Download Detecting Transits of Extra

Detecting Transits of
Extrasolar Planets and
their Moons
Alea Smith
Advisor: Dr. Bill Romanishin
Outline
Background

What is an Extrasolar planet?
Motivation
Detection Methods
TrES-3b
Detecting Moons
Results
Conclusion
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

Relative size of Earth and Jupiter
Jupiter is 11 times radius
of Earth & 300 times mass
of Earth
Jupiter ~1/10 size of Sun
www.wikipedia.org
Exoplanet Discoveries
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
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?
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
Indirect Detection Methods
Doppler Technique


Exoplanet in Orbit
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
Indirect Detection Methods (cont)
Transits



Planet passes in front of
star
Periodic dimming seen
in light curves
A few percent dimming
~ 30 planets discovered
Relative Flux

Brightness Variations of Two Stars with
Transiting Exoplanets
Phase
www.eso.org
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
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
Lowell Observatory
Telescopes like OU’s able to focus on candidate stars
and find what is causing the dimming
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


OU’s Telescope
16 inch LX-200
telescope
AP7p charge-coupled
device (CCD) camera
Observatory.ou.edu
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
Star Field of TrES-3 b
TrES-3
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
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
Data for TrES-3
TrES-3
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
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
Center of Mass (COM)
Moon changes COM of planet

Produces changing transit times that can be observed
If planet first, planet transits earlier
XCM
Direction of orbit
Moon
Exoplanet
If moon first, planets transits later
Star
XCM
Direction of orbit
Exoplanet
Moon
Star
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
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
Limiting Magnitudes
After finding minimum number of photons needed to
detect, determine limiting magnitude for system
 min # of photons / sec
mag  log 
 P QA r n t m 10 0.4 Xk
 Vega tel





 0.4
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
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
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
Varying Period of Planet
Period
Planet(s)
(s)
Planet
of of
Period
1000000000
100000000
10000000
1000000
100000
10000
8
9
10
Limiting Magnitude
11
12
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
Varying Period of Moon
Period
of Moon
(s)(s)
of Moon
Period
1000000000
100000000
10000000
1000000
100000
10000
8
9
10
11
Limiting Magnitude
12
13
14
Effect of Mass
Planet Much More Massive than Moon
moon
planet
Planet More Massive than Moon
moon
planet
Two Objects of Equal Mass
planet
moon
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
Varying Mass of Planet
(kg)
ofofPlanet
Mass
Mass
Planet (k
1E+29
1E+28
1E+27
1E+26
1E+25
1E+24
9
10
11
12
Limiting Magnitude
13
14
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
Varying Mass of Moon
1E+24
Mass
of Moon(kg)
(s)
of Moon
Mass
10
1E+23
1E+22
1E+21
1E+20
6
7
8
9
10
11
Limiting Magnitude
12
13
14
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
Varying Mass of Star
Mass
Star(kg)
(k
ofofStar
Mass
1E+32
1E+31
1E+30
1E+29
9
10
11
Limiting Magnitude
12
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?