Extra-Solar Planets

Astronomy 311Professor Lee

CarknerLecture 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

StarPlanet

Centerof Mass

Vstar

Vplanet

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, 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

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 (~MJupiter) 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