Exoplanetary Atmospheres:
Atmospheric Dynamics of Irradiated Planets
PHY 688, Lecture 24
Mar 23, 2009
Outline
• Review of previous lecture:
– atmospheric temperature structure of irradiated planets
• isothermal radiative region
• temperature inversion
• hot and very hot Jupiters
• Surface temperature gradients, winds, phases
• Radii
Mar 23, 2009
PHY 688, Lecture 24
2
Previously in PHY 688…
Mar 23, 2009
PHY 688, Lecture 24
3
From Lecture 17: H Phase Diagram
• temperature-pressure
(T-P) diagram
• for isolated planets,
temperature increases
monotonically toward
interior
Mar 23, 2009
PHY 688, Lecture 24
(Guillot 2006) 4
Effect of Irradiation
• balance between internal flux and flux incident from star
Teff4 = Tint4 + W T*4
• W – dimensionless “dilution” factor ~ 10–3
• incident light penetrates to depth τpen, such that
" pen
# T* & 4
= W % ( )1
$ Tint '
• for τ < τpen, Teff is governed by irradiation and is constant
– isothermal, radiative region
!
• for τ > τpen, Teff ≈ Tint and rises monotonically with τ
Mar 23, 2009
PHY 688, Lecture 24
5
P-T Profiles of Hot Jupiters
AU
• isothermal regions are radiative
Mar 23, 2009
PHY 688, Lecture 24
(Fortney et al. 2007)
6
Cloud-Free Hot Jupiters May Show
Only Tenuous Spectral Features
emission from
isothermal region
appears blackbodylike between 8–15
micron
•
H2O likely present,
but not detectable
•
note however, that
these are extremely
challenging
observations!
Mar 23, 2009
no H2O?!
Spitzer IRS spectrum of HD 189733b
model from Burrows et al. (2006)
Relative Flux
•
PHY 688, Lecture 24
(Grillmair et al. 2007)
7
Observational Challenges in
Extracting Secondary Eclipse Light
• star-planet contrast:
– ~ 0.01 % in mid IR
• time-varying response of IR detectors
• telescope pointing stability
– variations in pixel response, positioning
Mar 23, 2009
PHY 688, Lecture 24
8
Observational Challenges in
Extracting Secondary Eclipse Light
• Spitzer IR
detector
response is not
constant with
time
– figure shows
3.6-micron
background
variation for
TeES-4b
observation
Mar 23, 2009
(Knutson et al. 2008)
8 hours
PHY 688, Lecture 24
9
Observational Challenges in
Extracting Secondary Eclipse Light
raw TrES-4b data
• Spitzer pointing varies
– 3.6- and 4.5-micron (In:Sb array)
PSFs are barely Nyquist-sampled
• i.e., ≤2 pixel widths per PSF
FWHM
– PSF-pixel positioning affects
overall flux
• 5.8- and 8.0-micron arrays
(Si:As) have a time-varying gain
– depends on incident flux
8 hours
Mar 23, 2009
PHY
688, Lecture
24 2008)
(Knutson
et al.
10
Observational Challenges in
Extracting Secondary Eclipse Light
raw TrES-4b data
corrected data
8 hours
Mar 23, 2009
PHY
688, Lecture
24 2008)
(Knutson
et al.
11
Observational Challenges in
Extracting Secondary Eclipse Light
raw TrES-4b data
• planet signal
can not always
be extracted:
– 16-micron flux
measurement
gives only an
upper limit for
TrES-4b
corrected data
8 hours
Mar 23, 2009
(Knutson et al. 2008) PHY 688, Lecture 24
12
Some Planets Require Extra Opacity
at High Altitudes: TrES-4b
• extra opacity evident as excess >5 µm emission
• true for very hot Jupiters
• expected to cause a temperature inversion in the upper atmosphere
•
•
κextra – additional
opacity at high
altitude
Pn – fraction of
incident flux
redistributed to
planet’s night side
Spitzer photometry of TReS–4b
Mar 23, 2009
(Knutson et al.PHY
2008)
688, Lecture 24
13
Extra High-Level Opacity Creates an
H2O Emission Signature
• note 5.8µm
flux peak
• region of a
strong rovibrational
band of
water
Mar 23, 2009
PHY 688, Lecture 24
(Burrows et al. 2007)
14
Temperature
Inversions in Very
Hot Jupiters
• i.e., stratospheres
• gas-phase TiO / VO?
• S from H2S photolysis?
• tholins, polyacetylenes,
etc, produced through
photolysis of CH4 and
NH3?
Mar 23, 2009
(Fortney et al. 2008)
PHY 688, Lecture 24
15
The Earth’s Stratosphere
Earth’s stratospheric clouds:
an exception, not the rule
Mar 23, 2009
PHY 688, Lecture 24
16
Hot and
Very Hot Jupiters:
pL vs. pM Planets
•
distinction:
– based on lack or presence
of high-level TiO/VO
associated with a
stratosphere
– cf. L vs. M stellar spectral
types
•
•
transition at around
0.04–0.05 AU equivalent
separation from the Sun
note dependences on:
– observed planetary
hemisphere
– orbital phase for planets on
very eccentric orbits
(Fortney et al. 2008)
• HD 17156b, HD 80606b,
HD 147506b
Mar 23, 2009
PHY 688, Lecture 24
17
Outline
• Review of previous lecture:
– atmospheric temperature structure of irradiated planets
• isothermal radiative region
• temperature inversion
• hot and very hot Jupiters
• Surface temperature gradients, winds, phases
• Radii
Mar 23, 2009
PHY 688, Lecture 24
18
Opacities of pM and pL Planets
•
•
•
figure shows
approximate pressure at
photosphere (τ = 2/3)
emission from pM
photospheres comes
from ~10 times lower
pressures than in pL’s
<1-micron pM opacity
likely due to higher
TiO/VO abundance in
the upper atmosphere
– temperature inversion
•
>5-micron pM opacity
produces shallower
absorption signatures
– isothermal region
Mar 23, 2009
688,2008)
Lecture 24
(FortneyPHY
et al.
