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Astronomy 1 – Fall 2016 Lecture 7: October 13, 2016 Previously on Astro 1 • Properties of the Planets: – Orbits in the same plane and direction – Inner (terrestrial) planets are small and made of heavy elements – Outer (Jovian) planets are big and made of light elements • Other bodies in the Solar system – There are seven large satellites (like the moon) – Asteroids in Asteroid Belt between Mars and Jupiter – Outer solar system is populated by TNO and comets • How do we learn about solar system bodies? – – – – We send probes Spectroscopy reveals the composition of atmospheres Craters are erased by plate geological processes Magnetic fields reveal the presence of a rotating liquid core The Diversity of the Solar System Results from Its Origin and Evolution Today on Astro-1 What is the origin of the solar system? When did it form? What is it made of? How did it come to be this way? Do most stars have planetary systems? Are exoplanet systems similar to our Solar System? Are there exoplanets in the habitable zone of other stars? Meteorites: Rocks from Space • How do we know it came from space? Surface shows evidence of having been melted by air friction as it entered our atmosphere at 40,000 km/h (25,000 mi/h). • How old are these rocks? The ratios of various nuclei, a method called radioactive age dating, are used to determine the rocks formed 4.56 x 109 years ago. When did the solar system form? • The age of the universe, 13.7 x 109 years, has been measured from its expansion rate. •The universe reached two-thirds of its present age before our solar system came into existence. • The Sun and Earth are nearly as old as meteorites, the oldest objects in the solar system. The Solar System is Mainly H and He • All elements heavier than zinc (Zn) have abundances of fewer than 1000 atoms per 1012 atoms of hydrogen. • All elements heavier than Boron (atomic number 5) were made inside stars. • This composition is typical of the Universe as a whole. • Why is the composition of the Earth not very representative of the solar system? Dust that used to be in Antares Antares The dying star Antares is shedding material from its outer layers, forming a thin cloud around the star. Stars Lose Mass è Enrich Their Environment with Heavier Elements The dying star Antares is shedding material from its outer layers, forming a thin cloud around the star. Why is this nebula blue? To understand the formation of the planets, we need to look at the formation of the Sun. Most of the mass of the solar system is in the Sun. The Sun’s composition is close to that of the protostellar nebula. Protostellar Disks: Planets Are Likely Forming Here High Resolution Images of a Dusty Disk • The bands are almost certainly the result of planets forming in the disk. • Grain collisions create pebbles that ultimately grow into larger bodies called planetesimals (and planets). Planets disrupt the disk creating the rings. • Seen for first time! ALMA delivered 0.035” (5 AU) resolution. • How could such high resolution be achieved? • What part of EM spectrum? Sub-mm Interferometry Cool, but why are the young stars surrounded by disks? Gravity Causes Interstellar Gas Clouds to Collapse Planets Form Out of Gas Disks Conservation of Angular Momentum Taking a Closer Look at a Stellar Nursery Heat from the Hot Protosun Separated the Solar Nebular into Two Regions Inner Region: Only rocky and metallic materials remained solid Outer Region: Icy frost condensed beyond the snowline providing more mass for planet building. Accretion of the Terrestrial Planets Planetary Migration Planetary Migration The Kuiper Belt: The gravitational influence of the Jovian planets pushed small, icy objects to the outer reaches of the solar system past Neptune. The result shown in this artist’s conception is the Kuiper belt, a ring populated by trans-Neptunian objects like Pluto, icy planetesimals, and dust. Origin of the Chemical Elements (iclicker Question) How has the present mix of chemical elements in the Universe been produced? A) All the known elements have been formed by the radioactive breakup of the heavy elements formed in the initial Big Bang B) All of the known elements were formed in the Big Bang. C) H and He were formed in the Big Bang, while the heavier elements have been slowly forming by collisions in cold interstellar gas clouds D) H and some He were formed in the Big Bang, while the heavier elements have been slowly formed in the centers of stars over the life of the Universe. E) All the known elements were formed inside stars. How did the Earth form? (iclickers Question) • The formation of terrestrial planets around a star is thought to have occurred by what process? • A) Breakup of a large disk of matter which formed around the star • B) Condensation of gas from the original star nebula • C) Capture by the star of objects traversing the depths of space • D) Accretion or slow accumulation of smaller particles by mutual gravitational attraction Insight from other worlds Methods Direct imaging Radial velocity Transit photometry (light curves) Astrometric wobble Direct Imaging Presents a Contrast Problem Four Planets Orbit the Star HR 8799 First Extrasolar Planet Visible in a Telescope Image The Wobble of 51 Pegasi Detecting a Planet by Measuring Its Parent Star’s Motion Hot Jupiters!? Many planets about the size of Jupiter orbit closer to their star than Mercury does to our Sun! Early studies subject to a selection bias. Most sensitive to short period planets. NASA’s Kepler Mission Summary Solar System Formation: the nebular hypothesis. The Sun: formed by gravitational contraction of the center of the nebula. Terrestrial planets: formed through accretion of dust particles into planetesimals, then into larger protoplanets. Jovian planets: Began as rocky protoplanetary cores, similar in character to the terrestrial planets. Gas then accreted onto these cores. Alternatively, they formed directly from the gases of the solar nebula. In this model the cores formed from planetesimals falling into the planets. Homework (Due Tuesday 10/18) • • Do all review questions from chapter 8 on your own. For TAs, do • 8.46 – make measurements from image of disk • 8.47 – find the mass of a star from the orbit of its planet • 8.49 – compare the spectra from a planet and its star • 8.50 – estimate distance between a star and its planet from a picture then calculate the orbital period. • Note: For Tuesday October 18th, please read chapter 10 “Our Moon”. First Midterm Next Thursday! • You may use a calculator. Bring one. No devices that can connect to the internet. • Exam has 50 multiple choice questions. • You must bring a ParSCORE sheet. • Buy the version where you bubble in your NAME AND PERM #; it is red. • Check with your TA that you bought the right kind. • You must bring a #2 pencil. • Exam will include the equation sheet. • See example on course webpage. • Do not bring an equation sheet. How Should You Study? • • Go over the Key Ideas at the end of chapters 1-8. Go over the Review Questions for ch. 1-8. • Go over the homework solutions for ch. 1-8. • We only grade a subset of the problems. • You need to ‘check’ the solutions to the other problems yourself. • Exam tests whether you understand the concepts and can apply them. • It is not intended to be about memorization.