Extrasolar Planet Detection Using Transit Method
What is the transit method of extrasolar planet detection?
Using an array of CCD's (charged couple devices), thousands of stars can be viewed at once. CCD's are silicon chips which are used to detect light. Telescopes take timed exposures of the same field of view continuously for a period of time that can last many months, depending on the size and orbit of a planet being searched for. The data is run through software that corrects distortion and noise (from both the source (the stars in question) and the instruments used). The result is the production of light curves for the number of stars in the field of observation. Click this link for an animated transit and light curve. The number of stars being observed can vary greatly depending on the equipment and project. The alleged observed transit discovered must be viewed at least twice in order to be fairly sure that the periodic dimming of starlight is, indeed, periodic and follows a pattern.
What characteristics of a planet can transit photometry determine?
It is also possible for a planet's atmosphere to be, in a sense, examined during these transits by observing the change in starlight as it filters through the orbiting planet's atmosphere. The change in starlight that is needed to be observed in order to gather information about the chemical nature of a planet's atmosphere is not one of quantity, but a change in the color of light. Such changes can suggest chemical compositions in the atmosphere of the transiting star. This was done in the case of a sun-like star, HD209458, which is known to have an orbiting planet.
*the size of the star observed
What are some difficulties surrounding the transit method?
What are some benefits of the transit method?
Why care about the small puny planets, anyway?
Or, rather, it's probably a lot easier to have your mission funded if you are searching specifically for planets that can support life (like little green men and their robot assistants). You, yourself, would probably have more fun looking for planets that can support life--and the theory is, of course, that small earth sized planets are a good size for life. The extrasolar planet search project, the Kepler Mission specifically states that it's goal is focused on finding earth-like planets that are in the Habitable Zone. The Habitable Zone is bounded by the range of distances from a star for which liquid water can exist.
The Kepler Mission is designed to detect earth-size planets in orbits similar to earth's around stars similar to our own (mv=12). The wide angle telescope would monitor the brightness of about 100,000 dwarf stars, (like our sun), for four years. In searching for four years, the mission would detect three to four transits of each terrestrial planet detected.
STARE (STellar Astrophysics & Research on Exoplanets), in 1999, was the first program to detect an extrasolar planet using the transit method. This program is also ground-based.
Eddington (ESA is a program intending to search for extrasolar planets in the Habitable Zone, that is planned to be launched in 2005. It plans to use a 1.2 metre space telescope with a CCD imager for photometric imaging.
COROT is French project to study extrasolar planets using the transit method. This project is intended to be launched in 2004, and also would use spaceborne observation.
Over 100 extrasolar planets have been found since 1995, but we only know of one that transits it's parent star in the line of sight of the earth.
Bibliography
Alexander, A. Scientists Discover Extrasolar Atmosphere
http://www.planetary.org/html/news/articlearchive/headlines/2001/extrasolar_atmosphere.htm This website is about the atmosphere detected around a planet transiting a star.
Arizona Planet Search.
http://www.psi.edu/~esquerdo/asp/precision.html This website has information on the Arizona Planet Search and basic information about search methods.
Cowing, K. ESA's Hipparcos Satellite Observed an Extrasolar Planet 8 Years Ago.
http://www.spaceref.com/news/viewnews.html?id=44. Article about unnoticed news of an extrasolar planet during transit.
Doyle, L. Detecting Other Worlds: The Photometric Transit or 'Wink' Method. 2001.
http://www.space.com/searchforlife/seti_doyle_worlds_010809.html Brief description of what transit photometry is and the Kepler Mission.
Extrasolar Planets Searches:
Ongoing Programmes and Future Projects
http://www.obspm.fr/encycl/searches.html. This site contains links to many ongoing and future programs dealing with extrasolar planet search. It includes programs using other methods than transit photometry.
Goldsmith, D. Worlds Unnumbered: The Search for Extrasolar Planets. 1997. California: University Science Books.
The Kepler Mission http://www.kepler.arc.nasa.gov This website contains an elaborate and in depth description of the Kepler Mission, the science involved, and its objectives.
Terraciano, J. The Search for Another Earth. 2001.
http://www.wired.com/news/technology/0,1282,41483,00.html Discusses the Kepler Mission and its objectives, as well as its struggle to be funded.