19
Non-Uniform Planet Surface Brightness
• hot Jupiters are
tidally locked to their
host stars:
HD 189733 at 8 µm
– orbital and rotation
period are the same
(~1–5 days)
– sub-stellar point does
not change
• however, peak planet
brightness does not
coincide with
moment of secondary
eclipse
– redistribution of heat
Mar 23, 2009
PHY 688, Lecture 24
(Knutson et al. 2007)
20
Atmospheric Dynamics of Hot Jupiters
Mar 23, 2009
PHY 688, Lecture 24
21
HD 189733b Brightness Map
• brightest spot is
not at the substellar point
• brightest and
faintest spot on
HD 189733b are
on the same
hemisphere!
• temperature
difference is
~350 K
Mar 23, 2009
PHY 688, Lecture 24
(Knutson et al. 2007)
22
Non-Uniformity in Brightness
Depends on Incident Flux
• in fact, HD 189733b has a relatively homogenized daynight atmosphere
– ~350 K difference in temperature
– pL planet, no temperature inversion
• much larger day-night contrast inferred on υ And b,
HD 179949b
– ~1400 K at υ And b
– pM planets, temperature inversions
Mar 23, 2009
PHY 688, Lecture 24
23
!
Radiative
(Newtonian)
Cooling
•
temperature disturbance
relaxes toward radiative
equilibrium
exponentially, with time
constant trad
•
for atmospheric P, T:
t rad
P cP
~
g 4"T 3
Mar 23, 2009
PHY 688, Lecture 24
(Fortney et al. 2008)
24
!
Radiative
(Newtonian)
Cooling
•
temperature disturbance
relaxes toward radiative
equilibrium
exponentially, with time
constant trad
•
for atmospheric P, T:
t rad
P cP
~
g 4"T 3
Mar 23, 2009
PHY 688, Lecture 24
(Fortney et al. 2008)
25
!
Winds:
Cooling vs.
Advection
•
U
advection time scale
tadvec = Rp/U
– Rp – planet radius
– U – wind speed
•
balance of cooling vs.
advection decides wind
speed U
"Tday – night
~ 1# e#t advec / t rad
"Trad
•
winds of several km/sec
(~ sound speed) expected
from 2D and 3D
dynamical models
Mar 23, 2009
PHY 688, Lecture 24
(Fortney et al. 2008)
26
Winds: trad/tadvec
Ratio Depends
Also on Depth
•
ratio is higher in the lower
atmosphere
– especially in pM planets with
stratospheres:
t rad ~
!
!
•
P cP
g 4"T 3
"Tday – night
~ 1# e#t advec / t rad
"Trad
smaller day-night contrast (more
redistribution of heat) in:
– deeper layers
– pL planets
Mar 23, 2009
PHY 688, Lecture 24
(Fortney et al. 2008)
27
Observations in Optical Reflected Light:
Phases of Hot Jupiters
Mar 23, 2009
PHY 688, Lecture 24
(Rowe et al. 2006)
28
HD 209458b: No Phase Variation Seen
MOST satellite data
HD 209458: original time series
standard star: original time series
HD 209458: folded to P = 3.52 d
0
0.02
region of expected
secondary eclipse
0
HD 209458: folded, binned and zoomed
5×10–4
Mar 23, 2009
PHY 688, Lecture 24
(Rowe et al. 2006)
29
Hot Jupiters are Very Dark in the Optical
• 500–800 nm opacity dominated by neutral alkali lines
Mar 23, 2009
PHY 688, Lecture 24
30
Outline
• Review of previous lecture:
– atmospheric temperature structure of irradiated planets
• isothermal radiative region
• temperature inversion
• hot and very hot Jupiters
• Surface temperature gradients, winds, phases
• Radii
Mar 23, 2009
PHY 688, Lecture 24
31
!
From Lecture 17: Radius vs. Mass:
Comparison with Known Planets
•
for polytropes
R"M
•
•
•
•
•
1#n
3#n
n = 1.5 for brown
dwarfs
n = 0.5–1.0 for 0.1–1
MJup planets
(n = 0: uniform
density)
icy/rocky cores in
Neptune, Uranus?
the hot Jupiter HD
209458b has a larger
radius than nonirradiated planets
Mar 23, 2009
t
H 2O plane
oli
lanet
p
iO
S
4
)
Fe 2
vine (Mg,
PHY 688, Lecture 24
(Guillot 2006)
32
Sizes and Compositions of Hot Jupiters
Mar 23, 2009
PHY 688, Lecture 24
(Charbonneau et al. 2007)
33
Are Bloated Hot Jupiters Younger?
(Fortney et al. 2007)
Mar 23, 2009
PHY 688, Lecture 24
34
Jupiter’s Evolution in the Solar System
Mar 23, 2009
PHY 688, Lecture 24
35
Radii of Hot
Jupiters
• some large radii cannot be
explained even by coreless
planets with high-altitude
stratospheres:
– younger age?
• resetting of the age through
tidal heating?
– result of planetary
migration?
– preferential evaporation of
helium?
Mar 23, 2009
(Fortney et al. 2007)
PHY 688, Lecture 24
36