PHY6795O – Chapitres Choisis en Astrophysique
Naines Brunes et Exoplanètes
Chapter 6 – Transit
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
2
6.1 Introduction
 Principle
 Photometric method consisting of measuring the host star flux variation
due to the planet primary and secondary eclipses.
 Most prolific detection method.
 Provide an estimate of the planet radius/temperature through
primary/secondary eclipse.
 Powerful technique for atmosphere characterization.
 For a solar type star, transit depth ΔF/F is ~10-2 (10 mmag) for a
Jupiter, ~10-4 for an Earth.
 Detection much easier on small (M) stars (R★~0.1-0.5 R)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
3
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit Light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
4
6.2 Transit searches (1)
 Ground-based surveys can easily reveal giants but space is generally
required for detecting Earths and super-Earths.
 First mention of this technique (along with radial velocity) by Struve
(1952).
 Observing strategy is to monitor tens of thousands of relatively
brights stars (V<13) over weeks or months if not years (e.g. Kepler)
 Transit probability is typically 1%.
 Data processing
 Automatic field recognition matches to reference catalogues (e.g. 2 MASS),
providing an astrometric solution to a few arcsec.
 Differential aperture photometry + ‘’detrending’’, i.e. correction of non-Gaussian
noise (e.g. ramp effect on HST).
Kreidberg et al. 2014
PHY6795O – Naines brunes et Exoplanètes
6. Transit
5
6.2 Transit searches (2)
 Candidate identification
 Searches typically adopt a box-least squares algortithm (Kovacs et al.
2002) with coarse search grid to identify epoch, depth and duration of
strongest signals.
 Search refined using analytical transit profile models.
 False positives
• Eclipsing binaries w/o background stars. Equal mass grazing binaries.
Binaries including giant stars are excluded based on their reduced proper
motion H which is correlated with the absolute magnitude.
 Given an estimate of the stellar mass and radius, planet radius and
impact parameter are then derived from light curve models.
 Candidate confirmation
 Most promissing candidates subjected to radial velocity follow-up measurements
• Small RV amplitude combined with transit signature imply i~90 °
• A few RV measurements required to exclude double- and single-lined
binaries.
• A few tens of RV measurements typically required to confirm a single planet.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
6
6.2 Transit searches (3)
 Host star parameters determined from RV observations, more
generally from high resolution spectroscopy.
 Teff and log g derived from diagnostic lines such as Hα, Na I D and Mg I
 Then M★ and R★ from stellar evolutionary models.
 Planet parameters estimated combined χ2 fit to the light curve and
RV measurements. Details depend on eccentricity and adopted limb
darkening model.
 Exoplanet transit data bases
 NASA/IPAC/NExScI Star and Exoplanet Database
• www.nsted.ipac.caltech.edu
 Exoplanet Transit Database of the Czech Astronomical Society (ETD)
• www.var.astro.cz/ETD
 Amateur Exoplanet Archive (AXA)
• www.brucegary.net/AXA/x.htm
PHY6795O – Naines brunes et Exoplanètes
6. Transit
7
6.2 Transit searches (4)
Large-field searches from the ground
 The Hungarian Automated Telescope (HAT/HATNet)
 Hat north is six Cannon 11 cm cameras, 2kx2k CCD, all controlled and automated
with a single Linux computer.
 Precision of 3-10 mmag at I~8-11
 29 discoveries as of March 2015.
 OGLE (Optical Gravitational Lensing Experiment)
 1.3m telescope also used for micro-lensing.
 8 detections by the transit method so far out of 17.
 Trans-Atlantic Exoplanet Survey (TrES)
 Three 0.1m wide-field (6°) telescopes.
 Emphasis on bright stars.
 Five planets discovered so far.
 Wide-Angle Search for Planets (WASP/SuperWASP)
 Two-wide field cameras: La Palma (Canary Islands) and Sutherland (S. Africa)
 Each telescope uses 8 2kx2k CCDs with FOV of 15°x20° (RA, Dec)
 65 planets discovered so far.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
8
6.2 Transit searches (5)
Large-field searches from the ground
 XO
 Two 0.1m telescopes on Haleakala (Maui; Hawaii). Equipment similar to TrES.
 Drift scan mode.
