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Extrasolar Planets.I.
1. What do we know and how do we know it.2. Basic planetary atmospheres
3. Successful observations and future plans
Planets Orbiting Other Stars
• Total: 209 discovered to-date.
• Statistics:• Gas giant planets, like Jupiter & Saturn,
exist around >12% of stars (Marcy et al. 2005);
• Lower-mass planets (Super-Earths, 3 known to-date)
are significantly more common
(Rivera et al. 2005; Beaulieu et al. 2006).
• No Earth-like planets yet…
Planets Orbiting Other Stars:
aft
er
Go
uld
et
al .
(20
06)
First ‘Super-Earth’ discovered GJ 876d:-- Mass ~ 7.5 Earths
Also HD 69830b:-- Mass ~ 10 Earths
NASA Kepler mission:
… Radii in this range
M = MercuryV = VenusE = Earth, etc.
Atmosphere:
• In general - outer boundary for planet’s thermal evolution - the extrasolar planets have introduced conditions never imagined
• Clouds & (photo)chemistry• Evaporation (very hot & hot Jupiters)
Transits allow spectroscopic studies of the planet’s atmosphere
The Close-in Extrasolar Giant Planets
• Type and size of condensate is important
• Possibly large reflected light in the optical
• Thermal emission in the infrared
Seag
er &
Sas
selo
v 20
00
Atmosphere:
Theoretical Transmission Spectra of HD 209458 b
Wavelength (nm)
Occ
ulte
d A
rea
(%)
Seager & Sasselov (2000)
Transmission Spectra
)]/)(exp[exp(
)(
)exp(
0
00,
0
0,
∫
∫
−−−=
=
−=
L
L
dHRrII
dn
II
l
ll
λλλ
λλ
λλλ
σ
στ
τ
H = kT/gmH scale height extinction cross section L path length
How large is the planet atmospheresignal? It depends on theatmosphere annulus / star area
The actual detection (with the HST):
• a 5signal• 2x weaker than
model expected, but within errors
• Might indicate high clouds above terminator
Charbonneau et al. (2002)
planet/star flux ratio is: a
d
Rp
StarPlanet
Earth
ε ≡fplanet
f*
= pRp
2
a2
Reflected Light
p is albedo
Atmospheric Probe
● Sudarsky Planet types I : Ammonia Clouds II : Water Clouds III : Clear IV : Alkali Metal V : Silicate Clouds
● Predicted Albedos: IV : 0.03 V : 0.50
Sudarsky et al. 2000 Picture of class IV planet generated using Celestia Software
Photometric Light Curves Micromagnitude variability from planet phase changes
• Space-based: MOST (~2005), COROT (~2007), Kepler (~2008)
• m=2.5 (Rp/D)22/3/(sin() + (-)cos())
Seager et al. 2000
Mission Microvariability and
Oscillations of STars / Microvariabilité et Oscillations STellaire
First space satellite dedicated to stellar seismology Small optical telescope &
ultraprecise photometer goal: ~
few ppm = few micromag
MOST at a glance
Canadian Space Agency (CSA)
circular polar orbit altitude h = 820 km period P = 101 min inclination i = 98.6º
Sun-synchronous stays over terminator
CVZ ~ 54° wide -18º < Decl. < +36º stars visible for up to 8 wks
Ground station network Toronto, Vancouver, Vienna
MOST at a glance
MOST
orbit normal vector
to Sun
CVZ = Continuous Viewing Zone
Orbit
Lightcurve Model for HD 209458b
● Relative depths transit: 2% eclipse: 0.005%
● Duration 3 hours
● Phase changes of planet
Phase
Rela
tive F
lux
Eclipse Transit
The Lightcurve from MOST
45 days
0.03 mag
● 2004 data : 14 days, 4 orbital cycles● 2005 data : 45 days, 12 orbital cycles
● duty cycle : ~90%● 473 896 observations● 3 mmag point-to-point precision
2005 observations, 40 minute binned data
Albedo Results
● Best fit parameters: Albedo : 0.07 0.05 stellar radius : 1.346 0.005 RJup
● Other Parameters: stellar mass: 1.101 Msun inclination: 86.929 period : 3.52... days see Knutson et al. 2006
Geometric Albedo
Radiu
s (J
upit
er)
1,2,3 sigmaerror contours
Rowe et al. (in prep)
Atmospheres
MOST bandpass
Geom
etr
ic A
lbed
o● HD 209458b is darker than Jupiter● Rule out class V planet with bright reflection silicon clouds
Marley et al. 1999
Models Constraints
2004 1 sigma limit – or - ~2005 3 sigma limit
Spitzer Limit
Different atmospheres
blackbody
model
Rowe et al. 2006Rowe et al. (in prep)
best fit
Equili
bri
um
Tem
pera
ture
Direct Spectrophotometry
Proposed NASA Mission
• Nulling coronograph
• Can image Jupiter-like
planets in Earth-like orbits
Direct Spectrophotometry
• Could observe changing
cloud cover and atmospheric
conditions on gas giant
planets with highly eccentric
orbits, like HD 168443.
• Very exciting unique
opportunity to study rates for
photochemistry & forcing.