1 Chapter 12 The Origin of the Solar System

2 The solar system is our home in the universe. As intelligent species, we have the right and the responsibility to wonder what we are and where we live, to study the universe..

3 Humans have inhabited the solar system for at least a million years. However, only within the last 2400 years have we begun to realize that we live in a ‘cosmos’ and a ‘planetary system’ 400 years have we begun to understand the details of the true heliocentric ‘architecture’ of our solar system 300 years have understood its mechanics 200 years have we started to make detailed scientific models of its origin 100 years have sensed that it’s chaotic yet stable, and where it is located in a Galaxy

4 Humans have inhabited the solar system for at least a million years. However, only within the last 90 years have we completed the Copernican revolution: Harlow Shapley removed the Sun from the center of the Galaxy, and Edwin Hubble the Galaxy from the center of the Universe 50 years have we figured out how the sun ‘works’ and how the stars produced the atoms in our bodies

5 Humans have inhabited the solar system for at least a million years. However, only within the last 30 years have we begun to observe & understand details of star formation such as protostellar disks and their relevance to our origins 20 years have we begun to observe durect evidence for planetary systems other than our own 10 years have we placed our system in a wider context and compared it with hundreds of newly discovered extrasolar systems …and started filling the gaps in our theories of the origins

6 The Great Chain of Origins You are linked through a great chain of origins that leads backward through time to the first instant when the universe began 13.7 billion years ago. The gradual discovery of the links in that chain is one of the most exciting adventures of the human intellect.

7 The Great Chain of Origins In the course ASTA02 (next semester) you will have an opportunity to study some of that story in some detail, including: The origin of the universe in the big bang, The formation of galaxies, The origin of stars, and The production of the chemical elements. Here, you will explore the origin of planets.

8 The History of the Atoms in Your Body By the time the universe was three minutes old, the protons, neutrons, and electrons now in your body had come into existence. You are made of very old matter (13.7 Gyr old)

9 The History of the Atoms in Your Body Although those particles formed quickly, they were not linked together to form the atoms that are common today. Most of the matter was hydrogen (almost 75%) and about 25% was helium. Very few (< 1%) of the heavier atoms were made in the big bang. The big bang also formed the so-called dark matter, which permeates the planetary systems at very low density, but has very little interaction with them.

10 The History of the Atoms in Your Body Although your body does not contain helium, it does contain many of those ancient hydrogen atoms that have remained unchanged since the universe began. We are made mostly of water, and water is made mostly of hydrogen (counting the atoms), and of oxygen (which dominates our mass)

11 The History of the Atoms in Your Body During the first few hundred million years after the big bang, matter collected to form galaxies containing billions of stars. Our Galaxy was built ~8.8 Gyr ago (so-called thin disk) recent galaxy formation also occurs nuclear reactions inside stars combine low-mass atoms, such as hydrogen, to make heavier atoms.

12 The History of the Atoms in Your Body Generation of stars combined the original particles, fusing them into atoms such as carbon, nitrogen, and oxygen: CNO Those are common atoms in your body. Even the calcium atoms in your bones were assembled inside stars.

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14 The History of the Atoms in Your Body Most of the iron in your body was produced by: Carbon fusion in the explosions of stars called supernovae, and Decay of radioactive atoms in the expanding matter ejected by supernovae.

15 The History of the Atoms in Your Body Atoms heavier than iron, such as iodine, were created by rapid nuclear reactions that can occur only during supernova explosions.

16 Elements uncommon enough to be expensive – gold, silver, and platinum in the jewellery that humans wear – also were produced during the violent deaths of rare, massive stars. The History of the Atoms in Your Body

17 The History of the Atoms in Your Body Our galaxy contains at least 100 billion stars, of which the Sun is one. The Sun formed from a cloud of gas and dust about 4.56 billion years ago (4.56 Gyr ago). The atoms in your body were part of that cloud, and previously part of another cloud that formed an earlier star which no longer exists… You are stardust, reprocessed many times over the 13.7 Gyr, enriched in heavy elements inside stars and in exploding stars.

18 The History of the Atoms in Your Body How the Sun took shape, how the cloud gave birth to the planets, and how the atoms in your body found their way onto Earth and into you is the story of this chapter.

19 The History of the Atoms in Your Body As you explore the origin of our solar system, you should keep in mind the great chain of origins that created the atoms. As the geologist Preston Cloud remarked, “Stars have died that we might live.”

20 The Origin of the Solar System Astronomers have a theory for the origin of our solar system that is consistent both with observations of the solar system and with observations of star formation. Now, after the discovery of extrasolar planets they are refining the details of the theories of formation that would explain both our and other planetary systems.

21 The Origin of the Solar System The solar nebula theory supposes that planets form in the rotating disks of gas and dust around young stars.

22 The Origin of the Solar System Our own planetary system formed in such a disk-shaped cloud around the Sun. When the Sun became luminous enough, the remaining gas and dust were blown away into space by the solar wind (a stream of charged particles from the Sun), and UV (ultraviolet) radiation, leaving the planets orbiting the Sun.

23 The Origin of the Solar System – early concepts According to the solar nebula hypothesis, Earth and the other planets of the solar system formed 4.56 billions of years ago as the Sun condensed from the interstellar medium. Kant-Laplace nebula Immanuel Kant ( ) - dust & stones Pierre-Simon de Laplace ( ) - gas rings

24 Kant and Laplace nebular models

25 The Origin of the Solar System The theory predicts that most stars should have planets because planet formation is a natural part of star formation. Therefore, planets should be very common in the universe – probably more common than stars.

26 The Origin of the Solar System These young stars form at the centre of a rotating cloud of gas and dust that starts to contract due to gravity. Collisions between particles in the rotating cloud tend to flatten the cloud into a disk shape. In addition, as the cloud shrinks it spins faster due to the conservation of angular momentum. Most of the material in the spinning disk forms a star in the centre, while the remaining material forms the planets and other bodies such as asteroids and comets.

27 The Origin of the Solar System There is clear evidence that disks of gas and dust are common around young stars. The idea is so comprehensive and explains so many observations that it can be considered to have ‘graduated’ from being just a hypothesis to being properly called a theory. Bipolar flows from protostars were the first evidence of such disks.

The Origin of the Solar System Modern techniques, though, can image the disks directly.

29 Protoplanetary system HH30 with jets 400 AU

30 The Origin of the Solar System The solar nebula theory supposes that planets form in the rotating disks of gas and dust around young stars.

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32 Protoplanetary Disks in Orion nebula HST (Hubble Space Telescope) 100 AU