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Space Topics: Extrasolar Planets

Transit Photometry: A Method for Finding Earths

This method seeks to detect 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 based on its spectrum, and photometry therefore gives astronomers a good estimate of the orbiting planet's size, though 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, scientists can calculate the planet's density, and hence its composition.

Advantages


Photometry is one of the most sensitive methods for detecting extrasolar planets. The upcoming "Kepler" mission, due to launch in 2007 will use photometry to search for extrasolar planets from space. Its sensitivity will be such that it will be able to detect Earth-sized planets orbiting at an Earth-like distance from their star. No other method currently proposed can match this sensitivity.
 

Disadvantages


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. For most extrasolar planets this simply never happens - their orbital planes are such that they never pass in front of their star, as seen from Earth. In order for a transit to occur the orbital plane must be almost exactly "edge-on" when seen from the Earth, and this can be 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.

Search Strategies


The solution to both of these problems lies in observing vast stretches of sky containing many stars continuously 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


The most ambitious project currently under development is Kepler - a space-based photometry observatory, scheduled to launch in 2007. Kepler's photometer is designed to focus continuously on a single star-field of around 100,000 different stars.

The chances of any one fo these stars undergoing a transit is very small, but because of the huge number of stars being tracked, scientists hope to observe hundres if not thousands of transits. Kepler's sensitivity is such, that astronomers expect to detect several dozen planets of Earth-like mass, orbiting at an Earth-like distance from their star. But of course, no one will really knows until the spacecraft is launched.

Once in space Kepler will follow the Earth around the Sun, completing one revolution in each year for at least four years. This time-span will enable Kepler to track at least four transits of planets orbiting at Earth-like distances from their stars - quite engh to establish their presence with a high degree of certainty.

Most photometry searches, however, are Earth based, and make use of existing telescopes combined with state of the arts photometers of the highest sensitivity. One of these receives extensive support from The Planetary Society. Under the direction of Dr. Steven Howell, a team from the Planetary Science Institute (PSI), in consortium with three other institutions, is refurbishing the 50 inch telescope at Kitt Peak, and turning it into an RCT - a remote control telescope. Once completed, the PSI team, under the direction of Steven Howell, will be able to use the telescope for large stretches of time in its photometric search for extrasolar planets.

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. So easy, in fact, that it can often be accomplished by amateurs using commercially available equipment. 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 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.

--Amir Alexander