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Towards a Physical Characterization of Extrasolar Planets

Towards a Physical Characterization of Extrasolar Planets. Sara Seager Carnegie Institution of Washington. Image credit: NASA/JPL-Caltech/R. Hurt (SSC). Towards a Physical Characterization of Extrasolar Planets. Transiting Planets Models Data HD209458b Near Future Earths.

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Towards a Physical Characterization of Extrasolar Planets

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  1. Towards a Physical Characterization of Extrasolar Planets Sara Seager Carnegie Institution of Washington Image credit: NASA/JPL-Caltech/R. Hurt (SSC)

  2. Towards a Physical Characterization of Extrasolar Planets Transiting PlanetsModelsDataHD209458bNear Future Earths

  3. The Solar System Planet sizes are to scale. Separations are not. Characterizing extrasolar planets: very different from solar system planets, yet solar system planets are their local analogues

  4. Known Extrasolar Planets (As of 24 MAY 2005) Based on data compiled by J. Schneider

  5. Direct Detection Challenge • Nearby M dwarf star with brown dwarf companion • Jupiter would be • 10 x closer in • 1 million times fainter Gliese 229 and 229B - Hubble Space Telescope (Kulkarni, Golimowski, NASA)

  6. Solar System at 10 pc Fp/F* = p Rp2/a2 Fp/F* = Tp/T* Rp2/R*2 = (R*/2a)1/2[f(1-A)]1/4 Star Hot Jupiters J V E M Seager 2003

  7. P ~ (R*/a) P(0.05 AU) = 10% P(1 AU) = 0.5% P(5 AU) = 0.1 % 1 radial velocity planet is known to transit its star Geometric Transit Probability a Zone where transit can be seen from

  8. Transiting Planets Venus. Trace Satellite. June 8 2004. Schneider and Pasachoff. Mercury. Trace Satellite. November 1999. Transiting planets allow us to move beyond minimum mass and orbital parameters without direct detection. HD209458b. November 1999. Lynnette Cook.

  9. Survey thousands of stars simultaneously Measure drop in starlight due to transiting planet Huge number of false positives Over 20 groups running planet transit surveys Require radial velocity followup to determine mass Planet Transit Surveys Two OGLE transiting planets. Six short-period planets successfully discovered

  10. Survey thousands of stars simultaneously Measure drop in starlight due to transiting planet Huge number of false positives Over 20 groups running planet transit surveys Require radial velocity followup to determine mass Planet Transit Surveys Two OGLE transiting planets. Brown et al. ApJ 2001 Six short-period planets successfully discovered

  11. Why Transiting Planets? • Planetary bulk composition • H-He gas giant? • Super Earth? • Water world? • Rocky planet? • Evolutionary history • HD 209458b -- too big! • HD 149026 -- too small! Courtesy Jeremy Richardson

  12. Transiting Planets • Transit [Rp/R*]2 ~ 10-2 • Transit radius • Emission spectraTp/T*(Rp/R*)2 ~10-3 • Emitting atmosphere ~2/3 • Temperature and T • Transmission spectra[atm/R*]2 ~10-4 • Upper atmosphere • Exosphere (0.05-0.15) • Reflection spectra p[Rp/a]2~10-5 • Albedo, phase curve • Scattering atmosphere • Polarization Before direct detection Seager, in preparation

  13. Compelling Questions for Hot Jupiter Atmospheres • Do their atmospheres have ~ solar composition? • Or are they metal-rich like the solar system planets? • Has atmospheric escape of light gases affected the abundances? • Are the atmospheres in chemical equilibrium? • Photoionization and photochemistry? • How is the absorbed stellar energy redistributed in the atmosphere? • Hot Jupiters are tidally locked with a permanent day side • And are in a radiation forcing regime unlike any planets in the solar system

  14. Towards a Physical Characterization of Exoplanets Transiting Planets Models Data HD209458b Near Future Earths

  15. Giant PlanetSpectra 20 pc 0.05AU 0.1 AU 0.5 AU • dI(s,,)/ds = -(s,)I(s,,) + j(s,,); (s,) ~ T,P; T,P ~ I(s,,); • 1D models • Governed by opacities • “What you put in is what you get out” Seager, in preparation FKSI Danchi et al.

  16. Hot Jupiter Spectra Seager et al. 2000 • Scattered light at visible wavelengths • Thermal emission at IR wavelengths • Teff = 900 - 1700 K • H2O, CO, CH4, Na, K, H2 Rayleigh scattering • High T condensate clouds? MgSiO3, Fe? See also Barman et al. 2001, Sudarsky et al. 2003, Burrows et al. 2005, Fortney et al 2005, Seager et al. 2005

  17. Clouds • Spectra of every solar system body with an atmosphere is affected by clouds • For extrasolar planets1D cloud models are being used • Cloud particle formation and subsequent growth based on microphysical timescale arguments • Cloud models have their own uncertainties • Homogenous, globally averaged clouds Marley et al. 1999 Ackerman & Marley, Cooper et al. 2003; Lunine et al. 2001

