Section 14 Flashcards
What are the Jovian planets huge gaseous atmospheres made up of?
Hydrogen and helium
What is Pluto and how is it believed to have formed?
It is a planetesimal
Remnant from the planet-building phase of Solar System’s early history (from Kuiper belt)
What makes the inner terrestrial planets different from the Jovian planets?
They are smaller
They have rock surfaces surrounded by relatively thin atmospheres
More dense
Different internal structures
What do all Jovian planets have?
Rings (circumplanetary belts) however they do not reflect light as effectively as Saturn
What is the inner structure of Jupiter and Saturn?
Rock and ice core surrounded by metallic hydrogen and molecular hydrogen
What is the inner structure of Uranus and Neptune and what gives them their blue colour?
Rock and ice core surrounded by heavier elements: mantle (water, ammonia, methane ices) and hydrogen, helium and methane gas
Methane and ammonia
Why do some of the planets have a metallic hydrogen core?
Pressure is so high in core that there are free electrons in core
Where do asteroids orbit?
In Asteroid belt; 2 -3.5 AU from the Sun
Where do short-period comets orbit?
In Kuiper belt; beyond Neptune (> 30 AU from Sun)
What is the Oort cloud?
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)
Why are there asteroids located on Jupiter’s orbit in specific places?
Due to Lagrange points (points of stability from the Sun)
What is the solar system formed from?
A molecular cloud formed from remnants of a few stars
Cloud has mass of 2 - 3M_o and is 10,000 AU in size
What happened to the cloud that formed the solar system?
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
What are the extreme orbits of planets due to?
Due to dynamical interactions (exchange angular momentum) with Oort cloud (motion tracked and showed hyperbolic orbit which did not originate in solar system)
What is the cycle of formation of the solar system?
diffuse cloud -> dense cloud -> star with accretion disk -> stellar system -> mass loss (back to diffuse cloud)
(accretion disk material forms planets)
What is the Minimum Mass Solar Nebula (MMSN)?
Minimum mass required to build all the bodies orbiting the Sun (roughly a few dozen times the mass of Jupiter)
Where is the MMSN distributed and what does it contain?
What happens to the composition of the material in the disk over time?
In the original disk around the Sun
The disk contains dust and gas
The material changes as a function of distance from the star
What is the snow line?
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
What is the snow line dependent of ?
Different radius depending on spectral type of host star (usually within a few AU)
What process on the snow line helps grow larger grains/bodies?
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
Where do rocky planets form?
Before the snow line
To find the total disk mass using MMSN what do you need to consider?
the density profile of gas and da (2pi rdr)
What does the MMSN equation indicate?
That planet formation is not 100% efficient so not entire MMSN mass will go to form planets
At mm sizes, what are grains held together by?
Van de Waals forces and they feel gravitational pull in mid plane
What do dust grains do when they reach mm sizes?
They decouple from gas
What happens when grains condense?
The vertical component of the star’s gravity cause the dust to sediment out towards the midplane of disk
What does the growth from cm-size particle to km-size planetesimals depend on?
The relative motions between the various bodies
What is the gas in the disk, which is coupled to cm-sized materials, supported against?
Stellar gravity by a pressure in the radial directions and the gas orbits the star at slightly less than the Keplerian velocity (causing deceleration)
For circular orbits, what must be balanced?
The effective gravity and the centrifugal acceleration
What happens to large particles?
They encounter head wind which removes angular momentum and causes them to spiral inwards towards star
What is a consequence of the difference in velocities of particles
Small cm-sized grains can be swept up by larger bodies while gas drag on metre sized planetesimals into radial motion
Why does the time it takes for the transition from cm to km size planetesimal vary?
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
What is the hypothesis linked to a nebula being inactive?
The dust and small particles settle into a layer thin enough to be gravitationally unstable to clumping and planetesimals (1km) are formed
What is the hypothesis linked to a nebula being turbulent?
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
How was the solar system formed?
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
When does grain growth occur?
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)
What is the time frame of grain growth?
From 100 planetesimals to 4 planetesimals in 150 Myr
How long does it take for terrestrial planets to form?
takes less than 100Myr through giant impacts and depletes all available material
How are Jovian planets formed?
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)
What happens when a core mass of Jovian planet is greater than 10 M_o?
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
What does direct imaging use to detect planets?
Uses reflected light (this is very faint and difficult to observe) so successful for younger stars
Why can direct imaging mainly capture young stars?
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
What is classified as a planet?
Stellar objects which are not sufficiently massive for fusion to ever consume majority of their deuterium
What is brown dwarf?
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
What is the radial velocity technique for observing planets?
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)
What is the transit technique for detecting exoplanets?
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)
What are some key observed properties of exoplanets?
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
Why is each technique for observing exoplanets important?
Each technique finds planets at different distances (semi-major axis) from Earth and they are of different masses
(transit finds planets closest to us)
What do observations of exoplanets surprisingly show?
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
Where planets more likely to be found?
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)
What are the two ways in which exoplanets can form?
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
What is needed to estimate the conditions under which self-gravity wins over the stabilising effects of pressure forces and sheer?
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
When will a clump collapse?
When Σ is large (disk is massive)
c_s is small (disk is cool, lower temp)
Ω is small (disk is large, big radius)
What can’t grav instability explain?
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
In which scenarios does gravitational instability arise in a real disk?
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
What is the process of core accretion in the formation of exoplanets?
Core formation -> hydrostatic growth -> runaway growth -> termination of accretion
How can we account for hot Jupiters (need to be beyond snow line to form)?
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
What is Type I migration?
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
What is Type II migration?
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
How do planets migrate?
Through protoplanetary disk through tidal interactions (exchange of mass and angular momentum)
