The History of the Solar System. From cloud to Sun, planets, and smaller bodies

Transcription

1 The History of the Solar System From cloud to Sun, planets, and smaller bodies

2 The Birth of a Star Twenty years ago, we knew of only one star with planets the Sun and our understanding of the birth of stars was quite poor. There were several competing theories, but we couldn t see many of the details of how stars formed within the Milky Way galaxy. Today, we have much more powerful telescopes on the ground and in orbit. We can see the details of how stars form elsewhere in the Galaxy.

3 The Birth of a Star Astronomers knew that the Milky Way contained giant clouds of hydrogen and helium that were lit by stars much hotter and brighter than the Sun. A nearby example is the Orion Nebula, about 1400 light-years away in the constellation Orion. If you locate Orion in the winter sky, the sword that hangs from Orion s belt contains the Orion Nebula.

4 Orion Nebula (Hubble image) Stars that light up the nebula are very hot (35,000 K) and very bright (100,000 Suns).

5 From Cloud to Star Gas deep within the cloud is shielded from the light and ultraviolet of the hot stars. This gas is: very cold ( Kelvin) very low density (10-20 less density than air) Portions of the cloud about a light-year across undergo gravitational collapse.

6 Angular Momentum in Gas Clouds Conservation of angular momentum is at work in cloud of gas, too. What happens to the speed of rotation as the cloud collapses??

7 Why Flat? Think collisions! As the cloud collapses, the gas and dust is more crowded together collisions occur more often! Collisions tend to cancel out upward/downward motion and leave only the rotational motion

8 Thinking about Energy The cloud starts with lots of gravitational potential energy. What happens to this form of energy as the cloud collapses??

9 The Birth of Stars About one million years later, the center of the flat, rotating cloud becomes so hot and dense that nuclear fusion begins and a new star is born! Some of the gas and dust does not become part of the star: too much angular momentum. A disk of gas and dust orbits the star.

10 The Birth of Stars If we look carefully at the Orion Nebula, we can see many stars with disks of gas and dust.

11 The Birth of Stars We can see disks around stars that have formed in the last few million years, i.e. young stars

12 Disks of Gas and Dust What are these disks composed of? Hydrogen and helium are the most common: 98% of the entire cloud. Carbon, nitrogen, and oxygen make up 1.4%, so methane (CH 4 ), ammonia (NH 3 ), and water (H 2 O) are fairly common. Heavier elements are rarer: Rock makes up 0.4% and metals make up 0.2%.

13 From Gas and Dust to Planets The first solids to form are microscopic. But as more and more particles form, they stick together and larger and larger objects grow by accretion. Eventually, objects kilometers across formed. These planetesimals are very numerous in the disk around the Sun and collide frequently. Some collisions are slow enough that gravity holds the bodies together, forming larger and larger objects. These new larger objects have even stronger gravity and pull in other nearby small bodies and grow even faster. Protoplanets began to form

14 From Gas and Dust to Planets The process goes something like this: As time passed

15 From Gas and Dust to Planets Or, in terms of orbiting bodies, something like this: As time passed

16 From Gas and Dust to Planets The chemical composition of the new planets depended on their distance from the young Sun: At a distance of about 1 AU, itwashot. The only solid materials were metals and rock. Since metals and rock were rare, these protoplanets were relatively small. At distances greater than 4-5 AU, it was cold enough for ices. Since these compounds are much more common, the protoplanets far from the Sun were much more massive.

17 From Gas and Dust to Planets AstheworldsofrockandicefartherfromtheSun grew larger, they developed strong gravity, too. In addition to gathering up rock, metal, and ice, these protoplanets also gathered up the 2 gases (hydrogen and helium) that could not solidify. The thick hydrogen/helium atmospheres of the jovian planets we see today are a result of their rapid growth from ices and their strong gravity.

18 From Gas and Dust to Moons Planets grew from a disk around the Sun. Moons grew from disks around giant protoplanets. The whole process repeated itself on a smaller scale. The families of moons around the jovian planets formed through the same processes: collapse, flattening, & accretion It was cold enough that there was a lot of icy solid particles in the disks orbiting the planets. Most of the outer moons are covered in ices.

19 Sweeping It Clean The planets were the biggest and fastest-growing objects in the disk orbiting the Sun. Much of the leftover gas and dust was swept from the Solar System by sunlight and solar wind: sunlight pushes on atoms and very small dust particles, blowing them outward a wind of protons & electrons blows outward from the Sun, also pushing on atoms and dust (this stream of particles is known as solar wind)

20 The Leftovers However, there were many, many planetesimals that weren t swallowed up by a planet. The small bodies that survived we see today as the asteroids (rocky) and comets (icy). This process of accretion continued at a slower pace for about 1 billion years. The leftover planetesimals continued to strike the planets. We see the evidence of these impacts in the ancient cratered terrain on the Moon, Mercury, Mars, and the moons of the outer planets.

21 Period of Heavy Bombardment Heavily cratered, very ancient terrain Farside of the Moon Mars Mercury

22 Which physical property of the original cloud was responsible for the origin of the planets orbits? A. Temperature of the cloud B. Chemical composition of the cloud C. Angular momentum of the cloud D. Density of the cloud

23 Once the cloud collapsed to form a disk, which physical property controlled the types of planets that formed? A. Temperature at that location in the disk B. Chemical composition at that location in the disk C. Angular momentum at that location in the disk D. Density at that location in the disk

24 How many of the planets orbit the Sun in the same direction Earth does? A. None of them, Earth is unique B. The 3 terrestrial planets do C. The 4 jovian planets do D. All of the planets orbit the same way as Earth

25 How many of the planets rotate on their axes in the same direction Earth does? A. None of them, Earth is unique B. Two planets do C. All but 2 planets do D. All of the planets rotate the same way as Earth

26 Which planet listed below was the innermost world to form where the temperature was low enough for water to freeze? A. Mercury B. Earth C. Jupiter D. Uranus