Detecting Transits of Extrasolar Planets and their Moons

1. Detecting Transits of Extrasolar Planets and their Moons Alea Smith Advisor: Dr. Bill Romanishin

2. Outline • Background • What is an Extrasolar planet? • Motivation • Detection Methods • TrES-3b • Detecting Moons • Results • Conclusion

3. What is an Extrasolar planet? • Extrasolar planet (Exoplanet) • Planet in orbit around a star other than our sun • Over 300 discovered • Most much more massive than Earth; Jupiter-like • Jupiter is 11 times radius of Earth & 300 times mass of Earth • Jupiter ~1/10 size of Sun Relative size of Earth and Jupiter www.wikipedia.org

4. Exoplanet Discoveries

5. Motivation • Gather more data on a planet known to transit parent star • Show amateur telescopes are capable of observations • Large telescopes expensive to operate • Difficult to get observing time Image of Transit www.transitsearch.org

6. Extrasolar Planets • Estimated that 10% of sun-like stars (main-sequence) have orbiting planets • Jupiter formed far from sun • Exoplanets orbit close to stars • Not necessarily more common than Earth-like planets • Size and orbital period make them easier to detect?

7. Direct Detection Methods • Pictures • Can’t get images around sun-like stars • Seen around brown dwarfs • Only a few observed using this method • Not much information First extrasolar planet to have ever been directly imaged. http://en.wikipedia.org/wiki/Extrasolar_planet

8. Indirect Detection Methods Exoplanet in Orbit • Doppler Technique • Gravitational tug causes stellar wobble • Star not at center of mass http://en.wikipedia.org/wiki/Extrasolar_planet http://obswww.unige.ch/~udry/planet/method.html

9. Indirect Detection Methods (cont) Brightness Variations of Two Stars with Transiting Exoplanets • Transits • Planet passes in front of star • Periodic dimming seen in light curves • A few percent dimming • ~ 30 planets discovered Relative Flux Phase www.eso.org

10. Limb Darkening • Light curve not flat but curved • Limb Darkening • Decrease in intensity from the center of the star to the edge (or “limb”) • Caused by decrease in density and temperature as distance from center of star increases • Occurs just after star is fully transiting and right before it stops transiting • Even though completely transiting star, light not as intense at edges

11. Choosing an Object • Our object discovered by (TrES) Trans-Atlantic Exoplanet Survey • Three 4” telescopes • Low resolution wide-field survey cameras detect decrease in brightness from small area in sky around star • Brightest star fading a little • Fainter star fading a lot • Telescopes like OU’s able to focus on candidate stars and find what is causing the dimming Lowell Observatory

12. Research • Observe candidate star when possible transit expected to occur • Use differential photometry and interpret light curves • Differential photometry is measurement of changes in brightness of object compared to other nearby objects • Equipment • 16 inch LX-200 telescope • AP7p charge-coupled device (CCD) camera OU’s Telescope Observatory.ou.edu

13. TrES-3 b • First detected in 2007 • 800 light-years away • Located in constellation Hercules • 1.92 MJup &1.295 RJup • Orbital period of 1.306 days • Diminishes light from star by 2.98% • Parent star ~0.9 solar masses Hercules

14. TrES-3 Star Field of TrES-3 b

15. Reducing Images • Use IRAF (Image Reduction and Analysis Facility) software for image reduction and analysis of data • Flat-fielding • Take image of uniformly illuminated surface • Records imperfections in equipment such as difference in sensitivity of pixels or dust on CCD Flat Field Image

16. Reducing Images (cont.) • Dark frame • Image taken with the shutter closed • Records noise in CCD • To get calibrated image, divide dark subtracted raw image by flat image Dark Image

17. Data for TrES-3 TrES-3

18. Detecting Moons Around Transiting Exoplanets • Moons too small for direct observation • Need another method timing of planet transits • Use exoplanet transits to determine limiting magnitudes for being able to detect moons • Create star-planet-moon system • Vary one parameter at a time see how affects limiting magnitude • Period of planet, period of moon, mass of planet, mass of moon, mass of star

19. Original System • Parameters: • Mass of planet=mass of Jupiter • Mass of moon=mass of Earth’s moon • Mass of star=mass of sun • Period of planet=1 year • Period of moon=1 month • Goal: Observe how changing one parameter at a time affects types of stars moons can be detected around

20. XCM Moon Exoplanet Star XCM Moon Exoplanet Star Center of Mass (COM) • Moon changes COM of planet • Produces changing transit times that can be observed • If planet first, planet transits earlier • If moon first, planets transits later Direction of orbit Direction of orbit

21. Corresponding Light Curves Transit center for no moon Planet without moon Planet and moon with planet first in transit transit earlier Planet and moon with moon first in transit transit later

22. Can Moons be Detected with Small Telescopes? • Once change in transit time found, need to calculate minimum number of photons needed to observe change • Need minimum of 2 exposures per change in transit time • Typical drop in light from transit ~ 1% • Need an error less than ½% to resolve • Error = 1/(signal-to-noise)  S/N=200 • (S/N)² is number of photons detected = 40,000 • Ex: If 1 exposure is 10 seconds, need to detect minimum of 4,000 photons/s

23. Limiting Magnitudes • After finding minimum number of photons needed to detect, determine limiting magnitude for system • Gives faintest star that moons can be detected around for given system • Gather data for ranges of hypothetical cases • Vary period of planet: 1 day to 12 years • Vary period of moon: 1 day to 9 years • Vary mass of planet: 1/300th MJup to10 MJup • Vary mass of moon: 10^21 to 10^24 kg • Vary mass of star: 0.1 MSun to 40MSun

24. Magnitude System • Brighter stars  more negative • Logarithmic system • One magnitude interval corresponds to factor of 100^(1/5) or 2.512 times the amount of intensity • Apparent magnitude of Vega set to zero • Sun -26.7 http://www.astronomynotes.com/starprop/s4.htm

25. Period of Planet (s) Varying Period of Planet • Range 1 day to 12 years; original value 1 year • Fainter stars seen with planets of longer periods • Range of magnitudes ~8.5 to 11.6

26. Period of Moon (s) Varying Period of Moon • Range 1 day to 9 years; original value 30 days • Fainter stars seen with moons of longer periods • Range of Magnitudes ~8.2 to 14

27. Planet Much More Massive than Moon Planet More Massive than Moon Two Objects of Equal Mass Effect of Mass moon planet moon planet planet moon

28. Mass of Planet (kg) Varying Mass of Planet • Range 1/300-10 MJup; Original value MJup • Fainter stars seen with planets of smaller mass • Range of magnitudes ~9 to 14

29. Mass of Moon (kg) Varying Mass of Moon • Range 10^21 to 10^24 kg; Original value 7x10^22 kg • Fainter stars seen with moons of greater mass • Range of magnitudes ~6 to 13.5

30. Mass of Star (kg) Varying Mass of Star • Range 0.1-40 MSun; Original value MSun • Fainter stars seen with stars of smaller mass • Range of Magnitudes ~ 9.3 to 11.5

31. Conclusion • Using OU’s telescope and varying one parameter at a time, faintest star moons can be detected around has magnitude 14 • If real system, magnitude of star known, observe change in transit time and determine information about moon • Relationship between equations useful • can be used to solve for other variables • Questions?