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Planet Formation

• The Solar System

• Making a star

• Planet formation

• Rock and ice

• Stopping formation

• Extra-solar planets

Reminder: lectures at http://www.star.le.ac.uk/~pto/planets.html

What is in the Solar System?

Sun, planets, moons, asteroids, comets, dust…

Sun = 99.85% of mass! Comet West Eros Dust

Planets = 0.135% of mass!

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Asteroids – interplanetary debris

• Over 100,000 known - most between Mars and Jupiter

(>100m are asteroids; rest are meteroids)

• Total mass < 0.1 Moon. Largest is Ceres (940 km).

• Earth-crossing asteroids are of great interest!

• Source of most meterorites/meteors

Iron & Nickel –

rare, similar to

type M asteroids

Chondrite –

similar to

terrestial

mantle/crust

Gas GiantsJupiter Saturn

• Gas+ice+rock core

• Large & massive

• Low density

• Rapidly rotating

• Many moons

• Rings

Uranus Neptune

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Pluto – Dwarf Planet(God of the underworld)

• Discovered in 1930 during search for Planet X.

• Orbit is eccentric (0.249) and crosses Neptune’s.

• Atmosphere < 10-6 Earth (CH4, N2, CO). Cold!

• Low density – 2100 kg m-3; 70% rock, 30% ice.

• Moon Charon (1985) ~1/8 mass of Pluto (+2 small ones?)

The Kuiper Belt and Pluto (&TNOs)

Kuiper belt: ~1010 icy objects beyond Neptune (30–1000 AU)

Pluto is a large example.

Quaoar discovered in 2002 – half the size of Pluto. Other

large objects found since (e.g. Sedna, 2003 UB313 (Eris)).

Sedna (1200-2000 km size)

OC extends to 100,000 AU with total mass = 30 x Earth

The Kuiper belt (KBOs) and Oort cloud

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Interstellar Molecular Clouds

Orion nebula Eagle nebula

Clouds: ~1014 km, Tcentre~ 10 K, ncentre ~109 particles/m3

Sun: 1.5 x 106 km, Tcentre ~ 107 K, ncentre ~1032 particles/m3

Cloud fragments to form

multiple “protostars”

Protostar in <106 yrs – star in 107 yrs

Magnetic field helps

asymmetric collapse.

Lots of young stars have disks (+ jets)

Disks are “cool” but also have

regions emitting UV/X-rays these

which destroy many disks/planetsCloud to disc

Protoplanetary disks:

“Proplyds”

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Beta Pictoris “dust disk”, similar in

size and mass to the Solar System

Planet b orbit

Planet formation sequence

• Plot shows T vs. distance just

before main accretion stage.

• Only the most refractory

materials (e.g. Fe/Ni) remain

solid within ~1 AU.

• As protosun grows, contracts

and cools, dust re-condenses –

silicates first.

• At ~5 AU (~Jupiter) disk cool enough for H2O, CH4 etc.

to condense. This is the “ice (or snow) line” for the Sun.

Differentiation of the Solar System The Inner Planets• Within 5 AU dust grains grow to ~1 m in ~1000 years and

accrete into vast numbers of “planetesimals”.

• The biggest planetesimals undergo “runaway growth” to

form the terrestrial planet cores. Their exact chemical

composition depends on their distance from the Sun.

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The Outer Planets

• Beyond snow line ices dominate.

• Density is lower, so get fewer,

more massive embryos (~10xMEarth).

• Extreme runaway growth – they

accrete gas, ice and dust ⇒ giants.

N.B. The outer worlds are not gas-rich.

The inner worlds are light-element poor.

• Inner planetesimals perturbed by

Jupiter’s gravity. Fragmentation

follows rather than accretion.

• Some material ejected to large

distances – forms the Oort cloud

and Kuiper belt.

• Gas giants may also have moved

somewhat – migration.

Ending Terrestrial Growth

• Before H-burning, the Sun had an

unstable (T Tauri) phase – high

luminosity and intense solar wind.

• Sun lost ~10 % of mass. Nebula

dispersed halting gas-giant growth.

• Occurred at ~107 years – after

Jupiter/Saturn runaway but before

that of Uranus/Neptune.

• May be why MJ, MS > MU, MN

Ending Gas-giant Growth Extra-solar Planets

• Many extra-solar planets.

• Majority are large.

• Majority are close to their

parent star.

• Unlike Solar System.

• Hard to find low-mass

planets using radial

velocity.

• Can find using transits Kepler mission

http://kepler.nasa.gov

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How to make gas giants close in?

Don’t. Make them far out and migrate. Large planets can

interact with the disk – angular momentum transfer –

moving the planet and creating a gap in the disk.

Result – only a fraction of planets may survive!

The Habitable ZoneMain

Sequence

stellar

mass

range →

HZ is basically where water can be a liquid.

1604: Course test

• Test paper has 4 questions worth 25% each

• Paper available via Blackboard after noon on

Monday March 18

• Submit your answers electronically AND hand in

a printed copy to the teaching office by noon on

Tuesday April 30

• Marks will be deducted for plagiarism (i.e. you

must rewrite material in your own words)

The End