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 Blue shiftRed shift

Orbits of a Star+Planet System Star Planet Center of Mass V star V planet

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 Earth- sized 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

What is a Planet?  Star --  Mass > 0.08 M Sun (84 M Jupiter )  Brown Dwarf --  Mass > 10 M Jupiter  Planet --  Mass < 10 M Jupiter  Planets and brown dwarfs can be hard to tell apart

Known Exoplanets   More are being discovered all the time  Masses range from ~ M Jupiter  Orbits range from ~ 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, R orbit =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 (~M Jupiter ) 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