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Extra-Solar Planets
Astronomy 311
Professor Lee Carkner
Lecture 24
Finding Exoplanets
How do you find a planet around another star?
Planets are much too faint to be seen with a
telescope

Find exoplanets by looking at stars:
Doppler spectroscopy

Transits
Look for a dimming of the star when the planet
passes in front of it
The Doppler Effect
When you observe a moving object, the
wavelengths of light you observe change
Moving away -Moving towards --
Example: the change in a car’s sound as it
moves past you

By measuring the shift of lines in a spectrum,
you can determine how fast the object is moving
Doppler Effect
Doppler Spectroscopy

Line shifts position over time
As the planet moves around in its orbit, the
velocity of the star should go from positive to zero
to negative and back to positive again
Can plot the data to find the period of motion
Inducing
Stellar
Motion
No shift
Red shift
Blue shift
No shift
Vplanet
Center
of Mass
Star
Planet
Vstar
Orbits of a Star+Planet System
Light Curve of 51 Peg
Transits

The planet will block some of the starlight as
it transits

By measuring the degree and length of
the dimming the size and orbit of the
planet can be found
The Kepler mission may be able to find Earthsized planets
Transit Light Curve
Planetary Properties

From the period we can get the orbital
radius

From the velocity, we can get the planetary
mass

From the amount of dimming in a transit,
we can get the planetary radius
Bigger planets block out more light
Star --
What is a Planet?
Mass > 0.08 MSun (84 MJupiter)
Brown Dwarf -Mass > 10 MJupiter
Planet -Mass < 10 MJupiter
Planets and brown dwarfs can be hard
to tell apart
Known Exoplanets

More are being discovered all the time
Masses range from ~0.01 - 10+ MJupiter
Orbits range from ~0.02 - 6 AU

Large velocities and short periods are
easier to measure
Sample
Exoplanets
Data
Exoplanet Orbits

Most systems have only one known planet
but we are starting to find more

Long term observations are needed to see the
longer periods

Are the nearly circular orbits of our solar system
atypical?
Velocity Plots for Upsilon And
System
Orbits in Upsilon And System
A Multiple Exoplanet System
Orbit Evolution

It should be too hot close to the star to form
giant planets (no icy planetesimals)

The best theory holds that large planets form
in the outer protoplanetary disk and then
move inward due to friction in the disk
The magnetic field of the star may produce a
“hole” in the inner disk, stopping the motion
before the planet hits the star
Exoplanets and Habitability
Are any of the new planets habitable?
No,

They are almost all gas giants with no surface
However,

Example: 47 UMa, Rorbit=2.1 AU
We are just starting to be able to detect Earth
sized planets

Kepler working on this now
Planetary Spectra
Space Interferometry
One idea to find low mass planets is with an
interferometer
Combine the images from many small telescopes
to produce the effect of a large telescope

Would be able to detect the movement of a
star in the sky as it is being pulled by its
planets (astrometry)

Next Time
Read Chapter 28
Summary
Recently hundreds of planets around other
stars have been found
The planets are detected by:
measuring the motions they induce in the
central star
measuring the dimming of the central star when
the planet passes in front of it
The period and velocity of the motions
allows the determination of the mass and
orbit of the planet, the transit depth gives us
planetary radius
Summary: Exoplanet
Properties
Most known exoplanets are large
(~MJupiter) and in close orbits
They may form further out and then move
in
A key goal is to find Earth-sized
planets in the habitable zone
Many systems have detected multiple
planets