Transit Method of Planet Hunting - Starlight Dips
If a speck of dust crosses in front of a light-bulb, do you notice it? Probably not. But if the speck were a planet crossing over a distant star, you might squint a little more carefully.
When a distant planet passes in front of its host star, that star’s brightness drops a small amount— about .001%— for the duration of the planet’s crossing, or
Transit. It’s these tiny dips in brightness that astronomers often monitor in the search for exoplanets. To get an idea how tiny, San Jose University’s Natalie Batalha told National Geographic, “Imagine you have 10,000 light bulbs and you take one away. That's the change in brightness we're looking for.”
(Lovett, National Geographic)
The amount the star dims depends on the relative sizes of the star and the planet, and so by studying the light-curve, astronomers can determine the size of the planet relative to its star. We can also learn more about a planet by studying the flipside of transit— that is, when a host star blocks the planet, in what's known as a
secondary eclipse. By taking measurements of the star’s electromagnetic radiation during and after this eclipse, and noting the difference in value, astronomers can detect temperature and cloud formations.
Another advantage to this method is that once a planet has been detected using the transit method (alone, or in addition to other methods), it can in turn detect other nearby planets using a technique called
Transit Timing Variation (TTV). By noting the variations in the timing of the planets transit, TTV serves as an extremely sensitive method capable of detecting additional planets in the system as small as Earth.
(Charbonneau, Protostars and Planets)
However, the transit method also has its limitations. The first is, as previously mentioned, our limited angle. Only a fraction of those systems viewed will be tilted such that the planets will pass in front. More specifically, the probability our random alignment with a planet orbiting a sun-sized star at the same orbital distance as the earth is a mere 0.47%.
(Hidas, UNSW Extrasolar Planet Search- RAS)
Also, there is an unfortunately high rate of false detections; to really confirm a sighting, three or four transits must be observed. And while showing us a planet's size, transit reveals little of its mass, or the details of its orbit. For this, we turn to the second primary method of exoplanet detection, which employs one of the universe’s more reliable forces: gravity.
Next in the Planet Hunting series:
Tune in for the next part of our planet hunting series as we investigate the
The Radial Velocity Method, and trace the wobbling of distant stars for signs of a planetary companion.
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