Lecture 4 - Formation of our solar system

what does the sun’s disk consist of?

• 98.5% gas

• 1.5% dust

how long did it take for the solar nebula to form the protostar and planetsimals

100,000 years

what is age zero

when nuclear fusion takes over contribution to luminosity

what are the relevant factors for planet formation

• turbulence in disk

• temperature of disk

why is turbulence important

• Too turbulent: particles move too fast and
  bounce off each other

• Less turbulent: greater chance that particles
  collide and stick together

why is temperature important

it is critical in deciding which materials will be used to form which type of planets

• the temperature falls off roughly as 1/ (distance)2 as we move away from the new star

• the critical point is the ice line. beyond this line is so cold that hydrogen compounds condense into solid ice grains

what temperature happens between 0.8 and 1.3 AU

the dust is very hot and could not contain any residual water

what happens between 2 and 5 au

the temperature reamined low enough such that volatile organic substandces can stay solid (on dust grains)

what happens between 5 au

water no longer evaporates, surviving in the solid phase in empty space. this water-ice will be critical for forming the cores of the giant planet

how long does it take for plenetismals to form

• if there were no turbulence left then objects of a few kilometres diameter would form in a few tens of years through the weak action ofin a few tens of years through the weak action of their own gravitytheir own gravity
  

• But it is likely there would be some residualBut it is likely there would be some residual
  turbulence, so we expect the process to actuallyturbulence, so we expect the process to actually take 1000 – 100,000 years to produce objectstake 1000 – 100,000 years to produce objects larger than 1 kmlarger than 1 km

how are planetesimals formed

• after 100,000 yrs, we expect the central plane to
  contain 100 billion objects of 1-10 km in size, still
  surrounded by a thicker disk of nebular gas.

• Because they are all rotating in the same direction
  around the Sun (with different velocities), they
  undergo many gentle collisions. They gradually
  agglomerate into larger masses

• the iner planets are rocky in nature and have local gravity to retain any passing dust but not to capture any gas

how do gas gients form

• the outer planets contain large amounts of water-ice and rapidly reach much greater masses

  • this allows them to develop a gravitational field strong enough to capture and retain and imense gaseous atmsophere

what happens after about 1 million years for planet formation

• the inner zone contains about 20 objects the size of our moon

• the outer zone contains only a few large objects

• Then the 20 objects start to sufficiently influence the orbits of the other bodies

what happens in the later phases og agglomeration to massive objects

Massive objects will attract smaller objects from far, will attract smaller objects from far, inducing a larger number of collisions and hence speeding up the accumulation of larger objects

what happens in the later phases og agglomeration to smaller objects

• Smaller objects that avoid an initial collision but undergo a grazing pass, are deflected into more elliptical orbits.

• When these objects finally do collide,
  they have a much greater relative motion, andthey have a much greater relative motion, and
  fragmentation rather than conglomeration could occur

what planets are gas giants

• jupiter

• saturn

• neptune

• uranus

what are the properties of a gas giant

• Composed primarily of hydrogen & helium

• Low densities

• Rapid rotation

• Deep atmospheres

• Rings

• Lots of moons

how do gas giants have a solid core

• happens if beyond the ice line in a solar system

• having formed a core the gravitational field is sufficiently strong to attract and keep large amounts of hydrogen gas - a major difference to the terrestrial planets

what are the terrestrial planets

• mercury

• venus

• earth

• mars

what are the properties of rocky planets (6)

• Composed primarily of rock & metals

• High densities

• Slow rotation

• Solid surfaces

• No rings

• No or few moons

what is the rotation of jupiter

9h55m

what is the rotation of saturn

10h34m

what is the rotation of uranus

17h14m

what is the rotation of neptune

18h

what is the rotation of mercury

59 days

what is the rotation of venus

243 days

what is the rotation of earth

24 hours

what is the rotation mars

1.03 days