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Introduction to Astronomy • Announcements – Some notes on your homework: • USE YOUR OWN WORDS. • DO YOUR OWN WORK. Astronomy Image of the Day “Hanny’s Voorwerp” Survey of the Solar System Components Origin & History Exoplanets Components The Sun • Hot ball of dense gas – AH = 0.71 – AHe = 0.27 – Plus other trace elements (basically all of them) • Largest, most massive body in the solar system – More on the Sun later Planets • Massive bodies, but still too small to ignite nuclear fusion in their cores – “failed stars”… • Solar system = flattened, spinning pancake – 3 stacked CDs…roughly same relative thickness-to-diameter ratio – Exceptions to perfection • Orbital planes not exactly aligned • Tilt of rotation axes • Retrograde rotation (NOT retrograde motion) of Venus, Uranus & Pluto Inner vs. Outer Planets • Classified based on size, composition, location – Inner planets • Small, rocky bodies with thin or no atmosphere • SiO2 w/ Al, Mg, S, Fe & other heavy metals • Mercury, Venus, Earth, Mars – Outer planets • Large, gaseous, liquid, or icy bodies with no crust/atmosphere boundary • Gases “thicken” (get denser) with depth, eventually liquifying • H2O, CO2, NH3, CH4 • Jupiter, Saturn, Uranus, Neptune • Compositions – Using observations to infer properties we cannot directly measure • Kepler’s law for mass • Angular size – distance relation for volume • Then can calculate AVERAGE density 4 2 a 3 M m GP2 4 3 V R 3 M V • How do we “know” these compositions? – Have some direct measurements • Voyager 1 & 2, Pioneer, Cassini, etc… – Computer simulations – Gravity & Newton’s Laws of Motion • Caveats – Multiple combinations of substances may give the same average density – Gravitational compression – Assume initial dust disk was roughly uniform Satellites (Moons) • Mini models of the solar system – Recall capture theory, twin formation theory, fission theory, violent-birth theory – Mercury & Venus: only planets w/o moons! • Why? • Low mass, proximity to Sun – Any moon-forming material most likely would’ve been pulled into the Sun Asteroids & Comets • Asteroids – Rocky, metallic bodies – Size usually less than 1000 km – Asteroid Belt (between Mars & Jupiter) • Remnants of another planet that failed to fully form (?), because Jupiter’s enormous gravity interfered • Comets – Icy bodies usually less than 10 km in size – “dirty snowball” – Vaporized gases create comet “tails” Halley’s Comet Next opportunity to see it is ~ 2061 Shoemaker-Levy 9 fragments impacts Jupiter… …this was a wake-up call. • Where do comets come from? – Kuiper Belt • Disk-like region of comet nuclei (dirty snowballs) starting past Neptune’s orbit, extending out to ~ 60 AU (recall 1 AU = distance from Sun to Earth = 93 million miles) – Oort Cloud • Huge, spherical shell of comet nuclei that completely surrounds solar system • 40,000 AU < R < 100,000 AU (huge source of comet nuclei) Kuiper Belt & Oort Cloud estimated to contain >1 trillion comet nuclei combined! Origin & History • How did the solar system form? • What physical processes led to the formation of a star surrounded by so much “extra” material? Huge gas clouds gravitational collapse condensation accretion moon formation atmosphere formation Interstellar Clouds • Raw materials – Mostly H, H+, H2, He – “Interstellar Grains” • Silicates, Iron, Carbon & diamond (!) IS cloud Compose about 10% of visible matter in our galaxy Star cluster Collapse • Gravitational attraction pulls outer parts of slowly-rotating gas cloud toward center • Conservation of Angular Momentum – Like ice-skater – As cloud contracts, rotation speeds up • Causes cloud to flatten into a thick disk with a bulge at the center • Happens over few million years NOT to scale…final collapsed cloud would be nearly 100,000 times smaller than original size Condensation • A gas, when cooled, starts to form larger and larger “clumps” of its molecules – Gas to solid (flakes) – E.G. if temperature never falls below ~ 500 K, H2O molecules will never group together (water vapor stays in vapor state) Condensation • Heat from newly-forming Sun (at center) prevents H2O molecules from condensing into liquid water or water vapor – All the way out to Jupiter’s orbital distance – But…rocky materials (e.g. Iron & Silicates) can condense & settle even at high temperatures – RESULT: could not form solid ice particles in the inner parts of the solar system • Therefore, collapsing disk separates into rocky interior & icy outer regions You only see steam outside the tea kettle, because temperature outside is low enough for the water vapor to condense to visible-sized particles These are microscopic flakes of Aluminum produced by low-vacuum condensation of vaporized Aluminum Thickness scale (1nm = 10-9 m) Area scale (1μm = 10-6 m) Accretion • Building bigger objects out of smaller objects • Condensed particles attract each other – Initially, electrostatic forces (remember opposites attract) bind particles together • Atom/molecule-sized – As composite particles get bigger, collisions take over as binding force • Pebble/boulder-sized and larger • “Planetesimals” (building blocks of planets) – Collisions formed outer planets more rapidly, due to high amounts of ice particles • Large balls of rocky ice and heavy gases sweep up excess H & He by gravitational attraction – Collisions in the inner solar system melt the rocky bodies, allow differentiation (heavy Fe & Ni sink to core, lighter silicates remain near surface) – Few million years Ultimately, the properties of the planets were determined by large impacts with the material in the early solar system Moon Formation • Basically, just a scaled-down version of the formation of the solar system – Moon systems show same types of regularities as the planets in the solar system • Orbital planes • Compositions • But recall “violent birth theory” of our moon’s formation…possible to create a moon by different processes than those responsible for forming the solar system Atmosphere Formation • Outer planets probably captured