 5 discoveries so far.
 MEarth




Eight 0.4m automated telescopes at Mt Hopkins. In operation since January 2008.
Sample of 2000 nearby M dwarfs with masses between 0.1 and 0.35 M.
Sensitive to Earth-size planets.
One (spectacular) detection of a super-Earth: a 6.5 ME around GJ1214 (13 pc;
Charbonneau et al. 2009).
 Radial velocity discoveries
 Planets first discovered by RV then detected in transit.
 Ex: HD189733b (P=2.219 d, MP=1.15 MJ) with large transit depth (3%). Prime
target for atmospheric studies.
 Long-period planets found by small telescopes and amateur participation. Ex:
111d period of HD80606b (MP=4 MJ, e=0.93). First transit detected through
secondary eclipse from SPITZER at 8 μm. (Laughlin et al 2009). Prediction of
primary eclipse on 14 February 2009 confirmed by three independent groups.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
9
6.2 Transit searches (6)
Large-field searches from the ground
 Searches in open clusters
 Advantage of sampling host stars of known age.
 Disadvantage: relatively few bright stars available.
 No detection so far.
 Globular clusters
 Central core of 47 Tuc surveyed in a 8-d observation by HST. No candidate found.
 Ground-based observations of outer region of 47 Tuc (22 000 stars over 33 nights)
and for ω Cen (31 000 over 25 nights). No detection.
 Based on known planet frequency, more than 20 might have been detected in the
combined (core and halo) surveys of 47 Tuc.
 Planets in dense core with a=1 AU should survive disruptions by stellar
encounters. Should be stable despite several close stellar encounters.
 Null results likely suggest that planet formation has a significant dependency on
metallicity.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
10
6.2 Transit searches (7)
Large-field searches from space
 Space avoids limitations imposed by variable atmospheric extinction
(~0.1%) and scintillation (0.01%).
 Enable long uninterrupted observations.
 CoRoT
 Led by CNES (France). Launched on 2006 December 27th.
 Two science missions: transit and asteroseismology.
 0.27m telescope on polar orbit, 4 2kx2k CCDs. Long (150 d) continuous
observations towards Galactic center and anti-center.
 12 000 target stars in transit mode.
 Photometric precision of ~700 ppm at R=15
 28 detections so far including the super-Earth CoRoT-7b (P=0.853 d,
MP= 4.8 ME)
• Follow-up RV measurements have enveilled another (non-transiting) planets.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
11
6.2 Transit searches (7)
Large-field searches from space
 Kepler
 Launched on 2009 March 6 on Earth-trailing heliocentric orbit.
 0.95 Schmidt telescope, 42 2kx1k CCD , 115 sq degrees.
 150 000 main sequence stars (8-15) in Cygnus region, monitored
continuously over 3.5 years, four measurements per hour.
• Mission lasted nearly six years.
 All stars with K<14.5 were characterized before launch.
 Main mission was to determine ηEarth , the fraction of stars with a planet
in the habitable zone. Goal partly achieved.
 Very successful mission:
•
•
•
•
1000 confirmed planets
3600 candidates
2165 eclipsing binaries
Most typical planets are super-Earths
 K2 (Kepler extension)
 Mission extension with two reaction wheels + Sun radiation pressure.
 Enables a 83d continuous monitoring on the ecliptic.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
12
6.2 Transit searches (8)
Kepler FOV and photometric accuracy
PHY6795O – Naines brunes et Exoplanètes
6. Transit
13
6.2 Transit searches (9)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
14
6.2 Transit searches (10)
Follow-up observations from space
 Hubble Space Telescope
 Observations mostly in spectroscopic mode
 Recent observations with WFC3 enables spectroscopy at R~70 between
1.4 an 1.7 μm with an accuracy of 20-30 ppm in spatial scanning mode,
performance very close to the photon noise limit (as good as it can get).
• e.g. transit spectroscopy of GJ1214b (Kreidberg et al. 2014).
 Spitzer Space Telescope
 0.85 IR telescope launched in 2003
 Three instruments: the IRAC camera operating at 3.6, 4.5, 5.8 and 8 μm, the IRS
spectrograph (5.3-37μm) and the MIPS camera (24, 70 and 160 μm).
