Phys 214. Planets and Life

Transcription

1 Phys 214. Planets and Life Dr. Cristina Buzea Department of Physics Room (Please use PHYS214 in subject) Lecture 9. The nebular theory + Movie (Page 74-80) January 25

2 Contents Textbook: Pages The origin of our solar system Nebular theory Planetary nebulae Should habitable worlds be common? Movie Acknowledgments: NASA, ESA, Hubble

3 The origin of our Solar System- nebular theory The origin of our Solar system might give us some insight into finding habitable worlds in other star systems. Nebular theory our solar system was born from the gravitational collapse of an interstellar cloud, or nebula, of gas and dust. Carina nebula

4 Nebular theory

5 Nebular theory The formation of the solar system according to the nebular theory has four steps: 1. Contraction 2. Condensation 3. Accretion 4. Clearing

6 Nebular theory 1. Contraction The solar nebula began as a large, diffuse cloud, roughly spherical in shape. The initial cause of collapse is unknown, perhaps a nearby supernova. Similar clouds exist today and they show they can collapse and give birth to stars. Once the gravitational collapse begins the solar nebula heats up spin faster flatten into a disk shrinks in size

7 Nebular theory The solar nebula heats up <- law of energy conservation. Large gravitational potential energy -> kinetic energy & heat as they fall inward and collide. The cloud becomes hotter near the center, where the star forms. Spin faster <- conservation of angular momentum. The total amount of circling motion of an object must be conserved. A shrinking cloud spins faster as it contracts. Flatten into a disk <- consequence of the spin. When the particles collide, they tend to add to each other s motion when they move in the same direction. However, they cancel each other s motion in other directions.

8 Nebular theory The overall composition of the galaxy and Sun implies that: The composition of the solar nebula was: - 98% hydrogen and helium, & - 2% other elements (essential for planet formation) - metal, rock, and hydrogen compounds (water, methane, amonia). The process of planet formation in the early solar system is best described by condensation.

9 Nebular theory 2. Condensation The materials present in the solar system with the highest condensation temperatures were metals. Because the temperatures were high in the inner solar system, only materials with high condensation temperatures could become solid (metals and rock)

10 Nebular theory 2. Condensation Metal and rock compounds (with high condensation T) could condense within about the present location of the asteroid belt. Farther out, where temperatures were much lower, in addition to metal and rock, hydrogen compounds could condense to make ice.

11 Nebular theory 3. Accretion and terrestrial planet formation Solid particles grew larger = accretion. Particles orbit the forming Sun with orderly circular paths, each particle moving at about the same speed as neighbouring particles. Gentle collisions - due to electrostatic forces and not to gravity. Particles grew larger in mass -> gravity sticks them together into boulders - planetesimals (protoplanets or pieces of planets).

12 Nebular theory 3. Accretion and terrestrial planet formation Planetesimals grew to hundreds of km -a few million years (only 1/1000 the present age of the solar system). Dozens or even hundreds of planetesimals orbiting the Sun between the present day orbits or Mercury and Mars. Continued accreting - sometimes colliding violently. Computer simulations reproduce the collapse of clouds in spinning disks and the first stages of of accretion; cannot predict the results of late stages of accretion, especially the balance between shattering collisions and planetesimal growth. Illustration Credit: T. Pyle (SCSC),JPL-Caltech, NASA

13 Nebular theory 3. Accretion and terrestrial planet formation In the inner solar system, at least 4 objects grew to planetary size, becoming Mercury, Venus, Earth, and Mars ( ,000 km). At least a few other Moon (3,400 km) -to Mars (6,800 km) size objects might have been present in early times, but eventually smashed into one of the four planets that survived. Moon formed when a Mars-size object collided violently with the young Earth.

14 Nebular theory 3. Accretion and jovian planet formation (scientific( debate). The planetesimals in the outer solar system contained a larger amount of ice in addition to metal and rock. All solids objects that reside in the outer solar system today, such as comets, Kuiper belt objects, moons of jovian planets,, all show an ice-rich composition. The Jovian planets most likely formed from planetesimals of rock and ice attracting hydrogen and helium gas from the solar nebula

15 Nebular theory 3. Accretion and moons of jovian planet formation Similar process to the one that made the disk of the solar nebula (heating, spinning flattening). Each jovian planet - surrounded by its own disk of gas, spinning in the same direction that the planet rotates. Moons - accreted from ice-rick planetesimals within the disks, closed to the equatorial plane of the planet. This model explains why jovian planets have many moons.

16 Nebular theory 3. Accretion and jovian planet formation The model of accretion followed by gas capture explains the observed features of the jovian planets well. A competing model suggests that disturbances within the disk of the solar nebula led to clumps of gas to collapse and form jovian planets without the need of forming icy planetesimals first.

