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PH709 Extrasolar Planets - 5 Professor Michael Smith 1 5 Star & Planet Formation & Theory of Exoplanets What is the origin of our solar system? Descartes, Kant, Laplace: vortices, nebular hypothesis: importance of angular momentum. Star formation is on-going. In general: Gravity is fast-acting. Galaxy is old. But young stars are still being born. Stars don't live forever, they must continue to be "born". Where? Born in obscurity….needed infrared/millimeter/radio wavelengths. 5.1 Sign of Youth: Gas Disks around Young Stars Disks: During star formation, gas accretion occurs through a geometrically thin disk that is optically thick. The disks are cooler than the young star, and we thus see an infrared excess superimposed on the black body stellar spectrum: PH709 Proplyds: Extrasolar Planets - 5 Professor Michael Smith 2 PH709 Extrasolar Planets - 5 Professor Michael Smith 3 PH709 Extrasolar Planets - 5 Professor Michael Smith 4 Debris Disks Debris disks are remnant accretion disks with little or no gas left (just dust & rocks), outflow has stopped, the star is visible. Theory: Gas disperses, “planetesimals” form (up to 100 km diameter rocks), collide & stick together due to gravity forming protoplanets). Protoplanets interact with dust disks: tidal torques cause planets to migrate inward toward their host stars. Estimated migration time ~ 2 x 105 yrs for Earth-size planet at 5 AU. Perturbations caused by gas giants may spawn smaller planets: Start with a stable disk around central star. Jupiter-sized planet forms & clears gap in gas disk. Planet accretes along spiral Disk fragments into more arms, arms become unstable. planetary mass objects. Spiral density waves continuously produced by the gravity of embedded or external perturber. Debris Disks – Outer Disk AB Aurigae outer debris disk nearly face on – see structure & condensations (possible protoplanet formation sites? Very far from star) . (Grady et al. 1999) PH709 Extrasolar Planets - 5 Professor Michael Smith 5 Debris: not from original nebula but from recent collisions. After a few hundred million years, a planetary system is expected to have assumed its final configuration and has either set the stage for life, or will probably remain barren forever. It is difficult to probe this era. Most of its traces have been obliterated in the solar system. Only a minority of the nearby stars are so young. Even for them, planets—and particularly those in the terrestrial planet/asteroidal region—are faint and are lost in the glare of their central stars. However, when bodies in this zone collide, they initiate cascades of further collisions among the debris and between it and other members of the system, eventually grinding a significant amount of material into dust grains distributed in a so-called debris disk. Infrared (IR). Because the grains have larger surface area per unit mass compared to larger bodies, they (re)radiate more energy and therefore are more easily detected in the infrared compared to their parent bodies. In the absence of gas drag, a 10 m sized dust grain from the primordial, proto–planetary nebula cannot survive longer than 1 Myr within 10 AU of a star due to a number of clearing processes, such as sublimation, radiation pressure, Poynting-Robertson, and stellar wind drag. Therefore, any main-sequence star older than PH709 Extrasolar Planets - 5 Professor Michael Smith 6 10 Myr with an IR excess is a candidate to have circumstellar material supplied through debris disk processes. SUMMARY: 5.2 The Birth of the Solar System/Exoplanetary system? These three properties of the Solar System hold important clues to its origin: • • • Orbits of the planets and asteroids Rotation of the planets and the Sun. Composition of the planets, especially the strong distinction between Terrestrial, Jovian, and Icy planets. Clues from planetary motions: • Planets orbit in nearly the same plane. Planetary orbital angular momentum is close to direction of Sun’s spin angular momentum (within 7o) • Planet orbits are nearly circular. with a mean eccentricity of 0.06 and individual eccentricities ranging from 0.0068– 0.21. Planets & Asteroids orbit in the same direction. • Rotation axes of the planets tends to align with the sense of their orbits, with exceptions. 3 of 4 terrestrial planets but Venus is retrograde • And 3 of 4 giant planets have obliquities (angle between spin and orbital angular momentum) < 30o; but Uranus is tipped at 98o Sun rotates in the same direction in the same sense. The sizes of neighbouring orbits have ratios in the range 1.4–3.4. Jovian moon systems mimic the Solar System. PH709 Extrasolar Planets - 5 Professor Michael Smith 7 Clues from planet composition: • The Sun has eight planets, with the four smaller planets (Rp = 0.4–1.0 R⊕) interior to the four larger planets (3.9–11.2 R⊕) Inner Planets & Inner Asteroids: Small rocky bodies Few ices or volatiles Major facts in favour of a nebula hypothesis: Coplanar orbits of the planets. The orbits are nearly aligned, with a root-mean-squared inclination of 1.9 degrees relative to the plane defined by the total angular momentum of the Solar System (the “invariable plane”), and individual inclinations ranging from 0.33–6.3 degrees. All planets have prograde revolution (orbits) The revolution of rings and natural moons are all prograde (some moons of the outer planets are not prograde, but these are believed to be captured satellites) All planets except Venus and Uranus have prograde rotation The sun contains all the mass • The planets (especially Jupiter & Saturn) contain most of the angular momentum in the solar system (L⊙/Lorb ≈ 0.5%). Abundance gradient. Inner solar system is poor in light volatile gases such as H, He, but rich in Fe & Ni. Outer solar system is rich in volatiles H, He, etc. Abundances similar to that of the sun Small, dense, iron and silicate rich planets in the inner 2 AU. Slow rotors, few or no moons, no rings, differentiated (molten interiors) PH709 Extrasolar Planets - 5 Professor Michael Smith 8 Large, low density, gaseous planets