Transits of exoplanets – Detection & Characeterization

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 theplanet • and star centersduring • 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 forHD 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

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 http://www.exoclimes.com/topics/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 • Sodiumwas 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) http://en.wikipedia.org/wiki/HD_209458_b

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 ~ 10-8 !!! 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 http://www.spitzer.caltech.edu/about/ index.shtml

22. Spitzer IRAC Band pass

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

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 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

28. 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

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 ~ 10-8 !!! Also, the flux ratio is favorable where the flux from the star & planet is high (more photons) M-star 10-3 10-4

34. 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

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 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)

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

39. 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

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

41. 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 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

42. December 2011 data release • 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) http://www.nasa.gov/mission_pages/ kepler/news/kepscicon-briefing.html

44. Transit Timing Variations (TTV) http://kepler.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=60

45. TTV • Holman & Murray (2005) Science Delta t - Timing deviation M2 - 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. http://www.nasa.gov/mission_pages/kepler/multimedia/index.html

47. Pandora ?