their atmospheres by sweeping up H & He – Gravitational attraction + high amounts of solar nebular material in outer regions • Inner planets have (or had) volcanic activity to generate their atmospheres – Although recall “atmospheric delivery” by comets & asteroids Result of ocean-impact…the aftermath would be circularly-expanding tsunamis starting at about 1000 ft high. Once nearer shorelines, the crests steepen to 1-2 miles! Introduction to Astronomy • Announcements – Some notes on your homework: • USE YOUR OWN WORDS. • DO YOUR OWN WORK. Review • Solar System Formation – Initial ingredients: Hydrogen, Helium, trace others – Gravitational collapse – Condensation – Collisional accretion – Moon formation – Atmosphere formation Cleaning Up • Heat & solar wind emanating from sun blasts away remaining gases and small particles – Perhaps forming the parts of the Kuiper belt and the Oort cloud in the process? Extrasolar Planets (or Exoplanets for short) • Theories on solar system formation raised important questions – Do other stars form similar structures of nearby rocky & far-away icy bodies? – Can we find any? • Our solar system has formed this way, so is there any reason to expect that other “stellar systems” do not exist? – NO! We have observed (indirectly) many such systems Searching for Exoplanets • Many methods – Direct telescopic observation is still years away • Exoplanets just too faint • Upcoming Terrestrial Planet Finder (TPF) – Look for proto-planetary disks (gas clouds in the first stages of solar system formation) TPF Coronagraph: TPF Interferometer: Block out direct starlight, so we can see fainter objects that may be orbiting Operates in the IR to look for planetary heat signatures Funding for TPF was cut by Congress in early 2006. Public outrage ensued, and now TPF is back on track! In Orion Nebula: Proto-stars and surrounding disks of dust and gas Around star β Pictoris: Disk of dust and gas clearly visible Dust disk (seen here in IR) around the young star HR 4796A A small disk in the telescope blots out the light from the star itself so that it’s glare will not obscure the disk… …this is the same principle seen during solar eclipses, and it is how we are able to see the faint outer layers of our Sun. • Observe gravitational effects (indirect) – Star-planet system rotates about its common center of mass (CoM) – Causes parent star to “wobble” slightly • Recall Doppler Shift – When star wobbles toward us, see blue-shifted light – When star wobbles away from us, see red-shifted light – Amount of shift tells about speed of parent star’s orbit about the CoM – Speed of star’s orbit tells us the mass of the planet – This is two-body example, but still applies to more than one planet (Upsilon Andromedae) Center of Mass As unseen planet moves AWAY from observer, parent star moves TOWARD observer…this causes the starlight to be blue-shifted to shorter wavelengths Astrometric Stellar Wobble: observed position actually changes periodically Practically, this will only work for the nearest stars with the largest exoplanets. Spectroscopic Stellar Wobble: the positions of spectral lines (both absorption & emission) changes periodically. This can be translated into a velocity of the star, which then tells us that the star is rotating around it’s system’s center-of-mass • The Transit Technique – The best way to find exoplanets, but requires the right circumstances – An exoplanet passing between us and its star blocks some of the star’s light from reaching our telescopes • The amount and duration of this dimming tells size and speed of orbiting exoplanet • Also, can tell if the exoplanet has an atmosphere! – Starlight modified by atmosphere (if it exists) on limb of planet » New absorption lines not seen in the star itself » Tells chemical composition of any gases that are present • Current status of Exoplanetary Hunt – About 300 known exoplanets – Most around Sun-like stars – Statistics • At most, 1 in 3 Sun-like stars harbor a planetary system • At least, 1 in 14 Sun-like stars have one • According to this study – Last year, the most Earth-like planet found so far • Orbiting Gliese 581 (red dwarf, 21 ly away) at a distance that means liquid H2O could exist on it’s surface! Properties of Exoplanets • Most that we’ve seen have masses M ~ MJupiter • But unlike Jupiter, most exoplanets are VERY close to their star – Imagine replacing Mercury with Jupiter! • Here’s the kicker: No exoplanetary systems have been discovered that resemble our own solar system!!!! Wide variety of orbital geometries… • Does this mean our solar system (and hence Earth) is unique? • Nope. • Observational Bias: – High-mass planets close to their stars produce the largest “wobbles” – Some “wobbles” are too small to detect • HARPS to the rescue! – So, most easily-detected “wobbles” come from high-mass exoplanets very close to their stars What we see depends on how we look for it. Upsilon Andromedae • Multi-planet system • Very elliptical orbits – used to be thought that such elliptical orbits could not be sustained…too many gravitational perturbations would lead to planetary ejections either out of the stellar system, or into the parent star (planet consumption) Excerpts from space.com’s Top Ten Most Intriguing Exoplanets SWEEPS-10 orbits its parent star at only ¾ of a million miles. 1 year on this planet is only 10 Earth-hours long. An exoplanet orbiting Coku Tau 4 is less than 1 million years old, making it the youngest known exoplanet. An exoplanet orbiting a pulsar (a dead star that behaves much like a lighthouse) is roughly 12.7 billion years old. This is the oldest known exoplanet, and suggested that planets are very common in the Universe. A year on HD209458b is 3½ Earth-days long. It is being evaporated by it’s parent star, at an estimated rate of 10,000 Earth-tons per second. Gliese 581 C is the smallest exoplanet ever detected, and is the first to lie within its parent star’s Habitable Zone. Life could exist here. NEXT TIME • The Terrestrial (Inner) Planets – Mercury, Venus & Mars in more detail…