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Stars and Planet Formation > Section 14 > Flashcards

Section 14 Flashcards

1
Q

What are the Jovian planets huge gaseous atmospheres made up of?

A

Hydrogen and helium

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2
Q

What is Pluto and how is it believed to have formed?

A

It is a planetesimal

Remnant from the planet-building phase of Solar System’s early history (from Kuiper belt)

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3
Q

What makes the inner terrestrial planets different from the Jovian planets?

A

They are smaller

They have rock surfaces surrounded by relatively thin atmospheres

More dense

Different internal structures

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4
Q

What do all Jovian planets have?

A

Rings (circumplanetary belts) however they do not reflect light as effectively as Saturn

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5
Q

What is the inner structure of Jupiter and Saturn?

A

Rock and ice core surrounded by metallic hydrogen and molecular hydrogen

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6
Q

What is the inner structure of Uranus and Neptune and what gives them their blue colour?

A

Rock and ice core surrounded by heavier elements: mantle (water, ammonia, methane ices) and hydrogen, helium and methane gas

Methane and ammonia

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7
Q

Why do some of the planets have a metallic hydrogen core?

A

Pressure is so high in core that there are free electrons in core

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8
Q

Where do asteroids orbit?

A

In Asteroid belt; 2 -3.5 AU from the Sun

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9
Q

Where do short-period comets orbit?

A

In Kuiper belt; beyond Neptune (> 30 AU from Sun)

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10
Q

What is the Oort cloud?

A

A spherically symmetric cloud of cometary nuclei with orbital radii between 3000 - 10,000 AU (outside of the solar system)

source of all long period comets (which is how it was discovered)

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11
Q

Why are there asteroids located on Jupiter’s orbit in specific places?

A

Due to Lagrange points (points of stability from the Sun)

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12
Q

What is the solar system formed from?

A

A molecular cloud formed from remnants of a few stars

Cloud has mass of 2 - 3M_o and is 10,000 AU in size

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13
Q

What happened to the cloud that formed the solar system?

A

It collapsed inwards under gravity (triggered by supernova due to isotropic signatures)

Conservation of angular momentum and the magnetic fields lead to a flattened disk

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14
Q

What are the extreme orbits of planets due to?

A

Due to dynamical interactions (exchange angular momentum) with Oort cloud (motion tracked and showed hyperbolic orbit which did not originate in solar system)

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15
Q

What is the cycle of formation of the solar system?

A

diffuse cloud -> dense cloud -> star with accretion disk -> stellar system -> mass loss (back to diffuse cloud)

(accretion disk material forms planets)

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16
Q

What is the Minimum Mass Solar Nebula (MMSN)?

A

Minimum mass required to build all the bodies orbiting the Sun (roughly a few dozen times the mass of Jupiter)

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17
Q

Where is the MMSN distributed and what does it contain?

What happens to the composition of the material in the disk over time?

A

In the original disk around the Sun
The disk contains dust and gas

The material changes as a function of distance from the star

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17
Q

What is the snow line?

A

At a distance far away enough that the ice coatings on dust grains increase and material is formed which builds the core of planetesimals

The boundary between rock and rock + ice + gas on a density profile vs distance graph

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18
Q

What is the snow line dependent of ?

A

Different radius depending on spectral type of host star (usually within a few AU)

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19
Q

What process on the snow line helps grow larger grains/bodies?

A

Molecules collide with dust grains and coat the dust grains with ice mantle of water (e.g CO) so increase amount of solid material

The coating of ice increases the stickiness of dust grains and they grow larger bodies

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20
Q

Where do rocky planets form?

A

Before the snow line

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21
Q

To find the total disk mass using MMSN what do you need to consider?

A

the density profile of gas and da (2pi rdr)

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22
Q

What does the MMSN equation indicate?

A

That planet formation is not 100% efficient so not entire MMSN mass will go to form planets

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23
Q

At mm sizes, what are grains held together by?

A

Van de Waals forces and they feel gravitational pull in mid plane

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24
Q

What do dust grains do when they reach mm sizes?

A

They decouple from gas

25
Q

What happens when grains condense?

A

The vertical component of the star’s gravity cause the dust to sediment out towards the midplane of disk

26
Q

What does the growth from cm-size particle to km-size planetesimals depend on?

A

The relative motions between the various bodies

27
Q

What is the gas in the disk, which is coupled to cm-sized materials, supported against?

A

Stellar gravity by a pressure in the radial directions and the gas orbits the star at slightly less than the Keplerian velocity (causing deceleration)

28
Q

For circular orbits, what must be balanced?

A

The effective gravity and the centrifugal acceleration

29
Q

What happens to large particles?

A

They encounter head wind which removes angular momentum and causes them to spiral inwards towards star

30
Q

What is a consequence of the difference in velocities of particles

A

Small cm-sized grains can be swept up by larger bodies while gas drag on metre sized planetesimals into radial motion

31
Q

Why does the time it takes for the transition from cm to km size planetesimal vary?

A

For the material to survive to form a planet, the transition from cm to km size needs to be quick unless the material is confined to sub disk and dragged at same Keplerian velocity

32
Q

What is the hypothesis linked to a nebula being inactive?

A

The dust and small particles settle into a layer thin enough to be gravitationally unstable to clumping and planetesimals (1km) are formed

33
Q

What is the hypothesis linked to a nebula being turbulent?

