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ASTA01 @ UTSC – Lecture 16 Chapter 12. Origins. Extrasolar Systems Giant impact epoch Solar system: Theory vs. observations Dusty disks in other planetary systems: Beta Pictoris and other system Extrasolar planet discovery: - Pulsar planets - wobble method (radial velocity) - transit (occultation, eclipse) method -examples and statistics 1 Continuing Bombardment of the Planets • Astronomers have good reason to believe that comets and asteroids can hit not only Earth but other planets as well. Continuing Bombardment of the Planets • this bombardment represents the slow continuation of the accretion of the planets. • Earth’s moon, Mercury, Venus, Mars, and most of the moons in the solar system are covered with craters. Continuing Bombardment of the Planets • Most of the craters you see appear to have been formed roughly 4 billion years ago as the last of the debris in the solar nebula was swept up by the planets. • This is called the heavy bombardment. 4 Continuing Bombardment of the Planets • 65 million years ago, at the end of the Cretaceous period, over 75% of the species on Earth, especially many large animals like the dinosaurs and pterodactyls, went extinct over a short period of time (less than 1000 years) • Other earlier extinctions also were defining the boundaries of geological epochs 5 Cretacious-Tertiary extinction • Scientists have found a thin layer of clay all over the world that was laid down at that time. • It is rich in the element Iridium – common in meteorites, but rare in Earth’s crust. • This suggests that a large impact altered Earth’s climate and caused the worldwide extinction. 6 Continuing Bombardment of the Planets • Alvarez, L. W., et al. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1108. - in this paper the theory and observations were proposed for an asteroid impactor Mathematical models indicate that a major impact would eject huge amounts of pulverized/red hot rock high above the atmosphere. 7 Continuing Bombardment of the Planets • As this material fell back, Earth’s atmosphere would be turned into a glowing oven of red-hot meteorites streaming through the air. • This heat would trigger massive forest fires around the world. • Soot from such fires has been found in the final Cretaceous clay layers. 8 Continuing Bombardment of the Planets • Once the firestorms are cooled, the remaining dust in the atmosphere would block sunlight and produce deep darkness for a year or more, killing off most plant life. (so-called Nuclear winter model, since similar effect would result from nuclear war) • Other effects, such as acid rain and enormous tsunamis (tidal waves), are also predicted by the models. 9 Continuing Bombardment of the Planets • Geologists have located a crater at least 150 km in diameter centred near the village of Chicxulub in the northern Yucatán region of Mexico. Continuing Bombardment of the Planets • Although the crater is completely covered by sediments, mineral samples show that it contains shocked quartz typical of impact sites and that it is the right age. Continuing Bombardment of the Planets • The impact of an object 10 to 14 km in diameter formed the crater about 65 million years ago, just when the dinosaurs and many other species died out. • Most Earth scientists now believe that this is the scar of the impact that ended the Cretaceous period. • We still don’t fully know why some animal families survived while other died. • It took 3 Myr for marine ecosystems to recover 12 Continuing Bombardment of the Planets: SL9 • Earthlings watched in awe during six days in the summer of 1994 as 20 or more fragments from the head of comet Shoemaker-Levy 9 slammed into Jupiter. • This produced impacts equalling millions of megatons of TNT. • On Earth it would cause an almost C-T (cretecious-tertiary) type extinction 13 Continuing Bombardment of the Planets: SL 9 • Each impact created a fireball of hot gases and left behind dark smudges that remained visible for months afterward. 14 Continuing Bombardment of the Planets: SL 9 15 Continuing Bombardment of the Planets • The chance that a major impact will occur during your lifetime is so small that it is hard to estimate • However, the consequences of such an impact are so severe that humanity should be preparing. • One way to prepare is to find those NEOs (Near Earth Objects) that could hit this planet, map their orbits in detail, and identify any that are dangerous. • E.g., project Spacewatch 16 Explaining the Characteristics of the Solar System • Now, you have learnt enough to put all the pieces of the puzzle together and explain the distinguishing characteristics of the solar system in the table. Explaining the Characteristics of the Solar System 1. The orbits of the planets lie in the same plane because the rotating solar nebula collapsed into a disk, and the planets formed in that disk. Objects are co-eval (4.) 2. The division into small inner and giant outer planets rests upon the amount of solid material (mainly water ice and silicate rock) available in the inner/outer part of the disk. • There was more material beyond ice boundary • There was not enough material near the Earth for accretion of planets to proceed to the final stage quickly, in the runaway mode. Near Jupiter, protoplanets of several Earth masses could form quickly, before the gas dissipated. Reaching 10 Earth masses triggered gas flow. Explaining the Characteristics of the Solar System • The solar nebula hypothesis calls on continuing evolutionary processes to gradually build the planets. • Scientists call this type of explanation an evolutionary theory. • In contrast, a catastrophic theory invokes special, sudden, even violent, events. 