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 Fractional change in the stellar flux 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 mid-transit) 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 Atmosphere 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 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) http://obswww.unige.ch/~udry/planet/ hd209458.html

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)

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 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 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 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)

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 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

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 ~ 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. dusty stellar nurseries, the centers of galaxies http://www.spitzer.caltech.edu/about/ index.shtml

Spitzer IRAC Band pass

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

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

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)

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-12b Orbiting 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 + 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: Sun ~ 0.58 micron Hot-Jupiter > 3 micron Earth ~ 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) 10-4 M-star 10-3

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 Period = 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 1 Jupiter 0.1 M star 0.1-0.3 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 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 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 http://www.nmm.ac.uk/uploads/jpg/kepler.jpg

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 deviation M2 - Mass of perturber Kepler 9b & 9c

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

Pandora ?