1. Transits of exoplanets – Detection & Characeterization
Meteo 466

2. 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.

3. 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

4. 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 !

5. 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
Fractional change in the stellar flux
Image credit:
Jason Eastman
Ohio State Univ.

6. Transit geometry 2 (ingress), 3 (egress)
b – impact parameter (projected
distance between the planet
and star centers during
mid-transit)
Different impact parameter
(or inclination) results in different
Transit durations.
Seager & Mallen-Ornelas, 2003,
Astrophysical Journal

7. Limb Darkening Arises due to variations in temperature
and opacity with altitude in the stellar
Atmosphere
Light from the limb follows an oblique
Path, and reaches optical depth of unity
at a higher altitude where the temperature
Is cooler.

8. 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 to:
P = Rs/a
where
Rs = radius of star
= 7105 km for the Sun
a = planet semi-major
axis
= 0.04 AU (1.5108 km/AU)
= 6106 km
Hence
P  0.1
T. Mazeh et al., Ap. J. (2000)
hd html

9. 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
bigger
D. Charbonneau et al. Ap. J. (2000)
T. M Brown et al., Ap. J. (2001)

10. 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

11. Transmission spectroscopy

12. 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)

13. 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)

14. HST observations of HD209458b
Key: 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)

15. Transit of HD 209458 b observed in Ly 
Transit depth in visible: ~1.6%
Transit depth at Ly : ~14%
Ratio of areas:
ALy/Avis = 14/1.6  9
Ratio of diameters: ~3
Vidal-Madjar et al., Nature (2003)

16. 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)

17. Secondary Eclipse
Figure by Sara Seager

18. 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
Wavelength
Long-wavelength flux peak due to
Thermal emission and is estimated
by a black-body of planet’s effective
radiating temperature

19. Flux from the planet (a closer look)
Peak flux:
Sun ~ 0.58 micron
Hot-Jupiter > 3 micron
Earth ~ 10 micron
Flux ratio (~ 8 micron):
Hot-jupiter/Sun ~ 10-3
Earth/Sun ~ !!!
Also, the flux ratio is favorable where the flux from the star & planet is high (more photons)
10-3
10-8

20. Is there an instrument/telescope that is sensitive
in the thermal IR that can be used to observe
& study hot-jupiter atmospheres ??

21. Spitzer Space Telescope
0.85 m mirror, cryogenically cooled, Earth-trailing orbit
Intended to study dusty stellar nurseries, centers of galaxies, molecular clouds, AGN.
dusty stellar nurseries, the centers of galaxies
index.shtml

22. Spitzer IRAC Band pass

23. Secondary transit spectroscopy

24. HD 189733b Period = 2.2 days Radius = 1.1 Jupiter Radii
Flux drop on a 0.8 solar radii star
Is ~ 2.5 %
Secondary eclipse
(occultation)
Primary eclipse
(transit)
Longitudinal map
Flux varying ?
Knutson (2007), Nature

25. HD 209458b: Evidence for a thermal inversion
Data
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)

26. Conclusions from transit data on HD209458b
HST curves (visible/near-IR primary eclipse photometry) show H2O at approximately solar abundance
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 )

27. Tip of the iceberg
HD b & HD b, 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

28. Wasp-12b Orbiting a late F star (or early G) Mass = 1.41 MJ
Radius = RJ
Period = days ( AU)
Teq = K
Hottest, largest radius,
shortest period and most irradiated planet at the time of the discovery

29. Secondary Eclipse

30. Spitzer & Ground IR observations of WASP-12b
Madhusudhan et al.(2011), Nature, 469, 64

31. 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

32. Photochemical model for WASP-12b
Kopparapu, Kasting & Zahnle(2011), ApJ
Spectra by Amit Misra, U. Washington

33. Flux from the planet (a closer look)
Peak flux:
Sun ~ 0.58 micron
Hot-Jupiter > 3 micron
Earth ~ 10 micron
Flux ratio (~ 8 micron):
Hot-jupiter/Sun ~ 10-3
Earth/Sun ~ !!!
Also, the flux ratio is favorable where the flux from the star & planet is high (more photons)
10-4
M-star
10-3

34. GJ 1214b Star GJ 1214: M3 spectral type Mass = 0.157 M
Radius = R
Distance = 40 lightyears
Planet GJ 1214b:
Mass = 6.3 Earth mass
Radius = 2.67 Earth radii
Semi-major = AU
Period = 1.6 days

35. GJ 1214b spectrum

36. 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 atmosphere
Either a water-vapor atmosphere, or the presence of clouds or thick hazes in a hydrogen atmosphere
Photochemistry predicts methane & water dominant.

37. Finding M-star planets using transits
Presentation to the ExoPTF by Dave Charboneau (February, 2007)
Relative radii:
Sun 1
Jupiter 0.1
M star
Earth 0.01
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 b
(D. Charbonneau et al. Ap. J., 2000)

38. James Webb Space Telescope
JWST will be a 6.5-m thermal-IR (cooled) telescope
Scheduled deployment: 2018
JWST can be used to measure secondary transit spectra (like Spitzer) on planets identified from ground-based observations
Our first spectrum of a habitable world may come from a planet orbiting an M star!

39. Observing transits from space
Future space-based missions will be able to do transit studies at much higher contrast ratios
RJup/RSun   contrast = (0.1)2 = 0.01
REarth/RSun   contrast = (0.01)2 = 10-4

40. 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

41. Kepler Mission This space-based telescope will point at a patch of the
(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 Earth-
sized (and other) planets
105 precision photometry
0.95-m aperture  capable
of detecting Earths
Launched: March 6, 2009

42. December 2011 data release Candidate label Candidate size (RE)
Number of candidates
Earth-size
Rp < 1.25
207
Super-Earths
1.25 < Rp < 2.0
680
Neptune-size
2.0 < Rp < 6.0
1181
Jupiter-size
6.0 < Rp < 15
203
Very-large-size
15 < Rp < 22.4 55
TOTAL
2326
48 of these planets are within their star’s habitable zone

43. Kepler-22b 600 l.y. distant 2.4 RE 290-day orbit, late G star
Not sure whether this is a rocky planet or a Neptune (RNeptune = 3.9 RE)
kepler/news/kepscicon-briefing.html

44. Transit Timing Variations (TTV)

45. TTV Holman & Murray (2005) Science Delta t - Timing deviation
M Mass of perturber
Kepler 9b & 9c

46. Kepler -16b (Tatooine) 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
other.
Kepler-16b is only half
the binary star distance.

47. Pandora ?