Exoplanetary Atmospheres:
Temperature Structure of Irradiated Planets
PHY 688, Lecture 23
Mar 20, 2009
Outline
• Review of previous lecture
– hot Jupiters; transiting planets
– primary eclipses and atmospheric transmission
spectroscopy
– secondary eclipses and direct flux measurements
• Properties of irradiated giant planets
– atmospheric temperature structure
• isothermal region
• temperature inversion
Mar 20, 2009
PHY 688, Lecture 23
2
Previously in PHY 688…
Mar 20, 2009
PHY 688, Lecture 23
3
> 300 Planets
Now Known
•
•
•
•
(c.a. 2005)
vast majority discovered
through radial velocity
surveys
5 MEarth < M sin i < 15 MJupiter
1 day < P < 15 years
two dozen multiple planet
systems known
– 55 Cnc: 5 planets!
•
significant population of
“hot Jupiters”
– completely unexpected
before initial discovery
•
wealth of information on
orbital parameters, but
exact masses are scarce
Mar 20, 2009
(exoplanets.org)
PHY 688, Lecture 23
4
Geometry for Transit Probability
Mar 20, 2009
PHY 688, Lecture 23
(kepler.nasa.gov)
5
First Extrasolar Planet Transit:
HD 209458b
•
HD 209458b was a known extrasolar planet in a = 0.047 AU semi-major axis
Mar 20, 2009
PHY 688, Lecture 23
(Charbonneau et al. 2000)
6
Accurate Mass
and Radius
Determination
• i is nearly 90º; edge-on
• dominant source of
uncertainty is now
stellar parameters
– mass, radius
– also need good model
of limb darkening
Mar 20, 2009
PHY 688, Lecture 23
(Charbonneau et al. 2000)
7
Anatomy of a
Transit
• parameters:
– w, l depend on
R*/Rp, i, P
– d depends on R*/Rp
– c depends on limb
darkening
(Brown et al. 2001)
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PHY 688, Lecture 23
8
HD 209458b Transit Observed with
the Hubble Space Telescope
(Brown et al. 2001)
Mar 20, 2009
PHY 688, Lecture 23
9
Sizes and Compositions of Hot Jupiters
Mar 20, 2009
PHY 688, Lecture 23
(Charbonneau et al. 2007)
10
From Star
To Observer
Exoplanet Transit Spectroscopy
Planet
X
A ray may be wholly, partly, or negligibly absorbed, depending
upon its impact parameter and its wavelength.
Thus, the planet appears larger when observed at wavelengths
that are strongly absorbed.
Mar 20, 2009
PHY 688, Lecture 23
11
Differential
Na I
Absorption
•
•
measure difference of
transit depths in and
out of Na I doublet at
589 nm
gives increase in
planet radius at 589
nm due to extra Na I
opacity
(Charbonneau et al. 2002)
Mar 20, 2009
580
590
600
610
620
Wavelength (nm)
PHY 688, Lecture 23
630
12
>60 Transiting Planets Now Known
• hot Jupiters and Neptunes
– P < 5 days
–e=0
• but also some unusual systems
– HD 80606b:
• P = 111 days!!
• e = 0.94 (most eccentric known planet!)
• announced end of February, 2009
• Teff = 900 – 2000 K
– strong irradiation
Mar 20, 2009
PHY 688, Lecture 23
13
Detecting Thermal Emission From
Planet’s “Day” Side: Secondary Eclipse
Secondary Eclipse
See thermal radiation from
planet disappear and
reappear
Primary Eclipse
See radiation from star transmitted
Through the planet’s atmosphere
Mar 20, 2009
PHY 688, Lecture 23
14
Planet’s Thermal Emission Best
Detected in the Mid-Infrared
•
•
24-micron photometry with Spitzer of HD 209458
note:
– 0.015% primary eclipse, 0.004% secondary eclipse
– wavy nature of continuum: day-night variation
Mar 20, 2009
PHY 688, Lecture 23
(Deming et al. 2005)
15
Planet’s Thermal Emission Best
Detected in Mid-Infrared
TrES-1b Spitzer photometry
(Charbonneau et al. 2005)
• model (thick line) is not fit to data (solid diamonds)
• generally good agreement, although
model over-predicts 4.5-micron emission
Mar 20, 2009
PHY 688, Lecture 23
16
Current State of the Art: Emission
Spectroscopy during Secondary Eclipse
• Spitzer Space
Telescope:
• Hubble Space
Telescope
–1.6–2.5 µm spectra
• models include:
–day-night side heat
redistribution
–extra H2O and CO
opacity in upper
atmosphere, etc
Relative Flux
–3.6–8.0 µm and 16,
24 µm photometry
–8–15 µm spectra
HD 189733b
Mar 20, 2009
PHY 688, Lecture 23
(Swain et al. 2009 &
references therein)17
Outline
• Review of previous lecture
– hot Jupiters; transiting planets
– primary eclipses and atmospheric transmission
spectroscopy
– secondary eclipses and direct flux measurements
• Properties of irradiated giant planets
– atmospheric temperature structure
• isothermal region
• temperature inversion
Mar 20, 2009
PHY 688, Lecture 23
18
From Lecture 17: H Phase Diagram
• temperature-pressure
(T-P) diagram
• for isolated planets,
temperature increases
monotonically toward
interior
Mar 20, 2009
PHY 688, Lecture 23
(Guillot 2006) 19
From Lecture 20: P-T Profile of a
Brown Dwarf Atmosphere
L dwarf
Mar 20, 2009
T dwarf
PHY 688, Lecture 23
giant
planet
(Ackerman & Marley 2001)
20
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 20, 2009
PHY 688, Lecture 23
21
P-T Profiles of Hot Jupiters
AU
• isothermal regions are radiative
Mar 20, 2009
PHY 688, Lecture 23
(Fortney et al. 2007)
22
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 20, 2009
no H2O?!
Spitzer IRS spectrum of HD 189733b
model from Burrows et al. (2006)
Relative Flux
•
PHY 688, Lecture 23
(Grillmair et al. 2007)
23
Other Planets Require Extra Opacity
at High Altitudes
• 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 20, 2009
(Knutson et al.PHY
2008)
688, Lecture 23
24
Temperature
Inversions in Very
Hot Jupiters
• i.e., stratospheres
• gas-phase TiO / VO?
• tholins, polyacetylenes,
etc, produced through
photolysis of CH4 and
NH3?
Mar 20, 2009
(Fortney et al. 2008)
PHY 688, Lecture 23
25
The Earth’s Stratosphere
stratospheric clouds
Mar 20, 2009
PHY 688, Lecture 23
26
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 20, 2009
PHY 688, Lecture 23
27