rich in H, He and volatile elements at >= 5 AU Jovian Planets: Deep Hydrogen & Helium atmospheres rich in volatiles. Large ice & rock cores Rapid rotors, many moons, all have ring systems . Outer solar system moons & icy bodies: Small ice & rock mixtures with frozen volatiles. giant planets (Jupiter, Saturn) • composed mostly of H and He but enriched in metals and appear to have rock-ice core comprising 10-20 Earth masses intermediate or “ice” planets (Uranus, Neptune) • rock-ice core comprising most of mass surrounded by a gas envelope ; 5-20% H and He terrestrial planets (Mercury, Venus, Earth, Mars) • composed of rocky, refractory (high condensation temperature) material Formation of the Sun: back to the Primordial Solar Nebula Stage 1: All stars form out of interstellar gas clouds: Large cold cloud of H2 molecules and dust gravitationally collapses and fragments. Rotating fragments collapse further: Rapid collapse along the poles, but centrifugal forces slow the collapse along the equator. PH709 Extrasolar Planets - 5 Professor Michael Smith 9 Result is collapse into a spinning disk Central core collapses into a rotating proto-Sun surrounded by a rotating "Solar Nebula" Primordial Solar Nebula The rotating solar nebula is composed of ~75% Hydrogen & 25% Helium Traces of metals and dust grains Stage 2. Starts out at ~2,000 K, then cools: As it cools, various elements condense out of the gas into solid form as grains or ices. Which materials condense out when depends on their "condensation temperature". Condensation Temperatures Temp (K) Elements >2000 K Condensate All elements are gaseous 1600 K Al, Ti, Ca Mineral Oxides 1400 K Iron & Nickel Metallic Grains – Refractory, Rocky 1300 K Silicon Silicate Grains - Rocky 300 K Carbon, Oxygen Carbonaceous grains -Volatiles 300-100 K Hydrogen, Nitrogen Ices (H2O, CO2, NH3, CH4) The "Frost Line" or "Snow Line" Rock & Metals can form anywhere it is cooler than about 1,300 K. Carbon grains & ices can only form where the gas is cooler than 300 K. More exactly: At 10-4 bar; the ice lines are at: PH709 Extrasolar Planets - 5 H20 Methane CO and N2 Professor Michael Smith 10 182K 41K 24-25K Inner Solar System: Too hot for ices & carbon grains. Outer Solar System: Carbon grains & ices form beyond the "frost line". The location of the "frost line" is also a matter of some debate but current thinking holds that it is probably about 4 AU . A great deal depends on how much solar radiation can penetrate deep into the outer parts of the primordial Solar Nebula. Stage 3: From Grains to Planetesimals Grains that have low-velocity collisions can stick together, forming bigger grains. Beyond the "frost line", get additional growth by condensing ices onto the grains. Grow to where their mutual gravitation assists in the aggregation process, accelerating the growth rate. Can form kilometre-sized planetesimals after a few 1000 years of initial growth. Stage 4: From Planetesimals to Planets Aggregation of planetesimals into planets Terrestrial vs. Jovian planet formation. Terrestrial Planets Only rocky planetesimals inside the frost line: Collisions between planetesimals form small rocky bodies. PH709 Extrasolar Planets - 5 Professor Michael Smith 11 It is hotter closer to the Sun, so the proto-planets cannot capture H and He gas. Solar wind is also dispersing the solar nebula from the inside out, removing H & He. Result: Form rocky terrestrial planets with few ices. Jovian Planets The addition of ices to the mix greatly augments the masses of the planetesimals These collide to form large rock and ice cores:. Jupiter & Saturn: 10-15 MEarth rock/ice cores. Uranus & Neptune: 1-2 MEarth rock/ice cores. As a consequence of their larger masses & colder temperatures: Can accrete H & He gas from the solar nebula. Planets with the biggest cores grow rapidly in size, increasing the amount of gas accretion. Result: Form large Jovian planets with massive rock & ice cores and heavy H and He atmospheres Moons & Asteroids Some of the gas attracted to the proto-Jovians forms a rotating disk of material: Get mini solar nebula around the Jovians Rocky/icy moons form in these disks. Later moons added by asteroid/comet capture/impact. Asteroids: PH709 Extrasolar Planets - 5 Professor Michael Smith 12 Gravity of the proto-Jupiter keeps the planetesimals in the main belt stirred up. Never get to aggregate into a larger bodies. Icy Bodies & Comets Outer reaches are the coldest, but also the thinnest parts of the Solar Nebula: Ices condense very quickly onto rocky cores. Stay small because of a lack of material. Gravity of the proto-Neptune also plays a role: Assisted the formation of Pluto-sized bodies in 3:2 resonance orbits (Pluto and Plutinos) Disperses the rest into the Kuiper Belt to become Kuiper Belt Objects. Comets and other Trans-Neptunian objects are the leftover icy planetesimals from the formation of the Solar System. Stage 5: Mopping up... The entire planetary assembly process probably took about 100 Million years. • • Followed by a 1 Billion year period during which the planets were subjected to heavy bombardment by the remaining rocky & icy pieces leftover from planet formation. Solid planetary and satellite surfaces are heavily cratered; cratering rate must have been far greater in first 109 yr of solar system history than it is now (“late heavy bombardment”) PH709 Extrasolar Planets - 5 Professor Michael Smith 13 Light from the Sun dispersed the remaining gas in the Solar Nebula gas into the interstellar medium. Planetary motions reflect the history of their formation. Planets share the same sense of rotation, but have been perturbed from perfect alignment by strong collisions during formation. The Sun "remembers" this original rotation. Rotates in the same direction with its axis aligned with the plane of the Solar System. . 5.3 Now Exoplanets ……. Two obvious differences: Existence of planets at small orbital radii, where our previous theory suggested formation was very difficult. Substantial eccentricity of many of the orbits. No clear answers to either of these surprises, but lots of ideas...