Discovering the Universe Eighth Edition

1. Neil F. Comins • William J. Kaufmann III Discovering the Universe Eighth Edition CHAPTER 5 Formation of the Solar System

2. A montage of the planets in our solar system presented in correct relative sizes. The orbit in the background are also drawn to scale.

5. In this chapter you will discover… • how the solar system formed • why the environment of the early solar system was much more violent than it is today • how astronomers define the various types of objects in the solar system • how the planets are grouped • how the moons formed throughout the solar system • what the debris of the solar system is made of • that disks of gas and dust, as well as planets, have been observed around a growing number of stars • that newly forming stars and planetary systems are being observed

6. Big Bang! • Tiny gas & dust particles collide • Mostly H, He, small amount of Li created • Collect slowly, gather together • Progressively more violent collisions between larger particles • Heat increased • First stars form when gravity compressed H enough to fuse into He • Fusion converts small amount of mass into photons • Photons leak to surface, star begins to shine! • H -> He -> C -> sometimes N -> O -> Ne-> Si ->Fe • Matter ejected, including heavier elements • Continuous flow = stellar winds • Bursts = planetary nebulae • Detonations = supernovae

7. How Stars Lose Mass (a) Antares, is nearing the end of its existence. Strong winds from its surface are expelling large quantities of gas and dust, creating this nebula. The scattering of starlight off this material makes it appear especially bright. (b) The planetary nebula Abell 39 exhibits a relatively gentle emission of matter, the central star shed its outer layers of gas and dust in an expanding spherical shell now about 6 ly across. (c) The Crab Nebula: A supernova is the most powerful known mechanism for a star to shed mass.

8. The Formation of the Solar System

9. Formation of the solar system

10. Young Circumstellar Disks of Matter The four insets of the heart of the Orion Nebula as seen through the Hubble Space Telescope. are false-color images of protoplanetary disks within the nebula. A recently formed star is at the center of each disk. The disk in the upper right is seen nearly edge-on. Our solar system is drawn to scale in the lower left image.

14. This scale drawing shows the distribution of planetary orbits around the Sun. All orbits are counterclockwise because the view is from above Earth’s North Pole. Most of the orbits appear nearly circular. Mercury has the most elliptical orbit of any planet.

15. Planetary Orbits

16. The Sun and the planets are drawn to size scale in order of their distance from the Sun (distances not to scale). The four planets that orbit nearest the Sun (Mercury, Venus, Earth, and Mars) are small and made of rock and metal. The next two planets (Jupiter and Saturn) are large and composed primarily of hydrogen and helium. Uranus and Neptune, also mostly hydrogen and helium, are intermediate in size and contain much water.

17. Thousands of lunar craters were produced by impacts of leftover rocky debris from the formation of the solar system. Age-dating of lunar rocks brought back by the astronauts indicates that the Moon is about 4.5 billion years old. Most of the lunar craters were formed during the Moon’s first 700 million years of existence, when the rate of bombardment was much greater than it is now.

18. What else is out there? • Moon: planetesimals captured by another body’s gravity; formed from orbiting disks of gas and dust; or created as the result of a collision between early planets • Asteroids: rocky bodies typically less than a kilometer across; many orbit between Mars and Jupiter (asteroid belt) • Meteoroids: rocky or metallic bodies smaller than asteroids • Comets: icy chunks of rock, mostly in the Kuiper Belt beyond Neptune. Have highly elongated orbits

19. Dwarf Planets • Pluto: 1930 - its moon Charon is almost the same size (2380:1190km) • Ceres: 1801 – spherical, but tiny (1/4 the size of our Moon) found between Mars and Jupiter • Eris: 2003 – highly eccentric orbit, nearly the same size as Pluto with a moon 300km across

20. How do we know? • Computer simulations Astronomers use computer simulations to learn how the inner planets formed from planetesimals. A computer is programmed to simulate a large number of particles circling a newborn Sun. As the simulation proceeds, the particles coalesce to form larger objects, which in turn collide to form planets. By performing a variety of simulations, each beginning with somewhat different numbers of planetesimals in different orbits, it is possible to see what kinds of planetary systems would have been created under different initial conditions. A wide range of initial conditions lead to basically the same result in the inner solar system: Accretion continues for roughly 108 (100 million) years and typically forms four or five terrestrial planets with orbits between 0.3 and 1.6 AU from the Sun. • Observations of other newly forming solar systems

21. A Circumstellar Disk of Matter (a) An edge-on disk of material 225 billion km (140 billion mi) across orbits the star Beta Pictoris (blocked out in this image) 50 ly from Earth. Twenty million years old, this disk is believed to be composed primarily of iceberg-like bodies that orbit the star. (b) The central region of the disk is clearly warped in this image. This distortion is believed to be due to the gravitational tug of at least one planet that orbits Beta Pictoris. In (c) the solar system drawn to the same scale as the image in (b).

22. A Circumstellar Disk of Matter (d) Hubble visible light image of the disks of Beta Pictoris. The smaller disk is believed to have been formed by the gravitational pull of a roughly Jupiter-mass planet in that orbit. Because the secondary disk is so dim, the labeling for this image is added in (e).