 First detection of light from an exoplanet: HD208458b (Deming et al. 2005).
 Exoplanet studies is a significant scientific legacy of Spitzer even though the
mission was not designed for such science programs!
 Hipparcos
 ~100 photometric measurements between 1990-93 for 118 000 stars
 Enabled posteriori measurements of known transits, e.g. to improved the orbital
period (e.g. HD209458b and HD189733).
PHY6795O – Naines brunes et Exoplanètes
6. Transit
15
6.2 Transit searches (11)
Follow-up observations from space
 MOST (Microvariability and Oscillations of Stars)





Canadian-built. 0.15m telescope launched in 2003.
Allows 60d continuous observations between dec of -19 and +36.
Asterosesimology main science mission.
Important upper limits on the albedo of HD209458b.
Detection of a few transiting systems (e.g. 55 Cancri e)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
16
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
17
6.3 Noise limits (1)
 Limitations from space
 Stellar surface structures (star spots, plages, granulation and non-radial
oscillations)
• Strong functions of spectral type.
• Same limitations for RV and astrometry.
 For Kepler, at V=12, 10 min sampling, smallest detectable planetary
radii for 4.5 Gyr old G2, K0 and K5 stars, given a total of 3 or 4 transits,
were found to: 1.5, 1.0 and 0.8 RE.
 Limitations from the ground
 For nearby, relatively bright stars and moderate telescope apertures,
contribution from photon noise is generally negligible.
 Main limitations are:
• Atmospheric transparency variations
• Atmospheric scintillation noise
• Detector “granularity” (intra-pixel sensitivity).
– Can be corrected with high-precision autoguiding and/or defocussing the telescope
to spread light over several pixels.
 Accuracy limited to a few mmag over relevant integration times.
• Ground-based telescopes limited to transit depths up to about 1%.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
18
6.3 Noise limits (2)
Conjugate-plane photometry to improve scintillation noise
(Osborn et al. 2011)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
19
6.3 Noise limits (3)
Conjugate-plane photometry to improve scintillation noise
(Osborn et al. 2011)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
20
6.3 Noise limits (4)
Reduction of scintillation noise by a factor of ten possible with
conjugate-plane photometry to improve scintillation noise
PHY6795O – Naines brunes et Exoplanètes
6. Transit
21
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
22
6.4 Transit light curves (1)
 There are four observables that characterize the duration
and profile of the primary transit: the period P, the transit depth
ΔF, the time interval between 1st and 4th contact tT , and the time interval
between 2nd and 3rd contact tF..
PHY6795O – Naines brunes et Exoplanètes
6. Transit
23
6.4 Transit light curves (2)
 Three geometrical equations describing the light curve
 Transit probability
(6.5)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
24
6.4 Transit light curves (3)
 Simplified expression for tT (circular orbit)
(6.4)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
25
6.4 Transit light curves (4)
Theoretical light curves - limb darkening
 Limb darkening refers to the drop of intensity in a stellar image
moving from the centre to its limb. Intensity is represented by
functions of μ=cos θ, where θ is the angle between the normal to the
stellar surface and the line of sight to the observer.
 Limb darkening depends on spectral type and wavelength (weak in
the red)
 Limb darkening law
(6.6)
 The linear (c1=c3=c4=0) or
the quadratic (c1=c3=0) form is
usually a good approximation.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
26
6.4 Transit light curves (5)
Theoretical light curves - limb darkening
PHY6795O – Naines brunes et Exoplanètes
6. Transit
27
6.4 Transit light curves (6)
Light curve fitting
 Light curve equation (for small RP)
(6.9)
 Three parameters to fit (e.g. MCMC methods) :
 RP/R★
 a/R★
 b=a cos i/R★ (impact parameter)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
28
6.4 Transit light curves (7)
Physical parameters derived from the light curve
 Assumption of circular orbit, no limb darkening and no
contamination from background (blended) sources. Equations 6.1-6.3
can be rewritten as
(6.13)
(6.14)
(6.15)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
29
6.4 Transit light curves (8)
Physical parameters derived from the light curve
 Invoking Kepler’s third law
(6.16)
an expression for the stellar density ρ★ star can be derived from
equation 6.15, assuming MP<<M★,
(6.17)
A unique solution can be imposed on the dimensionless ratios by
invoking the stellar mass-radius relation
(6.18)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
30
6.4 Transit light curves (9)
Physical parameters derived from the light curve
where k is a constant, distinct for main sequence or giants, and x is
the corresponding power law. The five physical parameters R★, M★, i,
a and RP can be derived,
(6.19)
(6.20)
(6.21)
(6.22)
(6.23)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
31
6.4 Transit light curves (10)
Physical parameters derived from the light curve
(6.26)
 Estimate of the stellar density allows a discrimination between dwarfs
(ρ★~1 g/cm3) and giants (ρ★~0.1-0.01 g/cm3).
