With the near-daily announcements of newly discovered planets, one may wonder: How do astronomers find new planets? Intuitively it may seem that planet detection is simply a matter of viewing distant stars and trying to observe orbiting objects around them. And while there are processes that require actually seeing the planet to classify it-- known as direct detection methods-- there are other means of detecting planets where astronomers infer the existence of a planet by monitoring other features of distant stars.
One of these indirect methods is know as the Planetary Transit Technique. This process generally consists of observing slight changes in a star's brightness and determining whether or not the those changes were caused by the motion of an orbiting planet. In order to execute this process, one must set up a detection system that takes a spectrum of the star during the transit--when the planet is in front of the star--and again when the star is not transiting. By comparing the two spectra, one can discover whether or not certain elements are present during the transit that are not present when the star is not in transit. If such elements are discovered, then the change in brightness is likely caused by a planet in orbit.
Shifts in the star’s light emission are reflected in the following graph, with relative brightness on the y-axis and time on the x-axis. The visible dip in the brightness of the star results in a trough that shows when the planet is blocking light from the object.
Source: The Essential Perspective pg. 265 (1)
It is important to note that not all dips in the light received from a distant star are the result of a planet in orbit. Stars experience what are called “sunspots,” temporary dark spots on the surface of the star, which also block the light received from the star. This is why the periodicity of the light fluctuations are important. If the change in recorded light is a single occurrence and is not observed again, then the change in light was likely the result of a sunspot. However, if there is a pattern to the changes in relative brightness, then that is likely explained by the presence of a planet in orbit.
Other than providing us with the data needed to determine whether or not a slight light shift is the result of a planet, the transit method can provide us with some fairly important information about the planet-- if, of course, it is determined to be such.
For instance, the transit method can provide us the planet’s size, orbital period, and distance from Earth. And using the Kepler’s 3rd law, one can determine the star’s distance from its parent star, thereby giving us critical information as to whether or not the planet may be suitable for life. Compared to other methods, what is unique about the transit technique is that it is the only extrasolar detection method that can provide us with the size, or radius, of the planet.
There are, however, drawbacks to the transit technique. Most notably, in order for the planet to be detected, the relevant solar systems need to be aligned in such a way that one or more of the star’s planets need to pass between us and the parent star. Only about 1% of known systems are aligned in this way. (2) Therefore the transit technique is vastly limited in the number of systems it can analyze. Luckily, 1% of the solar systems in the universe is still a large number, so the transit technique is still widely used!
In fact, the transit technique is the primary method used by the Kepler telescope, which has contributed immensely in the search for Earth-sized planets. The number of detected planets has increased drastically with the addition of Kepler, and the transit method is instrumental in the satellite's process. And if you’re curious to see some of the fascinating data and amazing images from Kepler, visit https://kepler.nasa.gov/index.cfm
Lastly, the stars that Kepler searches for are tremendous in size when compared to their orbiting planets. Without the advanced technology of telescopes like Kepler, it is quite difficult to discern a planet in transit. See for yourself! The following image is of a transit of Mercury. Can you find the planet?
Mercury in Transit
Source: Colby’s 8″ Schmidt-Cassegrain telescopes. (3)
That's right! It's at the bottom right of the Sun!
If you enjoyed that and want to contribute to the planetary detection cause, there are many ways that we can put on our lab coats and be citizen-scientists. Visit planethunters.org to help discover more exoplanets!!
Citations: (1) Bennett, Donahue, Schneider, and Voit. The Essential Cosmic Perspective. Pearson Education, 2015: pg. 265. (2) Ibid.
(3) Colby’s 8″ Schmidt-Cassegrain telescopes. Mercury in Transit. 5/19/16 http://www.colby.edu/physicsastronomy/astronomy-events/
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