So far in this class we have discussed several techniques astronomers use to discover exoplanets. The "wobble method" was an easy way to describe what is officially called the radial velocity method. Here the wobble is detected by Doppler shifts in the spectral lines of the star. This was the first method astronomers used to discover exoplanets. The Kepler probe used the transit method to discover exoplanets. If the orbit is aligned in our line of sight, Kepler could measure the very slight dip in brightness as the small planet crosses in front of the star. In unit 2 (Einstein) we briefly mentioned a technique known as microlensing. Here an exoplanet can be detected as it passes between us and some distant light source (like a quasar). The gravity of the planet acts like a lens to produce a small brightening of the distant light source. This is how rouge planets (exoplanets not orbiting a star) have been discovered .
Your job is to discover 3 other techniques astronomers have used to discover exoplanets. Please explain each technique (in your own words) and report how many exoplanets were found with this technique. As always, include your sources.
As one of the three techniques, please include the technique known as astrometry. Many web sites bunch this together with the radial velocity method, but it really is different. You need to make it clear what these two methods have in common but more important, how they are different.
Pulsars are burnt out cores of dead stars which rotate many times a second. As they spin, they send out pulses of radiation at an extremely regular rate (hence the name). If a planet happens to orbit a pulsar, the gravitational tug it exerts on the pulsar will be picked up by receiving a oscillating change in the pulsation rate (very much like the Doppler effect). This is how the first exoplanet was discovered in 1992. This method is now known as Pulsar Timing.
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Sometimes two stars eclipse each other in a system (eclipsing binaries). A lot of information about the stars can be ascertained from this (as you will learn in unit 5). However, if the orbits of the stars lie on a plane in our line of sight it is likely any planets orbiting either star will also lie on the same plane and undergo a transit. It then becomes a bonus opportunity to discover an exoplanet as well. This one is really just the transit method but if you mention this example specifically, I'll take it.
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There is a technique similar to the transit method that does not rely on the exoplanet to transit the parent star. It is known as orbital brightness. Imagine a planet orbiting its star in such a way that is just misses a transit (from our point of view). As this exoplanet orbits its parent star, the amount of light reflected off the planet (towards us) will vary. Therefore, combined light of the star and light reflected from the planet will vary over time ... but unlike the transit method, you look for slight increases in brightness in a regular pattern.
A variation of this technique is when a planet is close to a star, it changes the shape of the star itself. So, if the shape of the star or the light from the star has changed over time, then it was caused by a planet.
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Technology has advanced so much that astronomers are now able to actually see the planets directly (since all planets reflect the light from their sun). This method is called direct imaging. I wouldn't expect you to know this but this technique requires some extremely sophisticated methods of eliminating the light of the star so the much fainter planet can be seen. This is like trying to find a bug flying around a lighthouse. Click here for more information.
A variation of this technique uses polarized light to detect planets with atmospheres. When light reflects off the atmosphere of a planet, it becomes polarized (much like light that reflects off shiny objects ... which is why you purchase polarized sunglasses).
I hope I'm around in 2025 to witness the
proposed
WFIRST space telescope. This project will take direct images of
exoplanets by deploying a "starshade" to block the light of the parent star.
Way cool!
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The last method is known as astrometry. This method is often lumped with the radial velocity method but is actually completely different. Let me explain. Both methods rely on discovering exoplanets by observing a wobble in the star the planet orbits. However, the radial velocity method relies on observing the Doppler shift (of the spectral lines) as a star approaches or recedes from the observer. It can instantly measure a star's speed with great accuracy. So how is astrometry different from this?
Please refer to the animations below. In the first two animations, the radial velocity method would work just fine since the orbit of the planet forces the star to wobble in your line-of-site. However, tilt the system so that the orbit is perpendicular to the line-of-site (bottom one) and the radial velocity method fails completely. This is because the wobble never causes the star to approach or recede from the observer and there is no Doppler shift observed in the star.
Astrometry is a painstakingly difficult process of actually observing the wobble on time lapse photos. Over time, the motion of the star traces out the portrait of a snake (because the star is also moving, as all stars do). However, it is like trying to observe the wobble of a slightly out of balance car wheel from a distance of 100 miles. This page does a good job showing astrometric methods (click on Jupiter to see our sun wobble).
The animations below show what an observer should observe over time (with the wobble highly exaggerated).
Imagine the blue planet is moving
towards and away from you.
Now imagine the blue planet stays on the plane of the screen (like the animation below)
Credit: Zhatt
Wikipedia
Can you see that if the system is pointed toward us
like this (face on), there is no Doppler effect observed from the star due to
the tug of the planet.
Note: Astrometry has been around for a long time and has been used to make claims of the discovery of exoplanets as far back as the late 1700's, .... but with little to show for it. Watch for a big change in the track record of astrometry when the space-based observatory GAIA (which has been launched) starts sending in data. As of 2017, there has been only one confirmed exoplanet discovered with this method. The updated page is here.
