–helium worlds — than like the rocky planets we’ve studied so far. And the most intriguing “places to stand” in these systems are not the planets themselves but the many moons orbiting them.

Humans could, in principle, walk on the moons of these giant planets: stroll along the shores of Titan’s methane–ethane seas in dim twilight, or (with serious engineering) dive through the icy crusts of Europa or Enceladus and swim in global saltwater oceans beneath the ice.

In this module, you’ll investigate what spacecraft have revealed about Jupiter and Saturn — their interiors, their atmospheres, their ring systems, and (especially) their moons — and you’ll see why these systems are unlike any of the inner, terrestrial planets you’ve met so far. You’ll explore data from the Cassini–Huygens and Juno missions, trace the path paved by the Voyager and Galileo spacecraft, and look ahead to the next generation of explorers, including NASA’s Europa Clipper and Dragonfly missions and ESA’s JUICE mission to Jupiter’s icy moons.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Compare Jovian planets with terrestrial planets in terms of bulk composition, physical characteristics, rings, and moons.
2. Describe how internal pressure and composition lead to metallic hydrogen and magnetic fields in Jupiter and Saturn.
3. Explain the mechanisms responsible for the formation and structure of planetary ring systems.
4. Investigate the formation and evolution of Jupiter’s and Saturn’s moons and rings, including evidence that some icy moons are geologically active.
5. Explore scientific results from spacecraft missions to the Jupiter–Saturn region (e.g., Cassini–Huygens, Juno, Voyager, Galileo) and preview upcoming missions such as Europa Clipper and Dragonfly.

Key Terms and Concepts

• Terrestrial / Jovian planets
• Planetary rings
• Roche limit
• Metallic hydrogen
• Differential rotation
• Tidal heating
• Galilean moons / icy moons
• Cassini–Huygens mission
• Galileo mission
• Juno mission
• Voyager missions
• Europa Clipper mission
• Dragonfly mission

Overview

Every planet you have studied so far in this course was known to the ancients. Mercury, Venus, Mars, Jupiter, and Saturn were all visible to the naked eye, their wandering motions across the sky inspiring myths and driving the development of astronomy itself. Uranus, Neptune, and Pluto are different: no one even suspected their existence until the modern era. The entire Scientific Revolution — from Copernicus through Newton — unfolded without a clue that more planets waited in the dark beyond Saturn.

Uranus was discovered by William Herschel in 1781 — the first planet ever found with a telescope. Neptune followed in 1846, its position predicted mathematically by Le Verrier and Galle after detecting small deviations in Uranus’s orbit — a landmark triumph of physics-guided discovery. Nearly a century later, Pluto was added in 1930, only to be reclassified as a dwarf planet after modern surveys in the 1990s revealed an entire population of icy worlds beyond Neptune in what is now known as the Kuiper Belt.

In this module, you’ll explore this outer frontier — the region where the Solar System blurs into interstellar space. Although Uranus and Neptune have each been visited only once (by Voyager 2 in 1986 and 1989), modern telescopes such as Hubble, the Very Large Telescope, and the James Webb Space Telescope (JWST) have transformed our view of these worlds. Webb’s infrared images reveal storms, high-altitude hazes, and narrow rings invisible in earlier data. Ground-based observatories now track their seasonal cycles, while new mission concepts — including Trident (a proposed flyby of Neptune’s moon Triton) and the Uranus Orbiter and Probe recommended by the 2023 Decadal Survey — promise to explore them again in the coming decades.

Beyond Neptune lies the Kuiper Belt, home to Pluto, Eris, Haumea, Makemake, and countless smaller icy bodies. NASA’s New Horizons mission gave us our first up-close look at Pluto in 2015 and the contact-binary world Arrokoth in 2019, showing that the distant Solar System is rich, diverse, and still evolving.

By tracing how Uranus, Neptune, and the Kuiper Belt were discovered and explored, you’ll complete your survey of the Solar System and see how these faraway worlds help us understand planetary formation, migration, and the boundary between planets, dwarf planets, and comets.

Learning Objectives

1. Outline the discoveries of Uranus, Neptune, Pluto, and other Kuiper Belt objects, and describe how these discoveries reshaped the definition of a planet.
2. Investigate the physical characteristics and internal structures of Uranus and Neptune, including what modern telescopes and models reveal about their atmospheres, rings, and magnetic fields.
3. Explain the difference between ice giants and gas giants, and how their compositions reflect Solar System formation.
4. Explore scientific results from the Voyager 2 and New Horizons missions, and recent JWST and ground-based observations.
5. Describe the structure of the Kuiper Belt and its sub-populations (plutinos, twotinos, cubewanos, scattered-disc objects).

Key Terms and Concepts

• Ice giants / Gas giants
• Trans-Neptunian objects (TNOs)
• Kuiper Belt objects (KBOs): plutinos, twotinos, cubewanos
• Scattered disc objects (SDOs)
• Dwarf planets
• Planet X / Planet Nine
• Voyager 2 mission
• New Horizons mission
• Trident mission concept
• Uranus Orbiter and Probe mission
• James Webb Space Telescope (JWST)

Overview

In the last four modules, you explored the planets of our Solar System — their atmospheres, internal structures, magnetic fields, rings, and moons — and how they’ve evolved since their formation. You are now ready to bring that knowledge together and examine the smaller bodies that preserve the Solar System’s earliest history.

