extra-solar planets astronomy 311 professor lee carkner lecture 24
Post on 18-Dec-2015
217 Views
Preview:
TRANSCRIPT
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
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
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
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
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?
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
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
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
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
top related