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Lecture

# AST101H1 Lecture Notes - Exoplanet, Doppler Spectroscopy, Methods Of Detecting Exoplanets

Department
Astronomy & Astrophysics
Course Code
AST101H1
Professor
Michael Reid

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Ast101 Lecture
November 29, 2012
- Review Session - December 10, 2-4 OISE G162
- cumulative
- twice the length of the midterms but we have four times as much time - 3 hours
- the Doppler method works on the idea that when a planet is orbiting a star, it "drags"
the star around a little circle - they orbit their common center of mass
- we cannot actually see the planet, but rather notice changes in the star's spectral lines
- this means that the star is being dragged toward and away from us as it orbits the
common center of mass
- A star is orbited by an exoplanet. When the star is moving away from us, the star's
spectral lines will be redshifted
- A star is orbited by an exoplanet. When the planet is moving away from us, the star's
spectral lines will appear blue-shifted
- can be represented on a radial velocity curve (radial velocity is motion towards or
away from us)
- Imagine two exoplanets orbiting the same star. Planet A has a mass half that of Jupiter
and orbits at 5 AU. Planet B has a mass twice that of Jupiter and orbits at 5.1 AU. Which
one will produce a bigger Doppler shift in the star's spectrum?
- a more massive planet will have a greater force of gravity on the planet star
- since in the equation it is distance squared, a small change in distance will create a
larger change in gravity
- in this case though, the difference in mass matters more because it is a factor of
- the one that is more massive will produce a larger effect on the star
- we can use the magnitude of the Doppler shift to calculate the mass of the unseen
planet
- radial velocity planets: 496 planets in 384 solar systems
- most radial velocity planets are called "hot Jupiters" - they are typically gas giants very
close to their parent stars
- some of them are so close that they are evaporating when they get close to their parent
star
- the abundance of hot Jupiters is an observational bias - the methods of detecting
planets that we have is really good at ﬁnding these types of planets and really bad at
ﬁnding other types
- aside: what about multiple planets orbiting the same star?
Method 2: The Transit Method
- a transit is the phenomenon of a planet passing in front of a star and blocking out a
portion of its light
- transits are like eclipses