Transcript Transits

Transits of exoplanets –
Detection & Characeterization
Meteo 466
Transiting planets
• If a planet’s orbital plane is
nearly aligned with the observer
on Earth, then the planet may
transit its star, i.e., it passes in
front of the star (and behind it)
• The probability of a transit depends on the
size of the planet’s orbit relative to the size
of the star
Image credit:
Jason Eastman
Ohio State Univ.
Probability of transits
i = inclination of planet’s orbit to the plane of the sky
o = angle of planet’s orbit with respect to the observer (= 90o – i)
a = planet’s semi-major axis
Rs = stellar radius
Then, the probability that a planet will transit is given by
Probability of transits
To find one jupiter at 5.2 AU from a Sun like star,
one needs to look at ~ 1 / (0.1%) ~ 1000 stars !
To find one hot-jupiter around a Sun like star,
one needs to look at ~ 1 / (10%) ~ 10 stars !
Radius of the planet
The radius of the planet is related to the fractional
change in the flux of the star:
Radius of the planet
Radius of the star
change in
the stellar
Image credit:
Jason Eastman
Ohio State Univ.
Transit geometry
• 2 (ingress), 3 (egress)
• b – impact parameter (projected
distance between the planet
and star centers during
Different impact parameter
(or inclination) results in different
Transit durations.
Seager & Mallen-Ornelas, 2003,
Astrophysical Journal
Limb Darkening
• Arises due to variations in temperature
and opacity with altitude in the stellar
• Light from the limb follows an oblique
Path, and reaches optical depth of unity
at a higher altitude where the temperature
Is cooler.
Radial velocity curve for HD
209458 b
First transiting hot Jupiter
Planetary characteristics:
– M = 0.69 MJ
– Orbital period = 3.5 d
Odds of seeing a transit are equal
P = Rs/a
Rs = radius of star
= 7105 km for the Sun
a = planet semi-major
= 0.04 AU (1.5108 km/AU)
= 6106 km
P  0.1
T. Mazeh et al., Ap. J. (2000)
Transiting giant planet HD 209458 b
Ground-based (4-inch aperture)
Hubble Space Telescope
• In 1999, about 10 hot Jupiters were known; hence, the
chances that one would transit were good
• Jupiter’s radius is 0.1 times that of the Sun; hence, the
light curve should dip by about (0.1)2 = 1%
• Hot Jupiters have expanded atmospheres, so the signal is
D. Charbonneau et al. Ap. J. (2000)
T. M Brown et al., Ap. J. (2001
Primary transit spectroscopy
Habitable Planets book, Fig. 12-4
• Primary transit is when the planet passes in front of the star
• The planet appears larger or smaller at different wavelengths
depending on how strongly the atmosphere absorbs
• Hence, the transit appears deeper at wavelengths that
are strongly absorbed, allowing one to form a crude spectrum
Transmission spectroscopy
Transmission spectroscopy
Higher temperatures or lower mean molecular weight or lower
gravity increases the scale height
⇒ puffier atmosphere
Image Credit: NASA, ESA, and G. Bacon (STScI)
First detection of an extrasolar
planet atmosphere (HD 209458 b)
Sodium ‘D’ lines
• Sodium was detected in this
spectrum taken from HST
• H2O was also detected
(next slide)
Planetary radius vs. wavelength
D. Charbonneau et al., Ap. J. (2002)
HST observations of HD209458b
Green bars – STIS data
Red curves – Baseline model with H2O (solid) and without (dashed)
Blue curve – No photoionization of Na and K
T. Barman, Ap.J. Lett. (2007)
Transit of HD 209458 b observed in Ly 
• Transit depth in
visible: ~1.6%
• Transit depth at Ly :
• Ratio of areas:
ALy/Avis = 14/1.6  9
• Ratio of diameters: ~3
Vidal-Madjar et al., Nature (2003)
Artist’s conception of transiting
giant planet HD 209458 b
• Hydrogen cloud
observed in Ly ,
presumably from
planetary “blowoff”
(Vidal-Madjar et al.,
Nature, 2003)
• Note: Evidently, this
observation is
controversial (may not
be correct)
Secondary Eclipse
Figure by Sara Seager
Flux from the planet
Peak flux:
Sun ~ 0.58 micron
Hot-Jupiter > 3 micron
(1 micron = 10-4 cm
= 10,000 Ang)
= 1000 nano meter)
Short-wavelength flux peak due to
Scattered light from the star at visible
Long-wavelength flux peak due to
Thermal emission and is estimated
by a black-body of planet’s effective
radiating temperature
Flux from the planet
(a closer look)
Peak flux:
~ 0.58 micron
Hot-Jupiter > 3 micron
~ 10 micron
Flux ratio (~ 8 micron):
Hot-jupiter/Sun ~ 10-3
~ 10-8 !!!
Also, the flux ratio is favorable
where the flux from the star &
planet is high (more photons)
Is there an instrument/telescope that is sensitive
in the thermal IR that can be used to observe
& study hot-jupiter atmospheres ??
Spitzer Space Telescope
• 0.85 m mirror,
cryogenically cooled,
Earth-trailing orbit
• Intended to study
dusty stellar nurseries, the centers of
dusty stellar
nurseries, centers of
galaxies, molecular
clouds, AGN.
Spitzer IRAC Band pass
Secondary transit spectroscopy
HD 189733b
Period = 2.2 days
Radius = 1.1 Jupiter Radii
Flux drop on a 0.8 solar radii star
Is ~ 2.5 %
Longitudinal map
Secondary eclipse
Primary eclipse
Flux varying ?