 If M★ and R★ are assumed known from the spectral type, then the
problem is over-constrained, and Equation 6.26 can be re-arranged to
give an expression of the orbital period, even if only a single transit is
observed.
(6.27)
P can be derived this way to 15-20% if δt < 5 min and photometric
accuracy is < 0.0025 mag.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
32
6.4 Transit light curves (11)
Reflected light
 A planet of radius RP at distance a from the star intercepts a fraction
(RP /2a)2 of the stellar luminosity. For RP << R★ << a, the planet/star
flux ratio, ε, can be written
(6.38)
where p(λ) is the geometrical albedo, the ratio of brightness at zero
phase (seen from the star) to that of a fully reflecting, diffusively
scattering (Lambertian) flat disk with the same cross-section. For a
Lambert sphere, p(λ)=2/3. The phase function g(α) is
(6.39)
where α is the angle between star and observer subtended at the
planet, i the orbit inclination and ϕ is the orbital phase, with ϕ=0 at
the time of radial velocity maximum.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
33
6.4 Transit light curves (12)
Secondary eclipse
 Total light just before the time of the secondary eclipse is the sum of
the stellar flux and that of the planet. The difference corresponds to
the flux of the planet’s day-side region. During the secondary eclipse,
the total light is that of the star alone.
 Infrared photometry of the secondary eclipse provides an estimate of
the planetary temperature. In the Rayleigh-Jeans limit (
),
the depth of the secondary eclipse is
(6.41)
For RP=1 RJ, R★ =1 R and planet/star planet temperatures of 1500
K and 6000 K,
.
 Probability of an observable secondary eclipse depends on the orbital
parameters, and particular eccentricity and argument of pericenter.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
34
6.4 Transit light curves (13)
Rossiter-McLaughlin effect
 As the planet transits, the radial velocity signal is slightly altered as a
small positive or negative anomaly in the radial velocity curve, caused
by the progressive occultation of the rotating stellar disk.
 Provides information on the star’s spin axis orientation with respect to
the planet’s orbital plane.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
35
6.4 Transit light curves (14)
Rossiter-McLaughlin effect
 The maximum amplitude of the anomaly is
(6.42)
where v sin i★ is the projected stellar equatorial rotation velocity. For
a Sun-like star, v sin i★ ~ 2 km/s, and the maximum size of the effect
is around 20 m/s for a Jupiter-like planet, and around 0.2 m/s for an
Earth.
 Spectroscopic monitoring of ΔV tracks the planet’s trajectory referred
to the sky-projected stellar rotation axis.
 Relevance to formation and migration.
 Close-in giants explained by migration. Disk migration acting alone may
largely preserve the initial spin-orbit alignment.
 Planet-planet scattering or Kozai migration would produce at least
occasionally large misalignments.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
36
6.4 Transit light curves (15)
Rossiter-McLaughlin effect
PHY6795O – Naines brunes et Exoplanètes
6. Transit
37
6.4 Transit light curves (16)
Transit timing variations
 The time of the transit event and its duration can be affected by
several factors:




gravitational bodies, including other planets and/or satellites
Tidal forces
Relativistic precession
Apparent effects due to changes in geometrical projection (proper motion
and parallax) as viewed by the observer.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
38
6.4 Transit light curves (17)
Transit timing variations – Kepler 9b/9c
PHY6795O – Naines brunes et Exoplanètes
6. Transit
39
6.4 Transit light curves (18)
Transit timing variations - satellite
 For a circular satellite orbit, the displacement of the planet
with respect to the planet-satellite barycentre is
(6.54)
where as and Ms are the satellite’s semi-major axis and mass. For
circular co-planar orbits, the peak-to-peak difference between the
mid-transit point for the planet and system barycentre is then
(6.55)
 Ex: a 1 ME orbiting HD209458b at a maximum distance of the Hill
radius, RH, the amplitude of the timing excursion about the mean
orbital phase is 13s, comparable to the present standard error on the
central time of a single transit with Kepler.
