Download Goal: To understand how Saturn formed and

Goal: To understand how we
were able to first find planets
orbiting other stars
Objectives:
1) To understand the Orbits of planets and stars
2) To understand how these Orbits produce
movements
3) To understand the Doppler Effect
4) To understand how to use observations to
pinpoint the mass and distance
5) To learn about what we have found.
Orbital velocity/period
• For any other planet in our solar system:
• Velocity = 30 km/s * (distance in AU)-0.5
• (Orbital Period)2 = constant * (distance in AU)3
http://antwrp.gsfc.nasa.gov/apod/image/0705/earth_galileo_big.mov
Problem:
• Stars are billions of times brighter than
planets.
• Using normal telescopes and normal
viewing methods spotting a planet directly
is very improbable.
Newton’s Third Law
• Newton’s Third Law states that for any
force there is an equal an opposite force.
• The star exerts a gravitational force on the
planet.
• The planet therefore has to exert the same
force on the star!
Quick question:
• If the planet exerts the same force on the star as
the star exerts on the planet, what will happen to
the star (which is thousands to millions times
more massive than the planet)?
• A) nothing
• B) it will be moved by a lot less than it moves the
planet
• C) it will be moved the same amount as the
planet
• D) it will be moved a lot more than the planet
How would we find this motion?
• Stars are very far away.
• So, side to side motion is not viewable.
• Long time frames make it difficult.
However, what about its motion towards and
away from you?
Radial velocity
• However we can observe radial velocity!
• How? By using the Doppler effect!
• When an object moves towards us, the
wavelengths of light it emits (or sound on earth)
decrease (because the object is closer to us
when the wave finishes than when it starts
• the shrink in the wave is the distance the object
travels in the time it takes to make the wave
• The fraction of the increase/decrease of the
wavelength just depends on the velocity of the
object!
So how do we know what it should
be emitting
• There are different elements in a star
which emit at specific wavelengths when
at rest.
• By observing the emissions around that
wavelength we can get the Doppler shift.
Diagram from
http://csep1
0.phys.utk.e
du/astr162/l
ect/binaries/
spectroscopi
c.html
First Discovery 1995
http://zebu.uor
egon.edu/5
1peg.html
Complications:
1) Stars move!
Stars have “proper motions”
which can be hundreds of KILOMETERS per
second which is hundreds of thousands of
meters per second
2) Stars rotate!
Stars spin with velocities that
are in the order of meters per second.
3) The earth moves! And rotates!
4) Observe at night
http://zebu.uor
egon.edu/5
1peg.html
What we can find
• The time it takes so do a full sine wave is
the orbital period of the planet around the
sun.
• To get anything more we would need to
know the mass of the star the planet is
orbiting.
Finding the mass of a star
• Masses of stars are found by examining
the spectrum of the star
• The spectrum tells you what type of star
the star is
• The spectrum tells you the temperature
the outside of the star is
• Since each mass/type has its own
temperature/spectrum then you can find
the mass of the star
Now we can find:
• The orbital distance
• The bigger a star the closer it orbits to a
star for some given period.
• That is because it pulls more on the planet
making it go faster.
Also can find:
• Mass of the planet
• The distance of the planet combined with
the orbital period will tell you how large the
planet has to be to make the star moves
as it does.
• The bigger the planet or the closer to the
star it is then the more the star moves.
Planets found
• Jupiter massed or bigger objects very near
to the star
• Hot Jupiters
Why?
• Large mass large effect
• Close to star shorter the period, less
telescope time
How
• Interactions with the planetary disc
• Terrestrials destroyed
• Life on moons?
The results:
•
•
•
•
•
http://planetquest.jpl.nasa.gov/
http://exoplanet.eu/catalog.php
1321 planetary systems
2074 planets
4696 candidates
• http://exoplanet.eu/catalog-RV.php (the
page with the list)
Conclusion
• We have learned how most of the
Exosolar planets have been detected even
if indirectly
• We have examined the complications with
this style of observation
• We have gazed on the rewards of the
efforts of great Astronomers to find many
times more planets around other stars
than orbit our sun.