An extrasolar planet is simply a planet orbiting a star other than our own, and the transit method of detecting such planets is one of several methods of extrasolar planet detection. This method simply detects when a planet is in transit in front of its star from the vantage point of the observer. This is detected by measuring variance in stellar luminosity. Transit photometry measures periodic dimming of light emitted from a star caused by an orbiting planet passing in front of its star along the observer's line of sight. The intensity in the drop of light perceived from a star depends on both the size of the planet and its parent star, as well as the distance of the planet from its star.
What are the origins of the transit method?
Artist's rendering of planet in transit of a star. Taken from exoplanets.org
This diagram illustrates the process the data from an observed star goes through in creating the light curve. Taken from the Arizona Search For Planets website.
The concept of observing planets in transit around their star originated about 300 years ago when astronomers noted a shimmer in sunlight when the planet Venus crossed the sun during its orbit.
Transit photometry is capable of determining *the size of the planet in transit (based on depth[change in brightness]), *the orbital period of the planet in transit (based on the period the planet remained in transit in front of the star during each observed transit), *and the inclination angle of the orbit. Used with the radial velocity method of extrasolar planet detection, the mass of the planet can potentially be determined.
What sizes of planets are detectable using the transit method?
The smallest detectable size of extrasolar planets is limited to planets about half of the size of earth for a 1 AU orbit around a star similar to our sun. Depending on the size and brightness of a star, the transit method is able to detect planets as small as Mercury.
Factors that need to be considered in determining minimum planet size detectable are:
*the brightness of the star observed
*the stellar variability (the inherent noise of the star)
*the observing instrument's noise
*the number of transits observed
*detection efficiency
*duration of the transit
(Kepler Mission)
The transit method of planetary detection requires years of observation in order to determine if a star has an orbiting planet. In order to be sure that a dimming in starlight from a certain star is, in fact, a transiting planet, the star must be observed for years, depending on the size of the planet and its orbit. Observing the transit of a planet across its star requires that the planet earth, the star, and its orbiting planet, be oriented in such a way that the orbiting planet is in the line of site of earth. Beyond that, discovering an extrasolar planet with this method requires that a telescope be pointed in the direction of a star with an orbiting planet, during the time in that planet's orbit that it transits across the sun in relation to its position and earth's. Ground-based exoplanet search using the transit method is largely limited to larger Jupiter sized planets. Transit method is in some ways better as a confirmation method, and as a way to determine the size and orbital period of an extrasolar planet. However, using spaceborne photometers capable of huge fields of vision increase the chances of discovering an extrasolar planet using the transit method.
Because of little green men and their robot assistants.
Some more about this....KEPLER MISSION
Little green man and his robot assistant. Picture taken from GIR.N3.NET Invader Zim rocks the hizzouse.
The Kepler Mission is an extrasolar planet search mission that would employ the transit method using a spaceborne photometer to measure repetitive stellar brightness changes. The fact that the Kepler Mission photometer is to be spaceborne would eliminate the day-night and seasonal cycle interruptions of ground based observing. From earth, a slight dip in what is perceived as stellar brightness of a star could possibly be the interaction with the earth's atmosphere. The Kepler Mission was originially named FRESIP, Frequency of Earth-Sized Inner Planets. The Kepler Mission project is not yet launched, and it plans to launch around 2007.
Other current as well as potential transit photometry missions:
Kepler Mission Spacecraft. From www.kepler.arc.nasa.gov
The TEP (Transit of Extrasolar Planets) Network is an ongoing earth based search for extrasolar planets. It's main focus is CM Draconis, a binary star. However, earth-based extrasolar planet search using the transit method is very difficult, especially in order to find smaller earth-sized planets.
Extrasolar planets actually discovered using the transit method:
The first extrasolar planet detected using this method was found in 1999 by the STARE program, but photometric data from eight years earlier was existent suggesting an extrasolar planet orbiting the star HD 209548. The ESA Hipparcos satellite, which had an intended mission of star-mapping, and not of exoplanet search, had among 89 of its observations, detected five occassions in which a dimming of starlight occurred. However, this data was not examined for the purpose of detecting planets, and so it went un-noticed until 1999.
