Transcript Formation of the Giant Planets

Timescales for Giant
Planet Formation
Dave Stevenson
Caltech
Harvard, May 17, 2004
The Important Questions
• How did our giant planets (J,S,U &N)form?
• How long did it take?
• How is this expressed in the structure of the
system (dynamics, interiors, satellites, etc.)?
• Is that mechanism of giant planet formation
universal or could it vary from system to
system? (Extrasolar planets.)
• What do we need to make further progress?
Here are the Main Points
• Most likely mechanism of Jupiter & Saturn
formation is the core accretion model. Uranus &
Neptune may have formed similarly (but without
the additional gas inflow)
• For J & S , the timescale may be set by the gas
accretion. For U&N, the timescale may be set by
core accretion. This requires core accretion before
the nebula is eliminated (5 Ma or even more?)
• Supported by current structure models
• May not apply to giant planet formation in other
systems…. Disk instability model attractive for
hot Jupiters.
Some Significant Facts
• Jupiter & Saturn are mostly gas. (So they must
have formed in the presence of a nebula).
• Jupiter may have a dense core & Saturn almost
certainly has a dense core. Both are enriched in
heavy elements throughout. [Ar/H] = 3 x solar in
Jupiter, implying delivery of T= 40K material.
• In situ formation of large satellites.
• Uranus and Neptune exist! And largely formed
while some nebula was around, because they have
several Earth masses of gas.This gas could only
come from the nebula
Gas
(H2,He)
J,S
Line of
cosmic ice
& rock
condensate
(variable
gas)
U,N
Ice (mainly
H2O)
LI
e.g., Ganymede
E
Rock (silicates,
oxides, met. Fe)
Gas
subJupiters
Not represented in our solar system
Superganymedes
sJ
Ice
J
Rock
M/MJ
Extrasolars
LI
=10-4
M/MJ=10-2
Increasing
mass
Gas
SG
E
M/MJ=1
Ice
Rock
(Tristan Guillot)
( Guillot)
• This shows the heavy element
abundance in the four major planets and
estimated uncertainties
• A major source of uncertainty is in the
equations of state.
D2
Hugoniots
• Discrepancy
is large and
important to
giant planet
models
• Latest
(Sandia) data
support less
severe
compression
Some Time Scales
Collapse from interstellar medium ~ few 105 yrs.
Disk orbital times; dynamical times ~10-1000yrs.
“Viscous” evolution times~106-107 yr.
Orbital migration (disk interaction) ~105-106 yr
Runaway Accretion of solid bodies & oligarchic
growth ~106-107 yr
• Loss of nebula gas ~5 x 106 yr but what does this
really mean?
• Rapid phase of gas accretion ~1000yr.
• Solid body growth by orbit crossing (inner solar
system)~107-109 yr
•
•
•
•
•
Boss, 2002
Boss, 2002
Common Viewpoint
Heavy element
core
Absence of Heavy
element core


Nucleated
instability
Gaseous Instability
Correct Viewpoint
Heavy element core


Nucleated
instability
Gaseous
instability
and
core rainout
But do you get an
acceptable core?
Absence of Heavy
element core