23. Off-Center Disk The star Fomalhaut, blocked out so that its light does not obscure the off-center disk, is surrounded by gas and dust in a ring whose center is separated from the star by 15 AU, nearly as far as Uranus is from the Sun. This offset is believed to be due to the gravitational effects of a giant planet orbiting Fomalhaut. This system is 25 light years from Earth. The dimmer debris in that system and between us and it scatters light that is considered “noise” in such images.

24. This infrared image of an almost-extrasolar object was taken at the European Southern Observatory. It shows the two bodies 2M1207 and 2M1207b. Neither is quite large nor massive enough to be a star, and evidence suggests that 2M1207b did not form from a disk of gas and dust surrounding the larger body, hence it is not a planet. This system is about 170 ly from the solar system in the constellation Hydra.

25. Detecting Planets that Orbit Other Stars (a) A planet and its star orbit around their common center of mass, always staying on opposite sides of that point. The planet’s motion around the center of mass provides astronomers with the information that a planet is present. (b) As a planet moves toward or away from us, its star moves in the opposite direction. Using spectroscopy, we can measure the Doppler shift of the star’s spectrum, which reveals the effects of the unseen planet or planets. (c) If a star and its planet are moving across the sky, the motion of the planet causes the star to orbit its center of mass. This motion appears as a wobbling of the star across the celestial sphere. (d) If a planet happens to move in a plane that takes it across its star (that is, the planet transits the star), as seen from Earth, then the planet will hide some of the starlight, causing the star to dim. This change in brightness will occur periodically and can reveal the presence of a planet.

26. This figure shows the separation between extrasolar planets and their stars. The corresponding star names are given on the left of each line. Note that many systems have giant planets that orbit much closer than 1 AU from their stars. (MJ is shorthand for the mass of Jupiter.) For comparison, the solar system is shown at top.

27. Microlensing Reveals an Extrasolar Planet (a) Gravitational fields cause light to change direction. As a star with a planet passes between Earth and a more distant star, the light from the distant star is focused toward us, making the distant star appear brighter. (b) The focusing of the distant star’s light occurs twice, once by the closer star and once by its planet, making the distant star change brightness. For these observations, the closer star and planet are 17,000 ly away, while the distant star is 24,000 ly away.

28. A Star with Three Planets (a) The star Upsilon Andromedae has at least three planets, discovered by measuring the complex Doppler shift of the star. This star system is located 44 ly from Earth, and the planets all have masses similar to Jupiter’s. (b) The orbital paths of the planets, labeled B, C,and D, along with the orbits of Venus, Earth, and Mars, are drawn for comparison.

29. Summary of Key Ideas

30. Formation of the Solar System • Hydrogen, helium, and traces of lithium, the three lightest elements, were formed shortly after the creation of the universe. The heavier elements were produced much later by stars and are cast into space when stars die. By mass, 98% of the observed matter in the universe is hydrogen and helium. • The solar system formed 4.6 billion years ago from a swirling, disk-shaped cloud of gas, ice, and dust, called the solar nebula. • The four inner planets formed through the accretion of dust particles into planetesimals and then into larger protoplanets. The four outer planets probably formed through the runaway accretion of gas and ice onto rocky protoplanetary cores over millions of years, but possibly by gravitational collapse in under 100,000 years.

31. Formation of the Solar System • The Sun formed at the center of the solar nebula. After about 100 million years, the temperature at the protosun’s center was high enough to ignite thermonuclear fusion reactions. • For 800 million years after the Sun formed, impacts of asteroid-like objects on the young planets dominated the history of the solar system.

32. Planets Outside Our Solar System • Astronomers have observed disks of gas and dust orbiting young stars. • At least 250 extrasolar planets have been discovered orbiting other stars. • Most of the extrasolar planets that have been discovered have masses roughly the mass of Jupiter. • Extrasolar planets are discovered indirectly as a result of their effects on the stars they orbit.

33. WHAT DID YOU THINK? • Were the Sun and planets among the first generation of objects created in the universe? • No. All matter and energy were created by the Big Bang. However, much of the material that exists in our solar system was processed inside stars that evolved before the solar system existed. The solar system formed billions of years after the Big Bang occurred.

34. WHAT DID YOU THINK? • How long has Earth existed, and how do we know this? • Earth formed along with the rest of the solar system, about 4.6 billion years ago. The age is determined from the amount of radioactive decay that has occurred in it.

35. WHAT DID YOU THINK? • What typical shape(s) do moons have, and why? • Although some moons are spherical, most look roughly like potatoes. Those that are spherical are held together by the force of gravity, pulling down high regions. Those that are potato-shaped are held together by the electromagnetic interaction between atoms, just like rocks. These latter moons are too small to be reshaped by gravity.

36. WHAT DID YOU THINK? • Have any Earthlike planets been discovered orbiting Sunlike stars? • Not really. Most extrasolar planets are Jupiter-like gas giants. The planets similar in mass and size to Earth are either orbiting remnants of stars that exploded or, in the case of Gliese 581, a star much less massive and much cooler than the Sun.