Knutson (2007), Nature
HD 209458b: Evidence for a
thermal inversion
Model (with H2O in absorption)
• High fluxes at 4.5 and 5.8 m represent emission by H2O,
rather than absorption
H.A. Knutson et al., ApJ 673, 526 (2008)
• Conclusions from transit data on HD209458b
– HST curves (visible/near-IR primary eclipse
photometry) show H2O at approximately solar
– Spitzer curves (thermal-IR secondary eclipse
photometry) show H2O in emission  the atmosphere
must have a thermal inversion
– Ly  data (Vidal-Madjar et al., Nature, 2003) show
evidence for escaping hydrogen (transit is 9 times as
deep in Ly )
Tip of the iceberg
HD 189733b & HD 209458b, both hot-jupiters, were extensively (and still are
being) studied by Spitzer
A whole range of hot-jupiters & low-mass planets were discovered after them
Only Warm Spitzer (3.6 and 4.5 micron) working now
Orbiting a late F star (or early G)
Mass = 1.41 MJ
Radius = 1.79 RJ
Period = 1.09 days ( 0.0229 AU)
= 2516 K
Hottest, largest radius,
shortest period and most irradiated planet
at the time of the discovery
Secondary Eclipse
Spitzer & Ground IR observations of
Madhusudhan et al.(2011), Nature, 469, 64
Model + observations
Major species : H2O, CO2, CO & CH4
With solar [C/O] = 0.54, H2O & CO are dominant
CO2 and CH4 are least abundant
The data indicates weak H2O features and strong CH4
& CO features.
Implies there is more carbon, possibly [C/O] >=1
Photochemical model for WASP-12b
Kopparapu, Kasting & Zahnle(2011), ApJ
Spectra by Amit Misra, U. Washington
Flux from the planet
(a closer look)
Peak flux:
~ 0.58 micron
Hot-Jupiter > 3 micron
~ 10 micron
Flux ratio (~ 8 micron):
Hot-jupiter/Sun ~ 10-3
~ 10-8 !!!
Also, the flux ratio is favorable
where the flux from the star &
planet is high (more photons)
GJ 1214b
Star GJ 1214:
M3 spectral type
Mass = 0.157 M
Radius = 0.211 R
Distance = 40 lightyears
Planet GJ 1214b:
Mass = 6.3 Earth mass
Radius = 2.67 Earth radii
Semi-major = 0.014 AU
= 1.6 days
GJ 1214b spectrum
GJ 1214b current status
HST and Spitzer space observations have shown that the transmission
spectrum is broadly flat from the near- to mid-infrared.
Exclude molecular features expected for a cloud-free hydrogen-rich
Either a water-vapor atmosphere, or the presence of clouds or thick hazes
in a hydrogen atmosphere
Photochemistry predicts methane & water dominant.
Finding M-star planets using
• Presentation to the ExoPTF
by Dave Charboneau
(February, 2007)
• Relative radii:
M star
• Thus, the light curve for
Earth around a late M star is
about as deep (~1%) as for
Jupiter around a G star
• The HZ around an M star is
also close in  transits are
reasonably probable
• Transiting giant planet HD 209458b
(D. Charbonneau et al. Ap. J., 2000)
James Webb Space Telescope
• JWST will be a 6.5-m
thermal-IR (cooled)
• Scheduled deployment:
• JWST can be used to
measure secondary
transit spectra (like
Spitzer) on planets
identified from groundbased observations
• Our first spectrum of a
habitable world may
come from a planet
orbiting an M star!
Observing transits from space
• Future space-based missions will be able
to do transit studies at much higher
contrast ratios
RJup/RSun  0.1
 contrast = (0.1)2 = 0.01
REarth/RSun  0.01  contrast = (0.01)2 = 10-4
COROT mission (ESA)
• 30-cm aperture
• Launched Dec. 27, 2006
• Must point away from the
Sun  can only look for
planets with periods <75
days, i.e., a < 0.35 AU
around a G star
• Planetary radius:
R > 2 REarth
• Could conceivably find
“hot ocean planets”, i.e.,
water-rich rocky planets
orbiting close to their
parent stars
Kepler Mission
(Will be discussed in detail later)
• This space-based telescope
will point at a patch of the
Milky Way and monitor the
brightness of ~100,000 stars,
looking for transits of Earthsized (and other) planets
• 105 precision photometry
• 0.95-m aperture  capable
of detecting Earths
• Launched: March 6, 2009
December 2011 data release
Candidate size
Rp < 1.25
1.25 < Rp < 2.0
2.0 < Rp < 6.0
6.0 < Rp < 15
Very-large-size 15 < Rp < 22.4
Number of
• 48 of these planets are within their star’s habitable zone
• 600 l.y. distant
• 2.4 RE
• 290-day orbit, late G
• Not sure whether
this is a rocky planet
or a Neptune
(RNeptune = 3.9 RE)
Transit Timing Variations (TTV)
Delta t - Timing deviation
- Mass of perturber
Kepler 9b & 9c
Holman & Murray (2005) Science
Kepler -16b
Mass = 0.3 Jupiter
Radius = 0.75 Jupiter
Period = 228 days
For a stable orbit, a
circumbinary planet
has to be 7 times as far
from the stars as the
stars were from each
Kepler-16b is only half
the binary star distance.
Pandora ?