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
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 0.phys.utk.e du/astr162/l ect/binaries/ spectroscopi c.html
First Discovery egon.edu/5 1peg.html
Complications: egon.edu/5 1peg.html 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
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: planetary systems 2074 planets 4696 candidates (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.