Exploring Extrasolar Planets: HST Studies of Transiting Planets and Their Atmospheres

1. Planets In Transit:The Shadow Knows! David Charbonneau California Institute of Technology www.astro.caltech.edu/~dc STScI May Symposium – 3 May 2004

2. Overview: Transits and Atmospheres • By making use of the transiting geometry of one system, HST has enabled the first direct studies of an extrasolar planet, including: • Accurate determination of the planetary radius • Searches for planetary moons and rings • Studies of the planetary atmosphere and exosphere • These studies also serve to develop these HST-based techniques, in anticipation of planets yet-to-be-discovered: • Wide-field, ground-based surveys • The Kepler Mission • HST is also being used to search for new transiting planets

3. The Radial Velocity Surveys

4. Transit Characteristics • Probability pt = Rstar / a = 0.1 • Depth d I / I = (Rpl / Rstar)2 = 0.01 • Period P = 3 – 7 days • Duration t = 3 hours

5. The First Transiting Planet:HD 209458b • Numerous groups (Henry et al.; Charbonneau et al.; Jha et al.; Deeg et al. ) have presented ground-based photometry (~0.2%) • Uncertainty in Rp is dominated by uncertainty in Rs Charbonneau et al. (2000)

6. The First Transiting Planet:HD 209458b • Numerous groups (Henry et al.; Charbonneau et al.; Jha et al.; Deeg et al. ) have presented ground-based photometry (~0.2%) • Uncertainty in Rp is dominated by uncertainty in Rs • HST photometry (~0.01%) breaks this degeneracy • Best estimates are: Charbonneau et al. (2000) Brown et al. (2001)

7. Understanding the Planetary Radii • Radius results from slowing of contraction, not due to “puffing up” (Burrows et al. 2000) • Initial models assumed energy was deposited deep in the atmosphere • “Colder” models require additional energy source • tidal circularization (Bodenheimer et al. 2001) • atmospheric circulation (Showman & Guillot 2002) • Discrepancy is increased if a large planetary core is included Showman & Guillot (2002) Bodenheimer et al. (2001)

8. HST STIS Photometryof HD 209458

9. Unphased HST STIS Photometryof HD 209458

10. Planetary Satellitesfrom Photometric Residuals

11. Planetary Satellitesfrom Transit Timing

12. FGS Transit Timing • Schultz et al. (2002) have observed several transits with HST Fine Guidance Sensors • FGS provide very rapid cadence (S/N = 80 in 0.025 s) • These data target times of ingress and egress Schultz et al. (2002)

13. Atmospheric Transmission Spectroscopy • Compare transit depth at various wavelengths (Seager & Sasselov; Hubbard et al.; Brown) • Where strong atmospheric opacity is present, the planet will appear larger, and hence the transit will seem deeper Brown (2001)

14. Detection of anExtrasolar Planet Atmosphere

15. Detection of Sodium Absorption • The transit appears deeper by 2.3 x 10-4 when observed at the sodium resonance lines near 589nm • This is ~1/3 the expected value for a cloudless atmosphere with a solar abundance of sodium in atomic form Charbonneau, Brown, Gilliland, & Noyes (2002)

16. HST/STIS Transmission SpectrumNEAR FUTURE Charbonneau et al. (2002)

17. New STIS Observations(290 – 1020 nm) Charbonneau, Brown, Gilliland, & Noyes (2003)

18. Detection of an Evaporating Atmosphere of Hydrogen • Vidal-Madjar et al. (2003) detect a very large (15%) transit depth at Ly a • At this radius, hydrogen atoms are no longer gravitationally bound – planet is losing mass • Liang et al. (2003) model the photochemical processes and determine that photolysis of H2O could result in a concentration of atomic H 3x greater than Jovian atmosphere • More recently, Vidal-Madjar et al. (2004) have claimed a detection of C & O in the exosphere Vidal-Madjar et al. (2003)

19. NICMOS Search for Water in HD209458b • 3 visits of 5 orbits each; 1st visit has occurred • Search for water features in 1.1 – 1.9 mm bandpass • Orbit-to-orbit sensitivity could reach S/N ~ 36,000 (if instrumental effects can be corrected) Brown (2003) Gilliland (2004)

20. A Second Transiting Planet:OGLE-TR-56 b • First extrasolar planet discovered by its photometric transit (2 additional OGLE planets have recently been announced) • 1.2 day orbital period • This system is at a much greater distance, hence it is much fainter and follow-up is more difficult • HST/ACS multi-color follow-up in progress to determine accurate planetary radius (Sasselov et al. 2004) • Best ground-based estimates are: Torres et al. (2003)

21. Sleuth: The Palomar Planet Finder There is only a single known system that is bright enough to study – we need more targets in a hurry to apply these HST-based techniques SLEUTH delivers high-cadence time series photometry on roughly 10,000 stars (9 < V < 15) in a typical field centered on the galactic plane. We obtain sufficient precision on 4,000 stars to detect a close-in Jupiter-sized companion.

22. The Search Is On:There are roughly a dozen similar wide-field surveys for transiting planets circling bright stars (8 < V < 12) G. Mandushev (Lowell Obs.)

23. HST Search for Planets in 47 Tuc • 34,000 main-sequence stars were monitored for 8.3 days • Benefit of a cluster: apparent magnitude implies a stellar (and hence planetary) radius • No planets were detected, whereas 17 would have been expected based on radial velocity surveys • Implies that planets cannot form and survive in this environment, likely due to crowding and low-metallicity Gilliland et al. (2000)

24. New HST Survey Toward Galactic Bulge • Bulge is not affected by low-metallicity or high stellar density • Sahu et al. monitored field in Sgr-I window for 7 days (Feb 2004), with additional epoch in cycle 14 (proper motion) • 100,000 stars to V=23, so several dozen planets could be detected • Possibility of studying planet rate as a function of stellar type and metallicity • Blends that mimic planetary transits are a concern, but effect can be mitigated by 2-color photometry, centroiding, and, for brightest candidates, RV work Sahu et al. (2004)

25. The Kepler-HST Connection • Kepler will monitor 100,000 stars in a 10 deg square f.o.v. with a precision of better than ~ 5 x 10-5 • Primary goal: Determine the rate of occurrence of terrestrial planets around Sun-like stars • Kepler could uncover dozens of transiting gas giants during the first year of operation: These would be great targets for HST follow-up study • Kepler could uncover dozens of transiting Earth-like planets. HST/STIS would be the only instrument capable of confirming these candidates

26. Summary: The Future of HST & Transiting Planets • HST enabled the first direct studies of an extrasolar planet, including an accurate determination of the radius, a search for satellites, and a study of the atmosphere • This work allowed the community to develop these HST-based techniques, but we are in need of new targets • We need to encourage the numerous wide-field ground-based surveys to deliver more transiting planets while HST remains available • HST may soon produce transiting planets of its own via the survey of the Galactic bulge • If HST can last into the Kepler Mission timescale, it could enable detailed studies of dozens of gas giant planets (2007), and the confirmation of terrestrial planet candidates (2010)