Photometry Details

Good photometric precision is necessary before we will have a realistic chance to detect planetary transits. What does this mean?

Our light curves are composed of many data points, each of which represents a single measurement for a star's brightness, or magnitude (there is one data point per exposure). Inherent to each data point is an uncertainty in how well it measures the stellar brightness. We call this the photometric precision.

The light curve of a non-variable star. This light curve shows the brightness of the star on 5 consecutive nights with about 4.5 hours of observing time per night. The precision in this light curve is 0.002 magnitudes (0.2%).

To be detectable, a transit must dim the star sufficiently so that its light curve shows a drop that is distinguishable from random noise. There are different detection thresholds for individual stars because they all have different brightnesses. Also, the brightest stars can be measured to the best precisions. Furthermore, the amount of diminution depends on the type of star being transited and the size of the transiting planet. A larger planet will block a larger fraction of a star's light while a planet of a given size will block a larger fraction of a small star's light than it would a large star. Consequently, the easiest planets to detect by transits are large planets orbiting small stars.

The size of a star depends on its mass and stage of evolution. Most stars seen with the unaided eye at night are low-mass dwarf stars like the Sun, commonly known as Main-Sequence stars. The same is true for most of the stars we observe through the telescope in our search fields. In the figure below, we plot curves showing the diminution in percent of a star's total light due to a transit by planets of 3 different sizes versus the type (size) of star that they transit.

Curves showing the diminution in percent of a star's total light due to a transit by planets of 3 different sizes versus the type (size) of star that they transit. The type of star is shown along the bottom of the plot by its spectral class, denoted by the letters F, G, K, and M, with subclasses given by numbers. The spectral classes range from stars somewhat larger and more massive than the Sun (F stars) to stars with lower mass and in some cases much smaller than the Sun (M stars). The three curves representing planets having the size of Earth, Neptune, or Jupiter (red, green and blue respectively) show that a larger percentage diminution is seen in the light curve of small M stars than the larger stars like the Sun (a G2 star). Larger planets also produce a larger diminution for transits across all type stars. Horizontal yellow lines in this figure show the precision of the light curves for the brightest (best) stars in our project. Roughly speaking, we can detect transits of equal or greater diminution than the precision of the light curve.

Currently we can detect the transits of Jupiter-sized planets across many types of stars and transits of Neptune-sized planets across M dwarf stars.

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