 PESTO @ OMM will have a timing resolution of ~0.1s !
PHY6795O – Naines brunes et Exoplanètes
6. Transit
40
6.4 Transit light curves (19)
Transit timing variations – parallax and space motion
 For an exoplanet orbit coplanar with that of the Earth-Sun system,
observers at the two extremes of the Earth’s orbit would register a
given transit event displaced in time by
(6.57)
Am: mean orbital radius (1 AU)
P: orbital period of the Earth
d★: distance to the star
P=400d, d★= 10 pc, Δt~5 sec.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
41
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
42
6.5 Transmission and emission spectroscopy (1)
Introduction
 Both transit and secondary eclipse probe an exoplanet’s atmosphere.
 In both situations, observations are made in the combined light of the starplanet system.
 Conditions in the planetary atmosphere are deduced from the differences in
flux as the planet moves in front/behind the star.
 Both measurements at the limit of what can be achieved from the ground.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
43
6.5 Transmission and emission spectroscopy (2)
Introduction
 The planet-star contrast varies significantly from the optical (10-5-106) to the mid-infrared (10-3-10-4).
 Optical-mid-infrared features several molecular bands of H20, CO,
CO2 and CH4.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
44
6.5 Transmission and emission spectroscopy (3)
Transmission spectroscopy
 Transmission spectroscopy probes the other regions of the
atmosphere.
 The area of the planetary atmosphere intercepted is approximately
an annulus of radial dimension ~5H where H is the scale height
(6.58)
with μm is the mean molecular weight, T the atmosphere
temperature, gp the planet surface gravity, and k the Boltzman’s
constant.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
45
6.5 Transmission and emission spectroscopy (4)
Transmission spectroscopy
 The fractional contribution of the transmission signal, δ, is given by the ratio
of the annular to stellar areas
(6.59)
 δ depends on wavelength through the mean molecular weight, related to the
chemical composition of the atmosphere.

 It it relatively difficult to detect atmosheric features characterized by a high
mean molecular weight.
 δ can also be written in terms of the planet density ρp:
with
PHY6795O – Naines brunes et Exoplanètes
6. Transit
46
6.5 Transmission and emission spectroscopy (5)
Expected transmission signal for various exoplanets
Host
Name
Tp
(K)

(g/cm3)
R★
(R)
δ
(ppm)
H2-rich
m=2
H2O-rich
m=18
Earth
m=29
Hot Jupiters/Neptunes
G0V
HD209458b
1130
0.37
1.14
700
-
-
M3V
GJ436b
700
1.5
0.42
800
-
-
Super Earths
M4V
GJ1214b
600
2
0.2
2300
250
160
K1V
HD97658b
800
3.4
0.7
150
20
10
Earths
M3V
TESS-xxx
600
5.5
0.2
-
95
60
M3V
TESS-xxx
300
5.5
0.2
-
50
30
PHY6795O – Naines brunes et Exoplanètes
6. Transit
47
6.5 Transmission and emission spectroscopy (6)
Emission spectroscopy – secondary eclipse
 Provides a measure of the planet’s thermal emission and associated spectral
features.
 Emission spectrum contains information about the atmosphere’s
temperature and gradient.
 Absorption lines indicate a temperature profile decreasing with height.
 An isothermal profile would produce a featureless emission spectrum.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
48
6.5 Transmission and emission spectroscopy (7)
Emission spectroscopy – secondary eclipse
 The equilibrium temperature of the planet is related to the star temperature
by
(6.60)
where T★ is the stellar effective temperature, R★ the stellar radius, a the
semi-major axis of the planet and AB is the Bond albedo. AB is the fraction of
bolometric flux (integrated over all wavelengths) scattered by the planet.
 Typical albedos (geometric and Bond) for Jupiter and gas giants: 0.3-0.5.