20 Explaining the Characteristics of the Solar System • Uranus rotates on its side and Venus rotates backward. • Both these peculiarities could have been caused by off-centre impacts of massive planetesimals [when] they were forming. • This is an explanation of the catastrophic type. 21 Explaining the Characteristics of the Solar System • Similarly, Earth’s unusually large moon could also be a result of a giant impact that stripped debris from the Earth, which accreted to form the Moon. 22 Explaining the Characteristics of the Solar System • The heat of formation – the energy released by infalling matter – was tremendous for all planets. • The Earth had an ocean of lava • Jupiter must have grown hot enough to glow with a luminosity of about 1 percent that of the present sun. 23 Explaining the Characteristics of the Solar System • However, because it never got hot enough to start nuclear fusion as a star would, it never generated its own energy though fusion of hydrogen into helium. • Jupiter is still hot inside. • In fact, both Jupiter and Saturn radiate more heat than they absorb from the Sun. • So, they are evidently still cooling. 24 Explaining the Characteristics of the Solar System • A glance at the solar system suggests that you should expect to find a planet between Mars and Jupiter at the present location of the asteroid belt. But it never formed there! Explaining the Characteristics of the Solar System • The bodies that should have formed a planet between Mars and Jupiter were broken up, thrown into the Sun, or ejected from the solar system. • This was due to the gravitational influence of massive Jupiter which formed first. • It induced high, destructive, collision speeds • This arrested the accumulation process 26 Explaining the Characteristics of the Solar System 3. All 4 Jovian worlds have ring systems. • You can understand this by considering the large mass of these worlds and their remote location in the solar system. • A large mass makes it easier for a planet to hold onto orbiting ring particles. • Also, being farther from the Sun, the ring particles are not as easily swept away by the pressure of sunlight and the solar wind. Explaining the Characteristics of the Solar Systems • Terrestrial planets – low-mass worlds located near the Sun – have no planetary rings. They are dynamically unstable due to closeness to the sun. • New research shows that we should not expect to find rings around ‘hot jupiter’ exoplanets • Probably no moons either 28 Growth of Protoplanets • Icy planetesimals have formed in the outer parts of the solar nebula. • They have been scattered by encounters with the Jovian planets. • Most escaped from the heliocentric orbit and now wander through the Galaxy • Some remained barely attached as Oort cloud of comets • Others were never strongly scattered and now form the Kuiper belt of comet-like bodies 29 Debris Disks: Other planetary systems • What you have learnt about the solar system: the formation of Kuiper belt of icy planetesimals, collisions between bodies, production of meteoroids and dust has now been observationally confirmed in other planetary systems! • The story of their discovery precedes the discovery of exoplanets. We have first found the disks of dust, derived from planetesimal collisions, and later full-fledged planets. 30 Debris Disks • Infrared astronomers in 1984 have accidentally spotted very cold, low-density dust disks around stars such as Vega, beta Pictoris, and epsilon Eridani. • Although much younger than the Sun, these stars are on the main sequence and have completed their formation, sometime long time ago. They are usually 20-200 Myr old. • So, they are clearly in a later stage than the newborn stars in Orion. 31 1993 1984 Beta Pic: comparison of visible and IR data yields a high brightness or reflectivity (albedo), like Saturn’s rings. However, the particles are olivines & pyroxenes, not ice. 32 33 Debris Disks = exo-zodiacal disks = dusty disks • These low-density disks dominated by dust not gas generally have innermost zones with even lowerdensity places where planets may have formed. β Pic ε Eri 34 Debris Disks = exo-zodiacal disks = dusty disks • Such tenuous dust disks are sometimes called debris disks. • This is because they are understood to be debris released in collisions and evaporation among small bodies such as comets and asteroids. 35 Debris Disks = exo-zodiacal disks = dusty disks • Zodiacal light: sunlight scattered by interplanetary dust particles (IDP) in our solar system 36 Debris Disks: Kuiper belt • Astronomers believe the Sun has an extensive debris disk of cold dust extending far beyond the orbits of the planets • Gerard P. Kuiper (1905-1973) But the Kuiper belt dust has not yet been detected 37 Debris Disks • Notice the difference between the two kinds of disks that astronomers have found. • The low-density dust disks such as the one around Beta Pictoris are produced by dust from collisions among comets, asteroids, and Kuiper belt objects. • Such disks are evidence that planetary systems have already formed (age = 10-1000 Myr) • The dense disks of gas and dust such as those seen around the stars in Orion are sites where planets could be forming right now (age < 10 Myr) 38 FEB = Falling Evaporating Bodies in Beta Pictoris FEB star H & K calcium absorption lines absorption line(s)39 that move on the time scale Beta Pictoris: IR disk and how the sky would look like Infrared image analysis (Lagage & Pantin 1994) 40 Debris Disks • If planetesimals are there, then you can expect that there are also planets orbiting those stars. • A planet is a 1000+ km body which clears a space where it orbits a star. • Many of the debris disks have details of structure and shape that are probably caused by the gravity of planets orbiting within or at the edges of the debris. 41 HD 1415969 Observations by Hubble Space Telescope (NICMOS near-IR camera). Age ~ 5 Myr, a transitional disk Gap-opening PLANET ? So far out? Only if migrated outward R_gap~350Ad R ~ 0.1 R_gap 42 HD 14169A disk gap confirmed by new observations (HST/ACS) 43 Alpha Pisces Austrini (α PsA) Fomalhaut A disk of a bright southern star 44 45