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ASTR 200 : Lecture 21 Exoplanet Detection Basics Assemble yourselves into work groups of 35 people 1 Extrasolar (or Exoplanet) detection • This is a subject that had a long history, full of `what turned out to be incorrect' claims. • It is technically very challenging to detect planets around other stars. Three main techniques currently 1) Radial velocity (spectral) method on parent star 2) Transit method 3) Direct imaging method There are also a few via other techniques 4) Pulsar timing 5) Gravitational microlensing 2 Spectral method • Assume there is a Jupiter-mass planet (10 -3 the mass of the Sun) in an orbit around a 1-solar mass star, with orbital a=4 au. • Assume we see the planet's orbit 'edge on' (i=90 deg) • The star/planet is a binary star, but we only see the light from the star. (a) How fast is the planet moving (km/s) in its orbit? (b) How fast is the star moving (km/s) in its orbit around the center of mass? 3 Stellar radial velocity method • Assume there is a Jupiter-mass planet (10 -3 the mass of the Sun) in an orbit around a 1-solar mass star, with orbital a=4 au. Planet speed~15 km/s, star ~15 m/s • Assume we see the planet's orbit 'edge on' (i=90 deg) • The star/planet is a binary star, but we only see the light from the star. (c) What will be the period of the star's Doppler shift pattern for its spectral lines? 4 (d) What will be the wavelength shift for a visible line (say with wavelength 500 nm) Stellar radial velocity method • Assume there is a Jupiter-mass planet (10 -3 the mass of the Sun) in an orbit around a 1-solar mass star, with orbital a=4 au. Planet speed~15 km/s, star ~15 m/s • Assume we see the planet's orbit 'edge on' (i=90 deg) • The star/planet is a binary star, but we only see the light from the star. 5 (c) What will be the period of the star's Doppler shift pattern for its spectral lines? P=a3/2 so P=8 yr (d) What will be the wavelength shift for a visible line? v 15m/s −6 −5 Δ λ=λ =(500 nm) =25 x 10 nm∼3 x10 nm 8 c 3 x10 m/s Signal of jovian planets around other stars • Expected signal was thus : – Stellar radial velocity ~10 m/s – Periods of roughly 1-2 decades – Need to, by luck or by doing a large number of target star, find one where planetary orbit is roughly edge on 6 The BIG surprise 7 Stellar radial velocity method • Assume the Jupiter-mass planet orbiting a 1-solar mass star has orbital semimajor axis a=0.04 au (not 4 au) (e) Now what will be the period of the star's Doppler shift pattern for its spectral lines? (f) What is the orbital speed of the star in its orbit around the center of mass? (g) What will be the wavelength shift for a visible line (say with wavelength 500 nm)? 8 9 This was a big surprise • There are jovian mass planets in orbits around stars, but closer than the distance at which Mercury orbits the Sun • These objects are called 'hot Jupiters' because their equilibrium temperatures will be high. • Conventionally, many (but not all) astronomers believe that these planets formed further out away from the star (past the snow line) but then migrated into their current position 10 Hot Jupiters 11 Transiting planets • Technique of watching the light output of a star, and observing a dimming because a planet moves in front of the star and blocks some of the light coming to Earth • Suppose we were near some star that lies along the ecliptic plane, and that one of our Solar System's planets passed in front of the star. What fraction of the star's light would be blocked if the planet was – (a) Jupiter ? – (b) Earth 12 It is just the fractional area covered • Light blocked is f= 2 p 2 ⊙ πR πR 2 Rp =( ) R⊙ • So the star's intensity is now I '=I −fI =I (1−f ) which as a fraction is 2 Rp I' = 1−f = 1−( ) I R⊙ 13 An exoplanet transit • Say this was Earth transiting our Sun as viewed by an astronomer elsewhere in the the galaxy. Roughly how long (minutes? hours? days?) would the transit last? 14 The Kepler space telescope was built explicitly to do just this • Stared at one field for a few years, measuring the `photometry' (amount of light arriving) of ~100,00 stars 15 Mostly transiting planets, mostly from Kepler 16 Directly imaging planets • Why doesn't one just take a picture?? • Perhaps it is angular separation. Ground based telescopes have trouble separating object if they are closer in angular separation than 0.5” – Suppose one was an astronomer on a planet in the Alpha Centauri system, 1.3 pc from the Sun, looking back at the Sun-Jupiter system. – What is the angular separation of Jupiter when Jupiter is as far away as possible (in an angular sense) from the Sun? 17 Directly imaging planets • So, the problem is NOT angular separation, when it comes to looking for planets within ~10 pc from the Sun • The problem is CONTRAST. One is looking for a faint object near a bright one (the star). Analogy is to see a firefly near a streetlamp; the light of the firefly is easy to see in the dark, but hard when there is lots of nearby light • This is why there are techniques used to block or cancel out the light of the star 18 HR 8799 4 planets around a star 40pc away Imaging is in the near infrared 19 Mostly directly imaged giant planets 20 Reflected light • What is the ratio of reflected optical light from Jupiter to that of the Sun? Take A_jup=0.5, R_jup=70,000 km, a_jup=5.2 au Thermal planetary light • What about the THERMAL planetary light? • Turns out contrast is better in the IR 21