 Appears to be very small for Hot Jupiters. Ex: HD209458b; AB < 4%.
 The factor f describes the effectiveness of atmospheric circulation, and the
degree to which the energy absorbed is transferred from the planet’s day to
night sides.
 f=1 means a perfect heat redistribution (uniform temperature between night and
day sides).
 f=2 means no circulation. Night-side remains cold.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
49
6.5 Transmission and emission spectroscopy (8)
Observations – HD209458b
Host star: V=7.7, G0V (M★=1.01 M) at 47 pc.
Planet’s properties: MP=0.69 MJ, RP=1.35 RJ, P=3.5d.
Most intensively observed transiting planets
Large transit depth (2%).
Density, surface gravity and escape velocity (43 km/s) indicate that
the planet is stable against disruption by tidal forces, thermal
evaporation or mass stripping by the stellar wind.
 Na, CO, H2O, VO, TiO and CH4 detected it its atmosphere.
 Evidence for clouds.
 Temperature inversion resulting from high-altitude absorbers.





PHY6795O – Naines brunes et Exoplanètes
6. Transit
50
6.5 Transmission and emission spectroscopy (9)
Observations – HD209458b
PHY6795O – Naines brunes et Exoplanètes
6. Transit
51
6.5 Transmission and emission spectroscopy (10)
Observations – HD189733b





Host star: V=7.7, K0V (M★=0.8 M) at 19 pc.
Planet’s properties: MP=1.13 MJ, RP=1.14 RJ, P=2.2d
Another intensively observed transiting planet.
Large transit depth (2.5%).
Day-side temperature mapped by Spitzer
PHY6795O – Naines brunes et Exoplanètes
6. Transit
52
6.5 Transmission and emission spectroscopy (11)
Observations – HD189733b
PHY6795O – Naines brunes et Exoplanètes
6. Transit
53
6.5 Transmission and emission spectroscopy (12)
Observations – HD189733b
PHY6795O – Naines brunes et Exoplanètes
6. Transit
54
6.5 Transmission and emission spectroscopy (13)
Observations – HD189733b
PHY6795O – Naines brunes et Exoplanètes
6. Transit
55
6.5 Transmission and emission spectroscopy (14)
Observations – Wasp-43b (Stevenson et al. 2014)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
56
6.5 Transmission and emission spectroscopy (15)
Observations – Wasp-43b (Stevenson et al. 2014)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
57
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
58
6.6 Properties of transiting planets (1)
Mass-Radius relation (as of 2010)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
59
6.6 Properties of transiting planets (2)
Mass-Radius relation (as of 2015)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
60
6.6 Properties of transiting planets (3)
Mass-Radius relation for stars and planet
 n ~3 for low-mass stars with large
radiative cores.
 n=3/2 for fully convective stars
(<0.4 M)
 Objects below H-burning minimum
mass supported by electron
degeneray
 n~1 for M~ MJ
PHY6795O – Naines brunes et Exoplanètes
6. Transit
61
6.6 Properties of transiting planets (4)
Kepler’s results
PHY6795O – Naines brunes et Exoplanètes
6. Transit
62
6.6 Properties of transiting planets (5)
Kepler’s results
PHY6795O – Naines brunes et Exoplanètes
6. Transit
63
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
64
6.7 Future projects – TESS
PHY6795O – Naines brunes et Exoplanètes
6. Transit
65
6.7 Future projects – TESS
PHY6795O – Naines brunes et Exoplanètes
6. Transit
66
6.7 Future projects – TESS
PHY6795O – Naines brunes et Exoplanètes
6. Transit
67
6.7 Future projects – TESS
PHY6795O – Naines brunes et Exoplanètes
6. Transit
68
6.7 Future projects – TESS
PHY6795O – Naines brunes et Exoplanètes
6. Transit
69
6.7 Future projects – TESS
PHY6795O – Naines brunes et Exoplanètes
6. Transit
70
6.7 Future projects – JWST
2018
PHY6795O – Naines brunes et Exoplanètes
6. Transit
71
6.7 Future projects – JWST
Four science instrument all with observing capabilities to
perform transit spectroscopy
NIRCam
FGS/NIRISS
2018
MIRI
PHY6795O – Naines brunes et Exoplanètes
NIRSpec
6. Transit
72
6.7 Future projects – JWST
JWST for transit/eclipse spectroscopy
HST
PHY6795O – Naines brunes et Exoplanètes
6. Transit
73
6.7 Future projects – JWST
FGS/NIRSS overview
 Two instruments in one box provided by CSA
 FGS (Fine Guidance Sensor)
 Provides fine guiding to the observatory
 0.6-5 μm IR camera. No filters, single optical train
with two redundant detectors each with a FOV of
2.3’x2.3’
2018• Noise equivalent angle (one axis): 4 milliarcsec
• 95% sky coverage down to JAB=19.5
 NIRISS (Near-Infrared Imager and Slitless Spectrograph)
 0.6-5 μm IR camera.