  18. Photochemistry Karkoschka Icarus 1994 • Jupiter and Saturn have hydrocarbon hazes--mute the albedo and reflection spectrum • Hot Jupiters have 104 times more UV flux = more hydrocarbons? • Much higher hydrocarbon destruction rate • normal bottleneck reaction is fast • less source from CH4 • additional consequence: huge H reservoir from H2O Liang, Seager et al. ApJL 2004 Liang et al. ApJL 2003

  19. Large Range of Parameters Seager et al. 2000 • Forward problem is straightforward despite uncertainties • Clouds • Particle size distribution, composition, and shape • Fraction of gas condensed • Vertical extent of cloud • Opacities • Non-equilibrium chemistry • Atmospheric circulation of heat redistribution • Internal luminosities (mass and age dependent)

  20. Towards a Physical Characterization of Exoplanets Transiting Planets Models Data HD209458b Near Future Earths

  21. Na (Charbonneau et al. 2001) Lyman-alpha (Vidal-Madjar et al. 2003) C and O* (Vidal-Madjar et al. 2004) CO upper limit (Deming et al. 2005a) Thermal emission 24 mm (Deming et al. 2005) TrES-1 at 4.5 and 8 mm (Charbonneau et al 2005) CH4 upper limit 3.6 mm (Richardson et al. 2003a) H2O upper limit 2.2 mm (Richardson et al. 2003b) MOST albedo upper limit (Rowe et al. 2005) Observations of HD 209458 b Primary Eclipse Secondary Eclipse

  22. Thermal Emission • Detected from two transiting planets during secondary eclipse • Brightness T • HD 209458 b 24 mm • 1130 +/- 150 K • TrES-1 4.5 and 8 mm • 1010 +/- 60 K/1230 +/- 110 K • Opens the door for many more measurements Deming, Seager, Richardson, Harrington 2005 Charbonneau et al. 2005

  23. Thermal Emission: NASA IRTF 2.2 m Constraint • Secondary eclipse • Spectral peak at 2.2 m due to H2O and CO • Data from NASA IRTF • R = 1500 • Richardson, Deming, Seager 2003; • Differential measurement only • Upper limit of the band depth on either side of the 2.2 micron peak is 1 x 10-4 or 200 Jy Richardson, et. al., in prep

  24. Transmission Spectra: HST STIS and Keck • Probes planetary limb • Na (Charbonneau et al. 2002) • CO upper limit (Deming et al. 2005) • Consistent with high clouds • Or low Na and CO abundance • H Lyman alpha (Vidal-Madjar et al. 2003)

  25. Transmission Spectra: HD209458b Exosphere • 15% deep Lyman alpha transit 4.3RJ • Requires exospheric T ~ 10,000K! • High exospheric T on solar system giant planets are not well understood (order of magnitude) • EUV heating • Upper atmospheric T, atmospheric expansion, and mass loss are coupled • Escape rates are high but atmosphere is stable over billions of years • No UV followup possible

  26. Secondary Eclipse: Albedo Upper Limit from MOST • Microvariability and Oscillations of STars • Space-based photometer for stellar seismology and exoplanet studies - ppm photometry • “Suitcase” in space • 54 kg, 60x60x30 • 15-cm telescope • Single broadband filter • 380 ≤ λ ≤ 750 nm • Launch 30 June 2003 • Russian Rockot = old ICBM • Cost • Can$10M US$7M Euro$6M PI Jaymie Matthews UBC

  27. Secondary Eclipse: Albedo Upper Limit from MOST • Microvariability and Oscillations of STars • Space-based photometer for stellar seismology and exoplanet studies - ppm photometry • “Suitcase” in space • 54 kg, 60x60x30 • 15-cm telescope • Single broadband filter • 380 ≤ λ ≤ 750 nm • Launch 30 June 2003 • Russian Rockot = old ICBM • Cost • Can$10M US$7M Euro$6M PI Jaymie Matthews UBC

  28. MOST Albedo Upper Limit Rowe et al. 2005 • HD209458 b albedo < 0.25 (1) in the MOST bandpass • Jupiter’s albedo is 0.5 • HD 209458 b is dark! • MOST will reach 0.13 in current observing campaign

  29. Towards a Physical Characterization of Exoplanets Transiting Planets Models Data HD209458b Near Future Earths

  30. HD209458b: Interpretation I • Basic picture is confirmed • Thermal emission data • T24 = 1130 +/- 150 K • The planet is hot! • Implies heated from external radiation • Transmission spectra data • Presence of Na • A wide range of models fit the data Seager et al. 2005

  31. HD209458b: Interpretation II • Models are required to interpret 24 m data • H2O opacities shape spectrum • T24 is not the equilibrium T • T24 = 1130 +/- 150 K • A wide range of models match the 24 m flux/T • Teq is a global parameter of model • Energy balance, albedo, circulation regime • E.g. Teq = 1700 K implies that AB is low and absorbed energy is reradiated on the day side only