Gaseous
Instability

Nucleated
Instability
and core
dredge up
But is this
efficient?
Nucleated Instability model
(“Standard” Case)
Truncated
by gap
formation
Embryo
formation
(runaway)
Embryo
isolation
Rapid gas
accretion
Pollack et al, 1996
Or is this
1.5 -3 Ma?
Or is this
2- 4 Ma?
The gas accretion could take place
even when the nebula is only 10%
(or even 1%?) of the so-called
minimum nebula
Gas Expected for Uranus &
Neptune
•Progressive increase in the
hydrostatically bound nebula as the solid
body grows
•Essentially instantaneous
•Amount depends on opacity and is only
logarithmically sensitive to the nebula
density! It is compatible with observation
Pros and Cons - The Nucleated
Instability
• There can be no
doubt that solid cores
can form: Existence
of Uranus and
Neptune
Pros and Cons - The Nucleated
Instability
• There can be no doubt
that solid cores can
form: Existence of
Uranus and Neptune
• But do they form fast
enough so that
massive gas accretion
takes place?
Pros and Cons - The Nucleated
Instability
• There can be no doubt
that solid cores can
form: Existence of
Uranus and Neptune
• Saturn (at least) has
a core that agrees
with the theory.
• But do they form fast
enough so that
massive gas accretion
takes place?
Pros and Cons - The Nucleated
Instability
• There can be no doubt
that solid cores can
form: Existence of
Uranus and Neptune
• Saturn (at least) has a
core that agrees with
the theory.
• But do they form fast
enough so that
massive gas accretion
takes place?
• A weak test,
especially since so
much heavy material
is delivered aside
from the core.
Pros and Cons - The Nucleated
Instability
• There can be no doubt that
solid cores can form:
Existence of Uranus and
Neptune
• Saturn (at least) has a core
that agrees with the
theory.
• Specificity of
published models is
artificial; shorter
timescales are
possible
• But do they form fast
enough so that
massive gas accretion
takes place?
• A weak test, especially
since so much heavy
material is delivered
aside from the core.
Pros and Cons - The Nucleated
Instability
• There can be no doubt that
solid cores can form:
Existence of Uranus and
Neptune
• Saturn (at least) has a core
that agrees with the
theory.
• Specificity of
published models is
artificial; shorter
timescales are possible
• But do they form fast
enough so that
massive gas accretion
takes place?
• A weak test, especially
since so much heavy
material is delivered
aside from the core.
• More models needed
Disk Instabilities
There is little doubt that
such instabilities can
arise in a model.
Need gasG/c > a few
Easier, early on than
later, but depends on
mass redistribution
within the disk.
Might depend on finite
amplitude disturbances
An example from the work
of Alan Boss
Gaseous
protoplanet
Contraction
& settling on
~103 yr
Ice/rock
particles
What’s Wrong with this Picture?
1. As you approach the state where core mass ~ included gas
mass, further “rainout” causes gas to compress (to support
the pressure) and heat adiabatically.
2. Large gravitational energy release
Consequence: evaporation
“Usual core”
Dense ice
& rock
core
Predicted “core” for
instability model
Dense non-ideal
mix of ice, gas &
rock. Comparable
mass fractions.
Pros and Cons - The Gas
Instability
• This process is fast!
Pros and Cons - The Gas
Instability
• This process is fast!
• You don’t even know
for sure if it happens!
Depends on the rate
at which you
approach instability,
etc.
Pros and Cons - The Gas
Instability
• This process is fast!
• Core rainout can
satisfy the need for a
core
• You don’t even know
for sure if it happens!
Depends on the rate at
which you approach
instability, etc.
Pros and Cons - The Gas
Instability
• This process is fast!
• Core rainout can
satisfy the need for a
core
• You don’t even know
for sure if it happens!
Depends on the rate at
which you approach
instability, etc.
• May not work; may
not have the right
mass
Pros and Cons - The Gas
Instability
• This process is fast!
• Core rainout can
satisfy the need for a
core
• Compatible with
extrasolar planets
• You don’t even know
for sure if it happens!
Depends on the rate at
which you approach
instability, etc.
• May not have the right
mass
Pros and Cons - The Gas
Instability
• This process is fast!
• Core rainout can
satisfy the need for a
core
• Compatible with
extrasolar planets
• You don’t even know
for sure if it happens!
Depends on the rate at
which you approach
instability, etc.
• May not have the right
mass
• Still need to make
Uranus and Neptune
This
parameter
NO GIANT PLANETS
GIANT
PLANETS
Gas
instability
Nucleated
instability
That
parameter
The Galilean satellites
were the first planetary
system (other than our
own) to be discovered.
We know a lot about it …..
But the effort to
understand it’s origin has
been small compared to
understanding the solar
nebula (at least until
recently)
Inflow may Cause Compact disk
Canup & Ward
(2002) ,from
Lubow(1999)
Accretion Disk
Viscous spreading
of gas
Solids &
gas infall
Cooling &
contraction
Good Features
• May be a natural
outcome of the
accumulation of a giant
planet
• Control is from the
“outside”
Bad Features
• Origin of viscosity is
unclear
• Permissive model (you
can get whatever you
want!)
Suggested Chronology
•Usual gas inflow from