A

Growth of solid body continues via two-body collisions and they grow very quickly from mm to km size (physics not understood)

Molecular forces lead to km size via coagulation (va der walls binging energies broken down)

Once size > 1km, gravity takes over

34
Q

How was the solar system formed?

A

Dust settles gravitationally to midplane

(it’s composition changes with distance to sun due to different condensation temperatures for materials in disk e.g high T for silicates and oxides, low T for molecules)

Close to sun only high T materials

H and He are mostly in gas form

35
Q

When does grain growth occur?

A

When disk gets very thin

There is collisions and sticking making meteor-type bodies

Collisions and grav attraction lead to formation of planetesimals (km size)

36
Q

What is the time frame of grain growth?

A

From 100 planetesimals to 4 planetesimals in 150 Myr

37
Q

How long does it take for terrestrial planets to form?

A

takes less than 100Myr through giant impacts and depletes all available material

38
Q

How are Jovian planets formed?

A

In outer disk where lower temp so grains move slower and there is an increase of solids available leading to more rapid core growth (form faster than terrestrial)

39
Q

What happens when a core mass of Jovian planet is greater than 10 M_o?

A

Gravitational accretion of gaseous envelope (a runaway process) leading to a gas giant planet

accretion stops when there is no more material and gap in disk is formed

formed in less than 10Myr

40
Q

What does direct imaging use to detect planets?

A

Uses reflected light (this is very faint and difficult to observe) so successful for younger stars

41
Q

Why can direct imaging mainly capture young stars?

A

If star still has disk, it is young and planet is even younger and its luminosity comes from gravitational contractions (energy source) making it brighter than it would appear otherwise and brighter than reflection alone

42
Q

What is classified as a planet?

A

Stellar objects which are not sufficiently massive for fusion to ever consume majority of their deuterium

43
Q

What is brown dwarf?

A

Object which is large enough for deuterium fusion but not massive enough to sustain hydrogen fusion (similar luminosity to young planet)

Grav contraction is major source of energy

44
Q

What is the radial velocity technique for observing planets?

A

Doppler-shifting of spectral lines due to orbital motion about centre of mass (tug from planet) cause periodic variations in speed

Star moving towards us blueshifts and away it redshifts

(sensitive to short-period massive planets)

45
Q

What is the transit technique for detecting exoplanets?

A

Planet disrupts light from star when it passes in front of stars and causes reduction in light curve

most successful technique (sensitive to massive stars)

46
Q

What are some key observed properties of exoplanets?

A

Masses larger than Jupiter (most common is between Earth and Neptune size) < 10M_j

They move on highly eccentric orbits (0 to 1)

Planets closer than 10 R_o (less than 5AU)

Planets orbiting components of stellar binaries

47
Q

Why is each technique for observing exoplanets important?

A

Each technique finds planets at different distances (semi-major axis) from Earth and they are of different masses

(transit finds planets closest to us)

48
Q

What do observations of exoplanets surprisingly show?

A

exoplanet distribution is very different from solar system

Hot Jupiters: massive planets orbiting at 0.1AU from star

Super Earths: inner planets have masses greater than Earths but less than gas giants in our solar system

Planet mass function declines towards large masses

At large radii do not have circular orbits

49
Q

Where planets more likely to be found?

A

With stars that have high metallicity (Fe/H) as metals are proxy for how much dust is around a star

> 0.20

(Gas giants have high metallicity)

50
Q

What are the two ways in which exoplanets can form?

A

Gravitational instability in the disk: direct formation of gas-giant planets (self gravity of disk causes disk to form denser clumps)

Core accretion scenario (same as formation of solar system): growth from dust to rocky planets; big rocky planets accrete gas and form gas giants

51
Q

What is needed to estimate the conditions under which self-gravity wins over the stabilising effects of pressure forces and sheer?

A

The timescale for collapse to be shorter than the time scale on which sound waves can cross a clump, or shear forces can destroy it

52
Q

When will a clump collapse?

A

When Σ is large (disk is massive)

c_s is small (disk is cool, lower temp)

Ω is small (disk is large, big radius)

53
Q

What can’t grav instability explain?

A

Why planets aren’t made of different elements to their host stars

can’t account for presence of small bodies

hard to explain the enhanced abundance of heavy species in giant planets

54
Q

In which scenarios does gravitational instability arise in a real disk?

A

The formation of a massive disk

Clumpy infall onto a disk

Cooling of a disk from a stable to an unstable state

Slow accretion of mass

Close encounters with other stars/disk

55
Q

What is the process of core accretion in the formation of exoplanets?

A

Core formation -> hydrostatic growth -> runaway growth -> termination of accretion

56
Q

How can we account for hot Jupiters (need to be beyond snow line to form)?

A

Grav attraction between planet and non-uniform arrangement of gas generate torques that alter planet’s orbit and causes planet to migrate towards or away from star and alters orbital eccentricity

Direction and rate of migration vary depending on the mass of the planet and properties of gas disk

57
Q

What is Type I migration?

A

When the perturbation on planet is small enough that it does not alter gas disk

It affects Earth-mass planets which induce a linear perturbation in surrounding disk

migration rate is proportional to mass of planet and surface density of disk

58
Q

What is Type II migration?

A

Massive planets exert torque on disk and increases until star modifies disk structure

the strong torque repels gas from the vicinity of planet orbit, creating a GAP (due to angular momentum and removal of interior gas)

Affects Jupiter-mass objects

59
Q

How do planets migrate?

A

Through protoplanetary disk through tidal interactions (exchange of mass and angular momentum)

Stars and Planet Formation (14 decks)

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