  32. HD209458b: Interpretation II • Models are required to interpret 24 m data • H2O opacities shape spectrum • T24 is not the equilibrium T • T24 = 1130 +/- 150 K • A wide range of models match the 24 m flux/T • Teq is a global parameter of model • Energy balance, albedo, circulation regime • E.g. Teq = 1700 K implies that AB is low and absorbed energy is reradiated on the day side only

  33. HD209458b: Interpretation III • Models with strong H2O absorption ruled out • Hottest models are ruled out • Isothermal hot model is ruled out by T24 = 1130 +/- 150 K • Steep T gradient hot model would fit T24 but is ruled out by 2.2m constraint • Coldest models are ruled out • High albedo required--very unusual • Cold isothermal model required to fit T24--doesn’t cross cloud condensation curves • Confirmed by MOST

  34. HD209458b: Interpretation III • Beyond the “standard models” • Low H2O abundance would fit the data • C/O > 1 is one way to reach this • See Kuchner and Seager 2005 • Solar System giant planets have 3x solar metallicity • Jupiter may have C/O >~ 1, but spectra look similar to C/O=0.5

  35. HD209458b C/O > 1

  36. HD209458b Interpretation Summary • Data for day side • Spitzer 24 microns • IRTF 2.2 micron constraint • MOST albedo upper limit • A wide range of models fit the data • Confirms our basic understanding of hot Jupiter atmospheric physics • Some models can be ruled out • Hot end of temperature range • Cold end of temperature range • Any model with very strong H2O absorption at 2.2 microns • Non standard models • C/O > 1 could fit the data

  37. Towards a Physical Characterization of Exoplanets Transiting Planets Models Data HD209458b Near Future Earths

  38. Hot Transiting PlanetsOrbiting Bright Stars • Transit [Rp/R*]2 ~ 10-2 • Transit radius • Emission spectraTp/T*(Rp/R*)2 ~10-3 • Emitting atmosphere ~2/3 • Temperature and T • Transmission spectra[atm/R*]2 ~10-4 • Upper atmosphere • Exosphere (0.05-0.15) • Reflection spectra p[Rp/a]2~10-5 • Albedo, phase curve • Scattering atmosphere Pushing the limits of telescope instrumentation Seager, in preparation

  39. Near Future Data from Seager et al. 2005

  40. Tracer Temp pv Cho et al. ApJL 2003 Near Future Data • New transiting planets orbiting bright stars • HD 209458 b • Spitzer thermal emission 3.6, 4.5, 8, 10 microns • HST/STIS primary transit • MOST albedo limit • HST/NICMOS: H2O • Spitzer • 3 transiting planets orbiting bright stars • 6 non-transiting planets • SOFIA, Kepler, JWST

  41. Hot Super Earths • New Super Earths • M=7.5 ME, P=1.9d, Rivera et al. 2005 • Msini =14 ME, P=9.5d, Santos et al. 2004 • M=18ME, P=2.8d, 4-planet system,McArthur et al. 2004 • Msini=21ME, P=2.6d, M star, Butler et al. 2004 • Solar System planet masses • Uranus: 17.2 ME • Neptune: 14.6 ME • Jupiter: 318 ME • Saturn 95 ME • What is the nature of these planets?? An Artist's depiction of the new planet orbiting Gliese 436. Credit: NASA/JPL. Credit: NASA/JPL.

  42. Towards a Physical Characterization of Exoplanets Transiting Planets Models Data HD209458b Near Future Earths

  43. Are We Alone? Are there Earth-like planets? Are they common? Do they harbor life?

  44. Terrestrial Planets • Evolution of the planetary atmosphere is determined by many factors: • atmospheric escape • gas-surface reactions • spectral energy distribution of host star • geologic activity • initial volatile inventory • active biology • atmospheric circulation will drive climate But, Venus and Earth look the same to Kepler and SIM

  45. NASA’s Terrestrial Planet Finder • Find and characterize Earth-like planets around nearby stars • Need to null out parent star by 106 to 1010 • Look for biomarker gases • Launch date: • 2014 TPF-C • 2019 TPF-I mid-IR spectra

  46. Earth as an Extrasolar Planet Modeling 1D Earth spectra is made easier by the right input data! Woolf , Smith, Traub, Jucks, ApJ, 2002

  47. Earth as an Extrasolar Planet High contrast between land and ocean causes changes in flux • rotational period • weather • presence of oceans • reconstruct map? Ford, Seager, & Turner, Nature 2001

  48. Vegetation as a Surface Biomarker S. Seager Institute for Advanced Study, Princeton, July 2002

  49. Vegetation as a Surface Biomarker S. Seager S. Seager

  50. Surface Biosignature • Chlorophyll causes strong absorption blueward of 0.7 m • Light scattering in air gaps between water-filled plant cells causes strong red reflectance • Plants absorb energy at short wavelengths for photosynthesis; reflect and transmit radiation at long wavelengths for thermal balance • Reflection favored over transmission? CO2 more accessible to plants with airgaps • Photosynthetic plants cause a global spectral signature even though Earth is not completely plant covered Clark 1993; Seager et al. 2004

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