Exoplanets
Edge on Face on Lab 5: Detecting Exoplanet Transit Lab 5: Detecting Exoplanet Transit
• Due: March 13 (Wed) 11:59pm (e-submission) • Target: GJ 1214 b • Data: from ACAM on the William Herschel Telescope • Lectures on Exoplanets and Differential Photometry • Group Presentations: Preferred Dates & Format? Exoplanets
Extra-solar Planets (Exoplanets) How Do We Detect? Exoplanet discoveries per year
2,000 confirmed exoplanets discovered so far! Exoplanets
So how do we detect them?
● Direct Imaging ● Stellar Motions − velocities, timing, astrometry ● Light curve − transient planets Direct Imaging
• Currently heating up with adaptive optics, high contrast imaging systems • Earth-like planet around a sun-like star is 10 billion times fainter than its star • Need to find a faint object very close to a bright star
Beta Pictoris
HR 8799 Direct Imaging
• Only about ~ 20 planets directly imaged (2017) • Technically challenging Stellar Motions
• Radial velocity, timing, astrometry Stellar Motions Radial Velocity Comparisons
1995
• Best measurements now at a level of 0.5 m/s (a slow walk) Stellar Motions Radial Velocity Comparisons
• 51 Pegasi b – First planet around sun-like star • P = 4.23 days! Hot Jupiter Exoplanets How do we detect exoplanets? Light Curve Method: Transient Planets A transient planet changes blocks the light from a star = eclipse!
● Eclipse depth in terms of planet radius and star radius? Exoplanets How do we detect exoplanets? Light Curve Method: Transient Planets A transient planet changes blocks the light from a star = eclipse!
● Eclipse depth in terms of planet radius and star radius?
● Transit probability in terms of star radius and distance between the planet and star? Exoplanets How do we detect exoplanets? Light Curve Method: Transient Planets A transient planet changes blocks the light from a star = eclipse!
● Eclipse depth in terms of planet and star radii?
● Transit probability in terms of star radius and distance between the planet and star? Light Curve – Transiting Planets
HD 209458
Precision Photometry Transit Transit
Secondary Eclipse
Orbital Period
Can reach 10 parts-per-million accuracy for the brightest stars from space. Exoplanets How do we detect exoplanets? Light Curve Method: Transient Planets A transient planet changes blocks the light from a star = eclipse!
Challenges and Advantages of Detecting Transient Planets? Exoplanets How do we detect exoplanets? Light Curve Method: Transient Planets
Large Transit Planet Survey: OGLE, Kepler, Corot, … Exoplanets How do we detect exoplanets? Transient Planets The Kepler Project
The Kepler Mission: Field
FOV The Kepler Mission The Kepler Mission The Kepler Mission Exoplanets How do we detect exoplanets? Transient Planets
Example: HD209458b (1999)
Small telescope discovery Hubble Space Telescope data Exoplanets How do we detect exoplanets? Transient Planets “Information on planet atmosphere!” Exoplanets How do we detect exoplanets? Transient Planets “Information on planet atmosphere!”
Example: HD209458b (1999) Exoplanets How do we detect exoplanets? Transient Planets “Information on planet atmosphere!”
Example: HD209458b (1999) Exoplanets How do we detect exoplanets? Transient Planets “Information on planet atmosphere!” Exoplanet Density Measurement: o Density is critical to understanding the nature of planets Density measurement: Useful Diagrams Density measurement: Example Kepler-78
Star KIC 8435766 (Kepler-78) Constellation Cygnus h m s Right ascension (α) 19 34 58 Declination (δ) +44° 26′ 54″
Apparent magnitude (mV) 12
Radius (r) 0.73±0.15 R☉ Temperature (T) 5143 (± 70) K Metallicity [Fe/H] -0.08 (± 0.13) A planet was discovered in 2013 by analyzing data from Kepler space telescope. The planet was found not only transiting the star, but its occultation and reflected light from the parent star due to orbital phases were also detected. Density measurement: Example
Kepler-78b (formerly known as KIC 8435766 b) is an exoplanet orbiting around the star Kepler-78.
Mass (m) 1.69-1.85 M⊕
Radius (r) 1.12 R⊕
Bond Albedo (%) 20-60 %
-3 Density (ρ) 5.3-5.6 g cm Exoplanet Density Measurement: o Density is critical to understanding the nature of planets o Part of Lab 5 is to measure the density of a transiting planet What do we learn from transit light curve analyses? o Transit Probability, Depth, Duration and Period o Limb Darkening Effect, Ingress and Egress Transit Probability, Depth, Duration: Simple Case
o Transit Probability R*/a Geometrical 2 o Transit Depth (Rp/R* ) Configuration o Transit Duration (R*/a)P Geometry & Time ( Mass) Transit Probability, Depth, Duration: Simple Case
Solid angle traced out by the two extreme transit configurations =
Transit Probability =
Blue Circles: two extreme planet-orbit inclinations, above and below which the planet does not transit.
2R*/a: angle separation between the two extreme orbits Transit Probability, Depth, Duration: Simple Case
Condition for full transit:
Condition for grazing transit: (i: inclination – see next slides)
True Anomaly (): angle between direction of periapsis (B) and the current position of a planet (P) on an ellipse (= angular parameter that defines the position of a planet in a Keplerian orbit)
Keplerian Planet Orbit
Larger star and/or closer planet gives a high transit probability For an eccentric orbit (e: eccentricity):
Higher probability with a large eccentricity Observer
Edge on (i = 90), so always transit
Face on (i = 0), so no transit i : the angle between the angular-momentum vector of the planet’s orbit and the line of sight. Transit Duration: time during which any part of the planet obscures the disc of the star, depends on how the planet transits the host star.
Transit Length: length the planet has to travel across the disk of the star
Transit Depth
Ingress Egress
Transit Depth
Limb Darkening
Transit Depth
A small planet (e.g., Earth) requires high-precision photometry for the planet to be detected due to its shallow transit depth.