AY 102 Lecture Notes - Lecture 17: Blueshift, Light Curve, Stellar Kinematics
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26 Feb 2018
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AY Lecture 17 2-26-18
Detection and Properties of Exoplanets
What is an exoplanet?
- A planet orbiting a star different from the sun
- More than 3,400 exoplanets have been found to date.
- In a galaxy with 200 billion stars, it is likely that exoplanets are everywhere
- Probably form when their parent star forms
- Exoplanets were expected to be common, but none were known prior to 1995
- Detecting exoplanets has difficulties not encountered with our solar system
Being close to the planets in our system
- We can observe them with telescopes
- We can send space probes to get better images or to land on the world
Can this be done for exoplanets?
- In general, no
- Planets shine by reflecting starlight and, as a result, are very dim compared to their
planet stars
- From a great distance, planets can be difficult to see in the glare of their star
- Stars are so far away that sending space probes is impractical at this time
Can we see exoplanets?
- Some can be seen
- If an exoplanet is large and orbiting very far from its star, it may be either out of the
star’s glare or detetale after the iage of the star is sutrated.
• This is best done in infrared
- Only a few of 3,422 confirmed exoplanets have been seen this way
There are two major ways to detect exoplanets even if the planets are too dim to be seen
- Radial velocity method
• A planet and its star orbit their common center of mass. Even if the planet is
unseen, you can detect the wobble (small degree of movement) of the parent
star around the center of mass
- Transit method
• If an eoplaet’s orit is earl edge-on to us, the planet will periodically pass in
front of its star, temporarily dimming the star a very small amount
- These effects are very small because planets are much less massive and generally
smaller than stars
What technological advances were needed to detect the small wobbling speeds and drops in
brightness?
- Our ability to detect exoplanets took off when new instruments were developed that
could reliably measure:
• Line of sight speeds on the order of a few meters per second
• Brightness changes of much less than 1%
The center of mass is like a balancing point between two co-orbiting objects:
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