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 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 = 7105 km for the Sun a = planet semi-major axis = 0.04 AU (1.5108 km/AU) = 6106 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 10-3 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) 10-8 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, the centers of dusty stellar galaxies nurseries, centers of galaxies, molecular clouds, AGN. 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 % Longitudinal map Secondary eclipse (occultation) Primary eclipse (transit) 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 M-star 10-3 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 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 Jupiter M star Earth 1 0.1 0.1-0.3 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 groundbased 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 (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 • 105 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 Earth-size Super-Earths Candidate size (RE) Rp < 1.25 1.25 < Rp < 2.0 Neptune-size 2.0 < Rp < 6.0 Jupiter-size 6.0 < Rp < 15 Very-large-size 15 < Rp < 22.4 TOTAL Number of candidates 207 680 1181 203 55 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 • Delta t - Timing deviation M2 - Mass of perturber Kepler 9b & 9c Holman & Murray (2005) Science 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 ?