One of the questions that comes up quite frequently on the planethunters.org forums is “what does a planetary transit look like”? That’s been partially answered by this post by Matt Giguere on the PH blog, but I’ve come across some more examples that planethunters might find useful.
You may remember that in January 2010, the Kepler team announced the discovery of the first five exoplanets from Kepler data. The lightcurves for the stars that these planets orbit are actually available online, and they’re available in a text format that makes it easy to import into a spreadsheet program! So, this is what the lightcurves for the transits of these confirmed planets look like! (click on them to see a larger image):
I’ve tried to make the graphs look as similar as possible to the ones that are presented on the planethunters website – the y-axis shows the normalised flux, autoscaled to fit all the data in the lightcurve (as they are on the PH site), and the x-axis shows the time in days. The only changes I made to the data were to convert the orginal times from “Heliocentric Julian Dates” to days (by subtracting 2454953 from the Julian Dates), and for Kepler-8b the values in the text file were normalised around 0 instead of 1 (unlike the other files), so I added 1 to all the data values there to put it at the same scale as everything else. Note that there’s about 44 days worth of data in these charts, rather than the 35 days or so on the PH site. There’s a brief data gap in all of these charts at around 11 days, and you can see a “shutter effect” on Kepler 7b (see this post on the PH blog for further explanation).
All five planets have transits that last a matter of hours, which means that they have short orbital periods (less than 5 days) and are therefore orbiting very close to their star – within 0.05 AU, since the stars are apparently all more massive than the sun (for comparison, the innermost planet in our own solar system – Mercury – orbits the sun at roughly 0.4 AU!). As such, it’s important to understand that these transits are much shorter than transits of planets on much wider orbits, which could take up to several days (again, see Matt Giguere’s post on the PH blog for those).
One other thing to take note of here is that the dip caused by Kepler 4b’s transit is much smaller than the dips in the other graphs, but this is somewhat obscured by the fact that the y-axis is autoscaled to fit the data. So here’s a re-scaled version of the Kepler 4b graph, shown to the same y-axis scale as Kepler 5b – the difference is much more obvious now! (this is why it’s always important to check the values on the y-axis, as I pointed out in my previous post). This means that Kepler 4b is quite a bit smaller than the other planets here – 4b is actually about the same size as Neptune, while the others are larger than Jupiter.
Hopefully that should help you understand at least what the short-period transits could look like while you’re looking through the Kepler data on http://planethunters.org!
If you want to play with the graphs yourself, you can download the Excel files I made below – for example, you can use these to adjust the x-axis scale and zoom in on individual transits to see what they look like in detail!
Kepler 4b lightcurve data (Excel 2007 spreadsheet)
Kepler 5b lightcurve data (Excel 2007 spreadsheet)
Kepler 6b lightcurve data (Excel 2007 spreadsheet)
Kepler 7b lightcurve data (Excel 2007 spreadsheet)
Kepler 8b lightcurve data (Excel 2007 spreadsheet)