Thanks very much for your help with this project. At last count, roughly 50,000 light curves had been sorted at planethunters.org. Many of you have requested more examples about how to classify stellar variability, so we’ll start with the easiest case. All of the light curves below are examples of quiet stars. Random variations in brightness occur because of photon noise (similar to shot noise in electronics). The number of photons that are collected are small enough that there random fluctuations that have nothing to do with the actual brightness of the star. Photon noise (or Poisson noise) produces scatter, but the data remain in a nearly featureless band of points.
If you look closely at the light curve data for these quiet stars, you will see light gray error bars associated with each data point. In any physical measurement, the error bar simply captures our ignorance about the true value of the measurement. In the Kepler light curves, the brightness is represented as a discrete dot, however, any and all points along an error bar are equally correct values for that particular brightness measurement.
In the quiet light curves above, should any of those low points be flagged as possible transits? Probably not. A deviant point or two can still just be noise. A true transit event should have a series of low brightness points that last for the time it takes the planet to cross in front of its stars (i.e., a few to several hours, represented by a few to several data points). Low dips that repeat are also good indicators of a transit, however some of the most exciting transits (from planets in wider, more habitable orbits) will only occur once per month (for example, a true analog of our Earth would just transit once per year).
The quiet light curves above may seem like duds, but they are an extremely important aspect of research for this project. Stars that do not vary in brightness are particularly important objects for exoplanet searches with other techniques. The work that you’re doing will feed into our understanding for the next generation instruments and space missions that could be built to detect planets.
Happy Holidays to All! Debra Fischer