17 Nebular theory 4. Clearing the disk As the planets formed, the Sun also formed and accreted the remaining gas. Young Sun had a strong solar wind, blowing off particles from its surface out into space. The wind swept away the remaining gas into the interstellar space, ending the era of planet formation.

18 Nebular theory clearing the disk Once nuclear ignition is achieved the star releases a massive wind -sweep out the remaining gas (T Tauri phase) The remaining planetesimals close to the Sun will almost all impact with planets in this region creation of the Moon About 20,000 of these objects left between Mars & Jupiter The rate of impacts was clearly much higher in the past than it is now Planetesimals farther out (mostly icy) interact with the Jovian planets and can be thrown out of the solar system! (comets) Disk of dust Three trillion mile-long jet from a star hiding in dust

19 Explaining the worlds planets rotation Nebular theory predictions: Planets rotate in the same direction as they orbit the Sun and in the same plane. Sun rotates in the same direction (born at the centre of the spinning cloud). The general cloud rotation explains why most planets rotate in the same direction and most of the large moons orbit their planets in the same direction.

20 Explaining the worlds planets rotation Nebular theory: The condensation theory does not explain the rotation rate of the planets.

21 Explaining the worlds almost circular orbits Planets have nearly circular orbits, because particles with more elliptical orbits would have suffered more collisions. Most planetesimals ended up in one of the eight major planets; many planetesimals shattered into pieces. Asteroids are the remaining planetesimals of the inner solar system. Most reside in the asteroid belt, others in Kuiper belt.

22 Explaining the worlds Oort cloud comets - more difficult to explain. originated as comets orbiting among jovian planets. When they passed near a jovian planet, they were flung out to a great distance by the planet gravity (similar to the way the scientists use Jupiter s gravity to accelerate spacecraft to planets beyond).

23 Explaining the worlds - Exceptions Uranus is tilted at 98 to the plane of the solar system Possibly an off-centre impact, or the fact that the solar nebular is less dense in the outer parts allowing a higher probability of being at an angle Pluto and Mercury lie at 7 and 17 degrees relative to the plane of the solar system Mercury probably suffered an impact during its formation (it is small and easy to perturb) Pluto seems to be a left over planetesimal so probably had many encounters to knock it into a strange position

24 Explaining the worlds - Exceptions Moons with strange orbits - Triton which orbits opposite to Neptune s rotation Probably a captured planetesimal Earth s moon orbits in the plane of the solar system, not in the plane of the Earth s equator Impact event occurred in the plane of the solar system

25 Should habitable worlds be common? Formation of the spinning disk - consequence of physical laws that operate everywhere -> most stars surrounded by spinning disks in which planets may form. Observations support this idea many young stars have such disks! Hubble telescope photo - flattened spinning disk around star AU Microscopii (edge on). Light reflected off dust around the young star when the star light is blocked. Hubble Space Telescope near-infrared picture of a disk around the star HD , located about 320 light-years away in the constellation Libra.

26 Planetary nebulae Hubble Space Telescope images of four protoplanetary disks around young stars in the Orion nebula, located 1500 lightyears away. Gas and dust disks can be seen in visible light.

27 Planetary nebulae Protoplanetary disks in the Orion Nebula as seen by Hubble Edge on. Artist impression - HD system - two pairs of double stars, with one pair surrounded by a disk of dust. Recent data from the Earth-trailing Spitzer Space Telescope in infrared light, indicate that the dust disk has gaps consistent with being cleared by planets orbiting in the disk. If so, one planet appears to be orbiting at a distance similar to Mars of our own Solar System.

28 Should habitable worlds be common? We expect to find many planetary systems with terrestrial and jovian planets laid out the same. However, most of the extrasolar planetary systems discovered to date are quite different than our own solar system: - jovian planets found close to their parent stars. - planets orbit closer to their star (closer than Mercury s orbit). Movie. The discovery of most Earth-like planet. The first image of an extrasolar planet. The planet roughly five times the mass of Jupiter is orbiting a brown dwarf. Artificial-colour Hubble Near Infrared Camera and Multi- Object Spectrometer (NICMOS) infrared-light view of the brown dwarf star 2M1207 and giant planet companion candidate - about five times the mass of Jupiter, is the magenta coloured spot at lower right. The brown dwarf s location is within the circle at image centre. The glare of the dwarf, which is 700 times brighter than the planet candidate has been greatly reduced through image processing

29 Should habitable worlds be common? We cannot say with certainty whether solar systems like ours should be rare or common. However, rare in Milky Way means large actually. If only 1 in 1 million star has a system like ours, this is 100,000 systems! Therefore, it is almost inevitable that our galaxy contains many worlds that have liquid water and would be suitable for life.

30 Movie Hubble 15 years of discovery Chapter 3. Planetary tales (9 minutes)

31 Next lecture Chapter 4. The habitability of Earth Geology