Extra-Solar Planets Astronomy 311 Professor Lee Carkner Lecture 24
End of Semester Observing project due Friday Should be neat, legible and organized Answer questions on a separate sheet of paper Final exam Monday, 3 pm
Finding Exoplanets How do you find a planet around another star? Planets are much too faint to be seen with a telescope As the planet orbits the star, the star also orbits the planet The motion of the star is quite small, but can be detected as a slight shift in the spectral lines of the star
Finding Exoplanets
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
Searching For Exoplanets Measurements are made over a long period of time and plotted 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 We find exoplanets by noticing changes in the spectral lines that indicate a planet tugging on the star
Orbits of a Star+Planet System Star Planet Center of Mass V star V planet
Light Curve of 51 Peg
Planetary Properties From the period you can get the radius of the orbit From the velocity you can get the mass It is easiest to find large planets in small orbits Large velocities and short periods are easier to measure
What is a Planet? Star -- large enough to have hydrogen fusion Mass > Brown Dwarf -- a star not large enough to have fusion reactions Mass > Planet -- object formed by accretion of planetesimals Mass < Planets and brown dwarfs can be hard to tell apart
Known Exoplanets About 300 exoplanets are known Masses range from ~ Orbits range from ~ Searches are biased towards large planets in tight orbits
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 The velocities they produce are too small to measured via Doppler shift
Transits For orbits seen edge on, the planet passes in front of the star once per orbit We can measure and time this slight dimming with CCD cameras By measuring the degree and length of the dimming the size and orbit of the planet can be found
Transit Light Curve
Planetary Spectra
Space Interferometry One idea to find low mass planets is with an interferometer A two telescope, Space Interferometry mission (SIM) may be launched in 2015 (?) Would be able to detect the movement of a star in the sky as it is being pulled by its planets (astrometry) Very large interferometers could take spectra of planets to look for signs of life
Next Time Read Chapter 28
Summary Recently many planets around other stars have been found The planets are detected by measuring the motions they induce in the central star The period and velocity of the motions allows the determination of the mass and orbit of the planet New missions in the next 20 years will allow for the detection of many new planets, including Earth-like, habitable ones
Summary: Exoplanet Properties Most known exoplanets are large (~M Jupiter ) and in close orbits They may form further out and then move in A few are near the habitable zone We are starting to find additional planets in the systems