2 Earth and Space Science Objectives and Expectations: Earth in space and timeObjectives and Expectations:TEK Objective 5: Earth in space and time. The student understands the solar nebular accretionary disk model. The student is expected to:analyze how gravitational condensation of solar nebular gas and dust can lead to the accretion of planetesimals and protoplanets;
3 Where did we come from?The origin of the planets in our Solar System has been debated since 1755 when Immanuel Kant proposed the first theoryScientists debated whether the origin of our Solar System was usual, or a common consequence of star formationDuring the 1900s, many believed we are not a unique occurrence because of the recent discovery of other planetary systems around other stars
4 The Protoplanet Hypothesis Two Main TheoriesThe Nebular HypothesisThe Protoplanet Hypothesis
5 Nebula Sun-like stars usually take around 100 million years to form. Nebula are star “nurseries”, where stars are born. This nebular photograph was taken by the Hubble Space TelescopeA nebula is the product of a supernova event. The death of one system, may well be the birth of another!
6 The Nebular Hypothesis In 1755 Immanuel Kant proposed that a solar nebulae was pulled together by gravity so that it collapsed into a flat, rotating disk that eventually coalesced into the Sun and planetsPierre Laplace expanded the theory in 1796 and proposed that the planets were formed by rings of matter split off a rotating nebulae by centrifugal forceSolar nebulae are huge clouds of dust and gas
7 The Nebular Hypothesis Laplace stated that after the matter split off, it coalesced into a planet. The process repeated itself, resulting in a planet each time. The matter left over was the Sun.
8 Much like a figure skater Much like a figure skater. You may have noticed that skaters can spin much faster if they pull their arms closer to their bodies. The more concentrated their body masses are, the faster they'll be able to rotate.The spinning nebula formed planets because as the masses became more concentrated the more they began to spin.The hypothetical explosion squeezed the unformed gas and dust together, which began to spin faster and faster in a circleAs the sun formed in the middle, the cloud started to flatten out into a disc, sort of like a Frisbee or a pancake, with tiny dust grains making up the rest of the disc.
9 Problems With the Nebular Hypothesis Used a flat, rotating nebula as the Solar System's origin to explain why all the planets orbit in nearly the same plane and in the same directionAll planets revolve around the Sun in a counterclockwise direction within a 7° band of the equatorial region of the Sun, and nearly all of them also turn on their individual axes in a counterclockwise direction as well.Contradicted the observation that the Sun contains most of the Solar System's mass but only a small fraction of its angular momentum.Angular momentum is the measure of the tendency of a rotating body to remain rotating
10 Problems With the Nebular Hypothesis If rings of matter were split off, as Laplace stated, they wouldn't be pulled together to form planets but would disperse into space.
11 Process of Nebular Accretion Disk Formation Hypothesis Starts with Solar NebulaeDisturbance (possibly a Supernova) cause gradual collapse of Solar NebulaeGravity causes cloud to shrink and compress the center which begins to heat upAccretion disk forms as matter spins around the center and forms a flat diskThermal energy evaporates the disk
12 Process of Nebular Nebular Accretion Disk Formation Hypothesis Protostar forms with opaque coreEnergy is given off by protostar causing a coolingCooling causes gas to condense into tiny specs of metal, rock, & ice “Stellar Debris”Stellar Debris begins to stick together to form PlanetesimalsAccrection of Planetesimals forms ProtoplanetsSome Planetsimals will form into asteroids, comets, and moonsProtoplanet – small body that could later form a planetPlanetesimals- Baby PlanetsAccretion – gradual growth of planets by the accumulation of other smaller bodies
13 The Protoplanet Hypothesis Because of faults in the nebular hypothesis, other explanations of planet formation were sought.After many failures, such as the encounter theory, astronomers returned to the nebular hypothesis to find improvements during the mid 1900s.
14 The Protoplanet Hypothesis Formed independently byCarl von WeizsackerGerard Kuiper
15 The Protoplanet Hypothesis The solar system begins to form as a rotating cloud, or nebulae, collapsesInstabilities in the nebulae cause dust particles to stick together and accrete into billions of planetesimals with diameters of about 10 meters. The planetesimals then collide and form protoplanets.Meanwhile, the protosun in the center of the nebular disk becomes massive and hot enough to "turn on" by fusing hydrogen.
16 The Sun begins to radiate energy and vaporize dust in the inner part of the Solar System. The remaining gas is blown away by solar winds.