 Four observing modes
 Main science drivers
• First Light: high-z galaxies
• Exoplanet detection and characterization
PHY6795O – Naines brunes et Exoplanètes
6. Transit
74
6.7 Future projects – JWST
NIRISS Single Object Slitless Spectroscopy (SOSS) mode
 Specifically optimized for transit spectroscopy
 Broad simultaneous wavelength range: 0.6-2.8 um
 Spectral resolution: 700 @ 1.2 um
 Grism with built-in defocussing weak lens to increase
2018
dynamic range and minimize systematic ‘’red noise’’ due to
undersampling and flatfield errors.
 Similar capability to HST’s scanning mode
PHY6795O – Naines brunes et Exoplanètes
6. Transit
75
6.7 Future projects – JWST
Harware implementation
2018
Monochromatic PSF measured in the lab
λ
20 pixels
PHY6795O – Naines brunes et Exoplanètes
6. Transit
76
Kreidberg et al, 2014
HST data. ~30 ppm noise level, within ~10% of the
photon noise limit !
77
6.7 Future projects – JWST
NIRISS will miss very few Earth/Super-Earths found by TESS
NIRISS Saturation
limit
2018
Figure courtesy of George Ricker (TESS PI)
PHY6795O – Naines brunes et Exoplanètes
6. Transit
78
6.7 Future projects – JWST
NIRISS transit spectroscopy simulations – Hot Jupiter
2018
JWST/NIRISS, HD189733 (K1V)
J=6.0, 2.7 hrs (1 transit)
Efficiency: 33%
HST
Noise level: 25 – 100 ppm
PHY6795O – Naines brunes et Exoplanètes
Model courtesy of J. Fortney
6. Transit
79
6.7 Future projects – JWST
NIRISS simulations – Water rich super-Earth
2018
JWST/NIRISS, GJ1214 (M4.5V)
J=9.8, 8.4 hrs (3 transits)
Efficiency: 83%
HST
Noise level: 25 – 100 ppm
PHY6795O – Naines brunes et Exoplanètes
Model courtesy of J. Fortney
6. Transit
80
6.7 Future projects – JWST
NIRISS simulations – Earth size HZ water-world + M3V
2018
JWST/NIRISS, Earth + M3V, 13 pc
J=8, 27 hrs (15 transits; 350 days )
Efficiency: 33%
Noise level: 10 – 20 ppm
PHY6795O – Naines brunes et Exoplanètes
6. Transit
81
Contents
6.1 Introduction
6.2 Transit searches
6.3 Noise limits
6.4 Transit light curves
6.5 Transmission and emission spectroscopy
6.6 Properties of transiting planets
6.7 Future projects
6.8 Summary
PHY6795O – Naines brunes et Exoplanètes
6. Transit
82
6.8 Summary (1)
 Most prolific detection method (~1000 known objects), most from
Kepler
 Powerful technique for exoplanet atmosphere characterization but
has to be done from space (HST, Spitzer, JWST)
 Transit depth
 Secondary eclipse depth
 Transit probability
 Physical parameters R★, M★, i, a and RP can be derived from the
transit light curve given a mass-radius relation.
PHY6795O – Naines brunes et Exoplanètes
6. Transit
83
6.8 Summary (2)
 Atmospheric signal from transit spectroscopy
 Equilibrium temperature derived from secondary eclipse
observations
 Future projects: K2, TESS, PLATO & JWST
PHY6795O – Naines brunes et Exoplanètes
6. Transit
84