Chapter 6: Planetological foundations for origins of life

2 Planet formation – magic in the residue of stellar formation! Kant-Laplace hypothesis: planets form in disks… verification 200 years later! Two major kinds: terrestrial (rocky) planets: like Earth giants (gaseous) planets: like Jupiter. Formation: terrestrial planets form by collisions of smaller bodies like asteroids? gas giants – gas accreting onto a massive rocky core; or by gravitational instability of disk? Emmanual Kant and Pierre- Simon Laplace: 18 th century giants

Gas Accretion & Gap-formation HH 30 (from HST) Flared, gaseous, dusty disk Protoplanet Star formation sets the stage for planet formation

Planet formation theories Giant planet formation; two mechanisms under intense investigation: 1. Core accretion model…. Coagulation of planetesimals that when exceeding 10 Earth masses, gravitationally captures gaseous envelope (eg. Bodenheimer & Pollack 1986) 2. Gravitational instability model …. GI in Toomre unstable disk produces Jovian mass objects in one go (eg. Boss 1998). For either 1 or 2 – final mass determined by gap opening in face of disk viscosity. Terrestrial planet formation; model 1 - do gaps open too?

Core accretion: 3 phases: rapid growth of rocky core, slow accretion of planetesimals and gas, runaway gas accretion after critical mass achieved (near 10 M E ) Problem: formation time still uncomfortably long: Jupiter at 5 AU forms in - 1Myr with 10 M E core - 5 Myr with 5 M E core Hubickyj et al 2005, Icarus

GI: rapid formation within few thousand yrs - disk must have Toomre Q < 1 - disk must cool quickly (less than ½ orbital period – Gammie 2001) Problem: latter point not satisfied in detailed simulations (eg. Cai et al 2004) Mayer et al 2002

Protoplanet Tidal Torque Viscous Torque Disk Gap opens in a disk when Tidal Torque ~ Viscous Torque When do giant planets quit growing?

Planetary masses: determined by gap opening - Gap-opening mass ~ Final mass of a planet - Two competing forces (Tidal vs Viscous) - Smaller gap-opening masses in an inviscid disk Depends on disk physics! - disk flaring (h/a) – governed by heating of disk (ie central star - disk viscosity: very low in central region or dead zone Disk Radius a [AU] Disk pressure scale height h [AU] Lin & Papaloizou (1993)

Disk Radius [AU] 0 2×10 6 4×10 6 6×10 6 8× Time [years] (w/o Dead Zone) Disk Radius [AU] 0 2×10 6 4×10 6 6×10 6 8× Time [years] (w/ Dead Zone) =10 -3 =10 -5 Dead Zone Migration of planets - by tidal interaction with disk: a planet moves in very rapidly (within a million years!) but can be saved by dead zone ( Matsumura, Pudritz, & Thommes 2006)

Detecting Jovian planets in other disks...close-up view with ALMA Wolf & DAngelo (2005) M planet / M star = 0.5 M Jup / 1 M sun Orbital radius: 5 AU Disk mass as in the circumstellar disk as around the Butterfly Star in Taurus 50 pc 100 pc astro-ph /

Birth of a Solar System: what ALMA can do….. ALMA band GHz = 1 mm resolution = 1.4 to AU = 0.3 at d=300pc ~ Highest resolution at 300 GHz = 1 mm (0.015 ) ~ Highest resolution at 850 GHz = 350 m

Condensation sequence: accounting for compositions of planets Temperature of disk drops as radius increases. -All materials whose condensation temperatures are higher than disk temperature at that radius can condense out into solids - so hot innner region of disk has metals – outer cool regions have ices

Biomolecule formation: organic molecules made in protostellar disks Organic chemistry in molecular layer – 3 layer vertical structure at r > 100AU 2D, stellar ultra-violet irradiation of disks: -molecules dissociated in surface layer, - abundant in gas phase in intermediate layer, - frozen out onto grains in densest layer. (Zadelhoff et al 2003, A&A). Delivery system of biomolecules to Earth? Water, and biomolecules: by asteroids? comets? Simulations: Typically find a few Earth oceans worth delivered by asteroids from beyond 2.5 AU.

Comets: Dirty snowballs Halleys comet as seen in May 1910: May 10 – 30 deg tail; May deg tail. Period of comet: 76 years Cometary nucleus – few km in diameter; passage near Sun heats up coma of dust and gas; coma can be 100,000 in size; hydrogen envelope extends millions of km;

Giotto images of Halleys comet Evaporating dust and gas from Halleys nucleus: 30 tons per second for comet inside 1AU – Halleys comet would evaporate in 5000 orbits In general: density 100 kg/ cubic metre; temperature, few 10s of Kelvins; mass ; composition, dust mixed with methane, ammonia & water ices

Cometary orbits – evidence for two distinct reservoirs of comets Isotropic distribution of comets at 50,000 AU: result of gravitational scattering? Oort cloud Disk-like distribution of comets beyond Neptune: remnant of original disk? Kuiper Belt

Origin of oceans…. delivery of water by comets or asteroids? Clue to origin of Earths water: HDO/H 2 O = 150 ppm = ½ of cometary value Asteroids (carbonaceous chondrites) beyond ice line (2.5 AU) can have high water content No more than 10% of Earths water from comets Perturbations by Jupiter of asteroid system perturbs their orbits into ellipses that cross Earths orbit and collide,… bringing in water. Do amino acids survive during this bombardment? Evidence for bombardment: craters on Moon and elsewhere… and formation of the Moon itself in late heavy bombardment…

Formation of the Moon – Impact Model 1. Mars – sized object collides with proto-Earth which has already formed iron core: much of impactor and debris encounters Earth a 2 nd time. 2. Collision tears off Earths mantle material – Moon ends up with composition similar to Earths mantle 3.Debris from collision in orbit around Earth collects together to form the Moon: < 10% of initial ejected material ends up accreting to form the Moon.

Brief history of the Moon a)Just after the end of the major meteoritic bombardment b) Lunar vulcanism floods maria with lava ending 3 billion years ago c) Original maria pitted with craters over last 3 billion yr