Selection Effects

There are some parameters or ranges of values that you can plot that might take you towards a misleading conclusion. 

For example, try plotting Orbital Inclination (linear) against Orbital Period (log). What do you notice about the data? You should see around 800 datapoints here (see Figure 1) but the majority appear to be grouped around an x-axis value of 90 degrees. You could narrow your selection to just those discovered by the transit method and most of the points will remain. This tells us that for the 700 or so systems for which we have an inclination value, most were discovered by transit and most have an inclination of nearly 90 degrees. Have a look at the transit page to remind yourself why this result might be expected.

Figure 1: A scatter plot of Orbital Inclination against Orbital Period.
Credit: exoplanet.eu

There is a second effect at work here, however. As we move to shorter orbital periods, we see a few datapoints with inclinations nearer 75 degrees (see Figure 2). Again, referring to Figure 1 on the transit page could help explain why.

Figure 2: As Figure 1, but here, the yellow oval denotes the spread of values away from an inclination of 90 degrees as the orbital period decreases.
Credit: exoplanet.eu

Another example would be to plot the co-ordinates of all the known exoplanet systems (i.e. RA against declination). Initially, you might expect to see a random distribution of your data here but what you may want to consider is whether the fact that we are likely to only find exoplanets around stars in our Galaxy.

How would this show up in your plot ? Can you see any evidence of this in your data ?

Look more closely at the data and try zooming in on an area bounded by 35 - 55 degrees of declination and 270 - 310 degrees of right ascension. What do you notice ? You should see a pattern of diamonds displayed on your plot. Does this distribution look random to you ? If not, can you identify what's happening here ? (hint: have a look at the names of the individual exoplanets).

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