Uncertainty

Hello, I’m John, a graduate student at Yale University and a member of the Science Team. We have had some questions about the non-light curve information on the star pages, including: how accurate is the data, and why is it missing occasionally? This post will give you some background on where this data comes from and how to interpret it.

Before the Kepler spacecraft was launched, a lot of thought went into finding the optimal patch of sky to observe. Then, many years were spent collecting as much data as possible, from the ground, about the stars in that patch of sky. Photometry (a measure of star brightness, like you have been looking at on planethunters.org) was taken through multiple filters of most stars and spectra (with more detailed information) were taken of the brightest stars. These measurements all went into selecting the most promising stars for the Kepler mission to look at.

From these measurements, it is possible to calculate certain values which are useful in interpreting the data from Kepler. This includes spectral type, effective temperature, surface gravity, and radius. All of these values were compiled into the Kepler Input Catalog, which we use to show you the information on the star pages. However, because this data was taken en-masse and many of the stars are quite dim there is sometimes a large uncertainty in these parameters or there may be no derived values at all. When you look at the stellar radius on a star page, that number could be off by as much as 50%!

After interesting stars are found, much effort goes into closely examining the star with larger telescopes, more frequent observations with Kepler, or both. This results in much more accurate determinations of all parameters. One recent paper (Metcalfe et al. 2010) determined the radius and age of a Kepler star to within 1%.

Some of you have already been calculating the size of the transiting companion using the duration of the transit and the stellar radius. It has been great to see the phenomenal work people are doing in seeking to understand these stars. Keep in mind though that until more accurate follow up data can be taken, there will be a large amount of uncertainty in those numbers.