Ever want to discover a new world? That's what we are planning to have folks do with PlanetQuest, a distributed computing screen saver that will allow anyone to find extrasolar planets on their own computer. Like the venerable SETI@home, but distinct from the new planetary-systems-generating program of the OKLO project at UC Santa Cruz, PlanetQuest will enable users to discover real planets around other stars using four different detection techniques. (By the way, there is also a NASA mission now named PlanetQuest. The two PlanetQuests are distinct, but we share a common educational goal to promote enthusiasm for planet detection.)
The first planet detection technique that PlanetQuest will use is the single star transit method. Sometimes a planet's orbit will align in such a way that it moves across its parent star creating a shadow, or transit. For a sunlike star, the brightness drops by about 1% for about three hours if the planet is about the size of Jupiter and orbits very close to the star (with a "year" of about a week). When a planet moves in front of a single star, the light drop is periodic, and fairly easy to recognize. A difficulty with the single star transit detection method is, however, that stars (like lightbulbs) are brighter in the center than at the edges (and bluer--hotter--in the center also). This is called "limb darkening." It means that when a transiting planet moves across the center of a star, the light blocked can also look like two eclipsing stars just grazing each other's outer edges. About two in three transit looking features will be grazing eclipsing binaries rather than planet transits, so one must find out if the star being observed is a double star or not.
A specialization of the transit method is the detection of transits across eclipsing binary star systems themselves. This can be very tricky since, in most cases, the planet cannot be said to be moving across the two stars as much as the two stars are moving behind the planet as they orbit each other. This produces a predictable but non-periodic signal that many times does not look much like a transit at all. The dimming and brightening will not be regular and their occurrence will, generally, not repeat in the same way because the stars and transiting planet will be at different places in their orbits when the planet moves in front again. PlanetQuest's approach to this problem is to use a matching filter that will compare all possible models of planet sizes and orbits with the observations. This takes a rather huge amount of computational time, so is a perfect opportunity for the public to participate with distributed computing.
The second detection technique PlanetQuest will use will be the eclipsing binary minimum timing method. This method relies on the fact that eclipsing binary stars, as they move in front of one another, are essentially a kind of "clock" that gives a time stamp to the observations. We watch the light of the stars (take pictures of them) and record the time exactly. (The plot of the brightness of a star as it varies with time is called the star's "light curve".) We have to correct for things like where the Earth is in its orbit at the time of the observations, so we generally work with "heliocentric" time--the time in the center of the Sun when the eclipse occurred. This way we don't have to keep correcting for the light travel time across the Solar System to the Earth.
This method will be able to detect circum-binary planets of Jupiter-mass or larger because these planets will offset the two eclipsing stars (like a see-saw) as they orbit them. The offset of the two stars due to a planet orbiting around them will be detectable because the eclipses will be early or late, depending on the direction of offset. PlanetQuesters will also then detect Jupiter-mass planets that don't even have to be orbiting across the disc of the two stars.
The third detection method is the gravitational lens planet detection. When there is a very close alignment between two stars, the star in front can bend the light from the star behind due to bending spacetime, according to general relativity. Any planets in the foreground star will also bend some light (for a shorter time) and so be detectable also. Since the data for this detection method are the same as that for detecting transits and binary eclipses (wide field, crowded star field images), then we shall also be able to detect planets using this method. The stars will, however, brighten instead of dim. One can tell, for example, a stellar flare from a gravitational lens event because gravitational lenses will be achromatic--that is, not of any color. Bending spacetime does not care what color the electromagnetic light wave is while stellar flares, for example, are brighter in ultraviolet than in red light.
Finally, PlanetQuest will use a new kind of SETI (search for extraterrestrial intelligence) detector that will compliment existing SETI projects. All SETI searches to date (as far as the author knows) are aimed at detecting the nature of the signal itself, rather than the content of the signal. In the case of radio SETI, a narrow-band radio carrier wave is detected, and in the case of optical SETI a nanosecond pulse is detected. In our approach, we classify the signals and produce a distribution of the frequency of occurrence of the signals. Then we compare this with the frequency of occurrence of known intelligent communications (this field of study is known as "information theory"). If there is a close match then we look at the structure of the signals and their dependence on each other. For example, if one recorded babies babbling, and tried to find a connection (syntax) within the message, one would not find such structure. But we know that if we record the vocalizations of an adult human, that there are grammatical and syntax rules that are causing certain words to occur at certain times with relationship to each other. This is what allows one to fill in missing words from a copy where the copier was low on toner and parts of the text are missing. We have applied this method extensively to vocalizations of ground squirrels, squirrel monkeys, dolphins, humpback whales, and humans so far. If an extraterrestrial signal is received--if they are transmitting information-- then they too will have to obey these rules of information theory.
We at PlanetQuest have already completed observations at Siding Spring Observatory in Australia and our second year of observations at the UC Lick Observatory in California, and are readying the alpha test software to begin our first tests of the whole system. We call the combination of our search engine and educational materials the "PlanetQuest Collaboratory" and hope you can join the search. Every PlanetQuester will discover something--the nature of a star, or a new planet, or many other possibilities--and get credit for that discovery in the PlanetQuest catalog, which will be accessable online. So check out our web site, and sign up for the PlanetQuest Newsletter. And soon you too can join the professional astronomers in discovering totally new worlds on your own.
- Shadows and Silhouettes: Looking for Transits
- Images: Venus Transit Gallery
- Detecting Other Worlds: The Transit or 'Wink' Method
- Way-Out World: New Technique Finds Most Distant Planet Ever
- Mercury Transits Sun, Images on Web