This method detects distant planets by measuring the minute dimming of a star as an orbiting planet passes between it and the Earth. The passage of a planet between a star and the Earth is called a "transit." If such a dimming is detected at regular intervals and lasts a fixed length of time, then it is very probable that a planet is orbiting the star and passing in front of it once every orbital period.
The dimming of a star during transit directly reflects the size ratio between the star and the planet: A small planet transiting a large star will create only a slight dimming, while a large planet transiting a small star will have a more noticeable effect. The size of the host star can be known with considerable accuracy from its spectrum, and photometry therefore gives astronomers a good estimate of the orbiting planet's size, but not its mass. This makes photometry an excellent complement to the spectroscopic method, which provides an estimate of a planet's mass, but not its size. Using both methods, combining mass and size, scientists can calculate the planet's density, an important step towards assessing its composition.
A Planetary Transit
An artist's impression of a Jupiter size extrasolar planet passing in front of its parent star
Transit photometry is currently the most effective and sensitive method for detecting extrasolar planets, particularly from an onservatory in space. The Kepler mission, launched in March of 2009, uses photometry to search for extrasolar planets from space. The spacecraft's sensitivity is such that it has already detected thousands of planetary candidates, including several that are Earth-sized and orbiting in their star's habitable zone. No other method currently proposed can match either the volume of the search or its sensitivity.
Transits can also provide scientists with a great deal of information about the planet that is not otherwise measurable. First and foremost the "dip" in a star's luminosity during transit is directly propotionate to the size of the planet. Since the star's size is known known with a high degree of accuracy, the planet's size can be deduced from the degree to which it dims during transit.
When combined with radial velocity data, a transit can also provide a good estimate of the planet's mass. This is because a transiting planet is necessarily in an "edge-on" position to an observer on Earth. Under these conditions, the minimum mass normally provided by radial velocity measurements is, in fact, the planet's true mass. Taken together, the planet's size and mass provide scientists with a crucial clue as to its composition: the planet's density.
But transits provide even more information. The light from the star passing through the planet's atmosphere is absorbed to different degrees at different wavelengths. This "absorption spectrum" depends on the different gasses present in the atmosphere. By monitoring the depth of the transit at different wavelengths, scientists can recreate the absorption spectrum and deduce the atmosphere's composition.
In addition to "primary" transits, which occur when a planet passes in front of its star, scientists are also interested in "secondary" transits, which occur when a planet completely disappears behind the star as seen from Earth. By deducting the star's light spectrum when the planet is hidden from the spectrum when it is visible, scientists can arrive at the planet's actual spectrum. From this they can deduce its temperature as well as the composition of its atmosphere.
Finally, transit photometry searches can operate on a massive scale. Ground based searches such as TrES, OGLE, HAT, and WASP, and space based searches such as Kepler track as many as a hundred thousand stars at a time for signs of a transiting planet. Given the scale of the searches, it is likely that transit photometry will soon surpass all other methods in the sheer number of planets detected.
NASA, ESA, G. Bacon (STSci)
Anatomy of a Planetary Transit
As the planet moves in front of its star, the star's luminosity dips, and then returns to its former level when the transit is complete.
The main difficulty with this method is that in order for the photometric effect to be measured, a transit must occur. This means that the distant planet must pass directly between it's star and the Earth. Unfortunately, for most extrasolar planets this simply never happens. In order for a transit to occur the orbital plane must be almost exactly "edge-on" to the observer, and this is true only of a small minority of distant planets. The rest will never be detected with photometry.
Another problem is that a planet's transit lasts only a tiny fraction of its total orbital period. A planet might take months or years to complete its orbit, but the transit would probably last only hours or days. As a result, even when astronomers observe a star with a transiting planet, they are extremely unlikely to observe a transit in progress. The problem is further compounded because in order to establish the presence of a planet, astronomers need to observe not one, but repeated transits occurring at regular intervals.
Finally, experience with transit photometry has shown that the method tends to produce "false positives" -- instances when a binary star is mistaken for a planet orbiting a star. Normally, the difference a planet transiting a star and one star transiting another is one of degree: the planet, being smaller, will create a much shallower dip in a star's luminosity than a transiting star would. But sometimes a binary star when viewed from Earth is positioned very close to a very bright star, making it difficult to separate between the two. In those cases the transiting binary will create only a relatively shallow dip in the combined luminosity of the binary and its apparent neighbor. From Earth, it would appear as if a planetary transit was taking place, creating a "false positive."
In order to find transiting planets at the moment of transit, searches must continuously cover vast stretches of sky containing many stars for long periods of time. It is highly probable that at least some of the stars observed will have planets, and that at least some of these planets will pass between their star and the Earth. Observing the same batch of stars for long stretches of time also makes it far more likely that if a planet does transit its star, the event will be observed and recorded.
Obviously, no planet hunter can accomplish such observations on his own, even if he or she is equipped with the most sensitive equipment for measuring starlight. Only an automated telescope that records its observations over long stretches of time can effectively detect planets using this method. Several such projects are underway across the world.
Selected Photometry Projects
Kepler, a space-based photometry observatory, launched in March 2009 and as of January 2013 has identified over 2,700 candidate exoplanets. The vast majority of these are expected to be confirmed as planets, making Kepler by far the most successful planet humting instrument to date.
Kepler operates by pointing its photometer continuously on a single star-field of around 145,000 different stars. The chances of any one of these stars undergoing a transit is very small, but because of the huge number of stars being tracked thousands of transits nonetheless take place. Thanks to Kepler's exceptional sensitivity, hundreds of the planets detected by the end of 2012 are of Earth-like diameter, and dozens are in the habitable zone.
Kepler follows the Earth around the Sun, completing one orbit each year for at least four years. This time-span enables Kepler to track at least four transits of planets orbiting at Earth-like distances from their stars - quite enough to ascertain their presence.
CoRoT (Convection Rotation and Planetary Transits) is a joint space mission of the French Space Agency (CNES) and the European Space Agency (ESA) launched in December 2006, aimed at detecting transiting exoplanets. The mission has already made several discoveries including a rocky planet only twice the diameter of Earth, announced in February 2009. Scientists hope that low-mass terrestrial planets as small as Earth may be found in the future.
Most photometry searches, however, are Earth based, and make use of existing telescopes combined with state-of-the-art photometers of the highest sensitivity. One of these, based at the the Planetary Science Institute (PSI) in Arizona received
extensive support from The Planetary Society. In consortium with three other institutions, PSI is refurbishing the 50 inch telescope at Kitt Peak, and turning it into an RCT - a remote control telescope. Once completed, the PSI team will be able to use the telescope for large stretches of time in its photometric search for extrasolar planets.
NASA/Kepler mission/Wendy Stenzel
Kepler in Space
An artist's depiction of Kepler in orbit around the Sun.
Photometry and Amateur Astronomers
While the discovery of a new planet with photometry requires the most advanced professional equipment (or an inordinate amount of luck), observing the transit of a known planet is much easier. This is because if one knows where to look and when, the effect of the transit itself can be quite substantial and easily detectable. In fact, a known transit can often be observed by amateurs using commercially available equipment.
In May 2001, for example, thousands of amateur astronomers around the world turned their telescopes towards a nearby red dwarf known as Gliese 876. This star was known to be orbited by two planets, both of which were discovered using the spectroscopic method. Since the star is small, and the planets orbiting it are large, the transit of the larger of the two dimmed the star substantially. This made it possible for amateurs the world over to observe the telltale signs of the presence of an extrasolar planet.
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