Unlike the major planets, which have undergone billions of years of geological change, asteroids and comets remain relatively unaltered. These remnants of the original solar nebula act as time capsules, recording the conditions that prevailed when the Sun and planets were born. By studying their orbits, compositions, and interactions, we can reconstruct how the Solar System formed and evolved.

This module begins with a first look at the nebular hypothesis — the idea that the Sun and planets formed together from a rotating disk of gas and dust. You’ll then explore the asteroid belt and the cometary reservoirs that lie beyond Neptune, comparing how these populations differ in composition and origin. We’ll examine key space missions such as Dawn, Rosetta, OSIRIS-REx (and its extended OSIRIS-APEX mission to Apophis), Hayabusa2, Lucy, and Psyche, which are providing close-up data on primitive bodies and testing models of planetary formation. You’ll also learn how fragments that reach Earth as meteorites reveal the chemistry of the early Solar System.

Finally, you’ll turn back to the question of origins. Using clues from small bodies, isotopic dating, and orbital dynamics, you’ll evaluate the Nice model and related theories of planetary migration — models that explain how the giant planets’ movements shaped the asteroid belt, Kuiper Belt, and ultimately, the conditions that allowed Earth to form.

Learning Objectives

1. Explain what asteroids, comets, and meteoroids are, and describe where they originate within the Solar System.
2. Explore scientific results from major space missions that have studied comets and asteroids, and discuss what these missions reveal about Solar System formation.
3. Distinguish between meteors, meteorites, and meteoroids, and explain how they connect to larger parent bodies.
4. Describe the nebular hypothesis and evaluate the evidence supporting modern models of Solar System formation and planetary migration.
5. Synthesize knowledge of individual Solar System components to explain the system’s overall structure and chemical diversity.

Key Terms and Concepts

• Asteroids / Asteroid belt
• Comets / Cometary nuclei
• Nebular hypothesis
• Planetesimals
• Protostar / Protoplanetary disk
• Conservation of angular momentum
• Pebble accretion
• Dawn mission
• Rosetta mission
• OSIRIS-REx / OSIRIS-APEX mission
• Hayabusa2 mission
• Lucy mission
• Psyche mission
• Yarkovsky effect
• Meteoroid / Meteor / Meteorite
• Nice model
• Planetary migration
• Solar System formation

Overview

Throughout this course, you have concentrated on the contents of a single planetary system — our own — and on the method used by humans to logically and systematically explore it: science. In this module, you’ll go beyond our Solar System and explore the planets that have been found orbiting other stars.

Since the discovery of the first confirmed exoplanets in the 1990s, astronomers have identified more than 5,600 confirmed planets in over 4,000 planetary systems. These discoveries reveal extraordinary diversity — from giant worlds orbiting scorchingly close to their stars, to frozen super-Earths drifting between them — and they continue to reshape our understanding of how planetary systems form and evolve.

By examining just the first two planetary systems ever discovered, you’ll see how dramatically our expectations were overturned. You’ll also explore how insights from these discoveries have improved our understanding of our own Solar System’s structure and history.

Building on those early surprises, astronomers have developed an expanding toolkit for detecting and characterizing exoplanets. You’ll investigate techniques such as the radial velocity and transit methods — the workhorses of exoplanet detection — along with microlensing, astrometry, and direct imaging. You’ll also learn how modern instruments now allow atmospheric spectroscopy, revealing the chemical compositions and even possible cloud layers of distant worlds.

The module highlights the role of space missions that have transformed the field. The Kepler mission first demonstrated that planets are ubiquitous. Its successor, TESS (Transiting Exoplanet Survey Satellite), continues to find thousands of nearby worlds suitable for follow-up. The James Webb Space Telescope (JWST) is now probing exoplanet atmospheres in unprecedented detail, while upcoming observatories such as PLATO (ESA) and the Nancy Grace Roman Space Telescope (NASA) will expand these efforts even further. Together, these programs are revealing how planets form, migrate, and evolve — and what makes some potentially habitable.

Finally, the module invites you to step back and reflect on the broader meaning of these discoveries. How does the search for other worlds connect with our place in the Universe? What responsibilities come with expanding our reach beyond Earth? You’ll consider how astronomy’s pursuit of knowledge intersects with human values, cultural perspectives, and the rights of traditional knowledge keepers as we chart the future of exploration.

Learning Objectives

• Examine the current status of our search for extrasolar planets, including confirmed discoveries, key properties, and upcoming observatories.
• Explore and compare the major techniques used to detect planets orbiting other stars.
• Analyze what recent missions reveal about the diversity and evolution of planetary systems.
• Reflect on the development of knowledge in astronomy through the scientific method and its interaction with cultural and ethical values.

Key Terms and Concepts

• Extrasolar planets (exoplanets)
• Pulsar planets
• Hot Jupiters
• Kepler mission
• TESS mission
• James Webb Space Telescope (JWST)
• PLATO mission
• Nancy Grace Roman Space Telescope
• Radial velocity method
• Transit method
• Microlensing method
• Astrometry method
• Direct imaging method
• Atmospheric spectroscopy
• Super-Earths / Mini-Neptunes
• Rogue planets
• Habitable zone