Transits of exoplanets – Detection & Characeterization

Meteo 466

• 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

Transiting planets

Image credit:Jason EastmanOhio 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 fractionalchange in the flux of the star:

Fractional change in the stellar flux

Radius of the planet

Radius of the star

Image credit:Jason EastmanOhio State Univ.

Transit geometry

Seager & Mallen-Ornelas, 2003, Astrophysical Journal

• 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 differentTransit durations.

Limb Darkening• Arises due to variations in temperatureand opacity with altitude in the stellarAtmosphere

• Light from the limb follows an obliquePath, and reaches optical depth of unityat a higher altitude where the temperatureIs 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

to:

P = Rs/awhere

Rs = radius of star = 7105 km for the Suna = planet semi-major

axis = 0.04 AU (1.5108 km/AU) = 6106 km

HenceP 0.1 T. Mazeh et al., Ap. J. (2000)

http://obswww.unige.ch/~udry/planet/hd209458.html

Transiting giant planet HD 209458 b

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

• 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

Ground-based (4-inch aperture) Hubble Space Telescope

Primary transit spectroscopy

• 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

Habitable Planets book, Fig. 12-4

Transmission spectroscopy

http://www.exoclimes.com/topics/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

Planetary radius vs. wavelengthD. Charbonneau et al., Ap. J. (2002)

• Sodium was detected in this spectrum taken from HST• H2O was also detected (next slide)

HST observations of HD209458b

T. Barman, Ap.J. Lett. (2007)

Key: Green bars – STIS dataRed curves – Baseline model with H2O (solid) and without (dashed)Blue curve – No photoionization of Na and K

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)

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)

http://en.wikipedia.org/wiki/HD_209458_b

Secondary Eclipse

Figure by Sara Seager

Flux from the planetPeak flux:

Sun ~ 0.58 micronHot-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 visibleWavelength

Long-wavelength flux peak due to Thermal emission and is estimated by a black-body of planet’s effectiveradiating temperature

Flux from the planet(a closer look)

Peak flux:

Sun ~ 0.58 micronHot-Jupiter > 3 micronEarth ~ 10 micron

Flux ratio (~ 8 micron):

Hot-jupiter/Sun ~ 10-3

Earth/Sun ~ 10-8 !!!

Also, the flux ratio is favorable where the flux from the star & planet is high (more photons)

10-3

10-8

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, centers of galaxies, molecular clouds, AGN.

http://www.spitzer.caltech.edu/about/index.shtml

dusty stellar nurseries, the centers of galaxies

Spitzer IRAC Band pass

http://www.nasa.gov/mission_pages/spitzer/news/070221/index.html

Secondary transit spectroscopy

HD 189733bPeriod = 2.2 days

Radius = 1.1 Jupiter RadiiFlux drop on a 0.8 solar radii starIs ~ 2.5 %

Primary eclipse (transit)

Secondary eclipse (occultation)

Knutson (2007), Nature

Flux varying ?

Longitudinal map

HD 209458b: Evidence for a thermal inversion

• 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)

Data

Model (with H2O in absorption)

• 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 )

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

Wasp-12bOrbiting a late F star (or early G)

Mass = 1.41 MJ

Radius = 1.79 RJ

Period = 1.09 days ( 0.0229 AU)Teq = 2516 K

Hottest, largest radius, shortest period and most irradiated planet at the time of the discovery

Secondary Eclipse

Spitzer & Ground IR observations of WASP-12b

Madhusudhan et al.(2011), Nature, 469, 64

Model + observationsMajor 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), ApJSpectra by Amit Misra, U. Washington

Flux from the planet(a closer look)

Peak flux:

Sun ~ 0.58 micronHot-Jupiter > 3 micronEarth ~ 10 micron

Flux ratio (~ 8 micron):

Hot-jupiter/Sun ~ 10-3

Earth/Sun ~ 10-8 !!!

Also, the flux ratio is favorable where the flux from the star & planet is high (more photons)

10-3

10-4

M-star

GJ 1214bStar GJ 1214:

M3 spectral typeMass = 0.157 MRadius = 0.211 RDistance = 40 lightyears

Planet GJ 1214b:Mass = 6.3 Earth massRadius = 2.67 Earth radiiSemi-major = 0.014 AUPeriod = 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 atmosphere

• 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 transits

• Presentation to the ExoPTF by Dave Charboneau (February, 2007)

• Relative radii:Sun 1Jupiter 0.1M star 0.1-0.3Earth 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 209458b (D. Charbonneau et al. Ap. J., 2000)

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!

http://www.jwst.nasa.gov/about.html

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

http://www.esa.int/esaSC/120372_index_0_m.html

Kepler Mission

http://www.nmm.ac.uk/uploads/jpg/kepler.jpg

• 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• 105 precision photometry• 0.95-m aperture capable of detecting Earths• Launched: March 6, 2009

(Will be discussed in detail later)

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

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)

http://www.nasa.gov/mission_pages/kepler/news/kepscicon-briefing.html

Transit Timing Variations (TTV)

http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=60

TTV• Holman & Murray (2005) Science

Delta t - Timing deviationM2 - Mass of perturber

Kepler 9b & 9c

Mass = 0.3 JupiterRadius = 0.75 JupiterPeriod = 228 days

Kepler -16b(Tatooine)

For a stable orbit, a circumbinary planethas to be 7 times as far from the stars as the stars were from each other.

Kepler-16b is only halfthe binary star distance.

http://www.nasa.gov/mission_pages/kepler/multimedia/index.html

Pandora ?