nebula, which accelerates
as mass increases.
dM/dt
•Gas fills the Roche lobe
but then contraction &
cooling allows a
protoJupiter (~700K, 2RJ)
•Last stages of inflow at
much lower fluxes (one
MMSN =0.02MJ per
106yr)
10-2M/yr
~106 yr
time
Rapid gas accretion
Declining accretion
as nebula gap
develops; onset of
satellite formation
Repeating the Main Points
• Most likely mechanism of Jupiter & Saturn formation is
the core accretion model. Uranus & Neptune may have
formed similarly (but without the additional gas inflow)
• For J & S , the timescale may be set by the gas accretion.
For U&N, the timescale may be set by core accretion. This
requires core accretion before the nebula is eliminated (5
Ma or even more?)
• Supported by current structure models
• May not apply to giant planet formation in other
systems…. Disk instabilty model attractive for hot
Jupiters.
Future Work
• Hydrogen Equation of state.
• Jupiter Polar orbiter & Probes.
• Better understanding of the connection between
giant planet formation time and terrestrial planets.
• Systematics for Extrasolar planets.
• Observations of young systems…when and how
does the gas actually leave?
• More realistic accretion models including orbital
migration (the argon problem), etc.
The End
Back-up Slides
Fluid Planets
• Gas Giants (primarily
hydrogen and helium)Jupiter and Saturn
• Ice Giants (everything,
but including large
amounts of H2O at
high P,T) Uranus and
Neptune
The Hydrogen Phase diagram
•
•
•
Jupiter & Saturn are
in the fluid region,
possibly crossing a
PPT phase transition.
Relevant conditions
encountered in
reverberation shock
experiments
Helium immiscibility
suggested by
observation & theory
but not well
understood.
What do we want to Explain (for
Jupiter & Saturn)?
• Consistency with our models of the Giant Planets
• Size of the System
– Mass
– Angular momentum budget
– Size of individual bodies
• Compositions & Structures
– Temperature and pressure of formation
– Processing of the nebular gas can change ice/rock? (CO/CH4 and
N2/NH3)
– Accretion timescale may affect initial differentiation
Some Basic Principles
• Some of the same basic physics should apply as
for planet formation
– Dynamics of disks (disk-satellite interaction, gas drag, accumulation of
solids, etc.)
• Satellite systems are compact compared to the
solar system
– Tidal truncation or even more? Hill sphere ~several hundred planet radii.
Satellite system ~ few tens of planet radii
– Rapid formation times
– Greater role of tides (orbital evolution, internal evolution, etc.)
• Not all aspects of satellite formation need follow
the solar nebula model!
The Callisto Constraint
• Simplest interpretation of
Callisto’s gravity is that the
interior is only partly
differentiated (I/MR2 =0.355).
• This requires a “long” accretion
time.
Tsurf ~250K.(106yr/acc)1/4
Normal accumulation time
~10,000 yrs or less!
• Presence of CO2 suggests low T
Constraints from
Io and Europa?
• Io is silicate rich (no evidence for
water)
• Water component for Europa is
consistent with hydrated silicates
• Heat flow on Io may not be simple
equilibrium orbital expansion.
(Reduces our ability to link to origin
but also permits primordial
resonance origin)
• Tidal evolution of Europa (antiquity
of an ocean)?
Constraint from Ganymede?
• Fully differentiated but we don’t know
when!
• Dynamo may
constrain history?
Perhaps not-all you
need is S and 40K
• Tidal evolution &
previous excitation
of eccentricity?
What about Amalthea?
• Very low density (~0.9 g/cc!). May require an icy component even
though it doubtless has high porosity
• Not related to primary satellite formation process? (There are other
small satellites in the vicinity). May be accretion from a later transient
disk…Testament to the role of impacts.
Constraints from the Saturnian
System
• Volatile -rich Titan my be a clue
to T,P of origin…or is it partly
that the fields & particles
environment was less severe
than for Ganymede.
• Extent of differentiation not
known- Cassini may tell us.
• Titan’s orbital eccentricity is
intriguing (~0.028 - Evidence
of disk-satellite interaction?)
• Absence of strong evidence for
differentiation of Rhea (for
example)? Might be a constraint
on accretion timescale (but too
soon to say).
Titan
(fantasy)
Rhea
Competing Scenarios
1.
Spin-out Disk
2.
Accretion Disk
3.
Impact-generated disk
4.
Capture & “Co-accretion” (collisions within the Hill
sphere) …will not discuss further because clearly not
relevant to multiple large regular satellites.
Pollack & Bodenheimer
Many workers (Ruskol, Coradini…)
Cameron….
Giant Impacts?
Cartoons indicate the giant
impact that may have led to
formation of Earth’s moon.
Has also been suggested for
the moons of Uranus (since
Uranus has large obliquity)
But not a natural explanation
for Jupiter
Spin-Out Disk
Cooling &
contraction
Good Feature
• May be a natural
outcome of the
accumulation of a giant
planet
Bad Features
• Does not offer a natural way
to explain rock at Ganymede
& Callisto orbits
• Material that is shed may
evolve too quickly
Cosmic (~Solar) Abundances
Elemen t
Number
F raction
Mass
F raction
H
He
O
C
Ne
N
Mg
Si
Fe
0.92
0.08
7 x 10- 4
4 x 10- 4
1.2 x 10- 4
1 x 10- 4
4 x10- 5
4 x 10- 5
3 x 10- 5
0.71
0.27
0.011
0.005
0.002
0.0015
0.001
0.0011
0.0016