The easiest way to imagine what Kepler is doing is to think of it as staring at the sky, taking measurements at how much light is being received from different spots of the sky at different times. Kepler is focused on one part of the sky, observing continuously, measuring for changes in the flux of light reaching the telescope.
What is Kepler looking for?
Kepler’s primary goal is to find planets. Specifically, Kepler’s aim is to identify planets and planet candidates that will help scientists understand the types of and distribution of planets in the universe. In order to find these planets, Kepler uses the transit method, with measures dips in light as a planet crosses a distant star.
What do these fluctuations in light mean?
Kepler is looking for stars that have a dip in luminosities, because this could indicate a planet transit. For a planet to ‘transit’ a star means that the planet is crossing in between us (the observers) and the star. One example of a transit closer to home is Venus. When Venus transited the sun several years ago, you might remember pictures of the transit showed a dark circle on the sun. That was the planet blocked part of the sun’s light from reaching earth.
When a planet transits a distant star, it decreases the amount of light reaching observers here. So, when Kepler measures a temporary decrease in light from a star, it could mean that a planet is transiting across the star.
Problems with the transit method
Although the transit method is conceptually simple, there are many problems that accompany it. For example, not every dim in light indicates a planet. Other possible causes for the dimming include a transiting binary star, three-star system, or two stars blending their light together. So, if Kepler detects a dip in light from a source, it COULD be, but is not necessarily, a planet. This is why the “planets” Kepler identifies are called “planet candidates.” Scientists have estimates the “false positive” rate – that is, how many sources that are not actually planets are accidentally identified as such by the Kepler mission[i]. This rate, although small, is not insignificant, thus the differentiation between confirmed planets and planet candidates.
In an ideal world...
As the name “direct imaging” implies, the best way to identify an exoplanet is to actually see it. There are two main reasons that this is hard to do, however:
High contrast ratio between luminous sun and planet (which reflects only a very small amount of light)
The atmospheric distortion blends light from distinct sources into what appears to be one source
Adaptive Optics can really help!
This is where adaptive optics comes in! Adaptive optics counteracts the atmospheric distortion. As a result, photons are received in a pattern closer to that by which they were emitted. If there were a dark spot on the star (say… caused by a transiting planet!) then ideally, that spot would also be received by the mirror. Without adaptive optics, it is very difficult if not impossible to see this spot.
Adaptive optics also allow the resolving of very bright stars and the planets next to them. Adaptive optics can measure sources within one arcsecond of each other, even if one of the soruces (a planet!) is 10 million times less luminous than the other (a star!)[ii].
Adaptive optics is really the most reliable way to confirm a planet candidate is actually a planet (or to show that it is NOT a planet).
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