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):
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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.
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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)
Hi EDG,
Thanks for the great follow-up work on the charts here. I found your blog through some links from planethunters.
I just made a posting on your discussion on half-a-transit at the planethunters talk page. I assume that if the y-axis is normalised flux, then the transit is 20%, hence a pretty largish object.
As for transits of an earth-sized planet in a habitable zone, I guess that’s going to be tough because I don’t see how we can spot a transit of 0.01% amidst all that noise, plus the fact that these transits happen so infrequently.
Finding large planets do help of course, and we can just zero in on those stars since it means the orbital planes are aligned just right for us to discover the smaller planets.
Seasons greetings and best of luck on planet hunting!
Thanks for the comment! Yeah, I got all excited when I saw that big transit and then frustrated when I could only see half of it!
I’ve done some more calculating on the PH talk pages and I now think it’s actually a detached eclipsing binary – as you point out it’s a huge object, and it’s just too big to be a planet (it’d have to be about 0.42 solar radii!).
I suspect we’re really looking at an M0 V red dwarf companion there – it’s like this detached eclipsing binary, but the objects have a longer orbital period (at least 35 days, and so they’re more widely separated) here and we just happened to have caught half of one star’s dip here.
Still, it’s a bit exciting, I hadn’t seen that before! 🙂
Nevertheless, it’s still an interesting discovery and shows how much we can glean from Kepler photometry.
Some speculations: if I counted 6 samples from 1st to 2nd contact (assuming that’s where the data ran out), and the star is 0.42 solar radii, then does that mean that the complete transit duration should be 3/0.42 or 7.14 hrs?
If that’s the case, can we work out the approximate distance of the dwarf from the primary? My math isn’t good enough to do it (it was 30 years ago since I left college 😉 – I guess it to be somewhere a little under Mercury’s orbit since Mercury takes 8 hours to make a full transit. If so, then the orbital period could be more than 70 days and <88. Because we should have seen a secondary transit within the 35 days but we didn't.
I tried plugging in some plausible values in http://astro.unl.edu/naap/ebs/animations/ebs.html like this:
Primary, mass = 0.73, radius = 0.9, temp = 5800K (I assume same density as Sol)
M0V dwarf, mass = 0.10, radius = 0.42, temp = 3700K (0.1 is the lowest allowed value for mass)
These values put the stars exactly on the main sequence so I think you're spot on about the secondary being an MV red dwarf.
I couldn't get the orbital separation to go above 60 radii, or around 0.28 AU. This gives a period of 59 days.
However, the dwarf's luminosity is so low there might be another issue here – taking the dwarf's temperature below 3700K made the secondary transit disappear so if it's much cooler than this it may well be hidden in the noise.
An M0 V would have a mass of about 0.45 Msol, not 0.1 (that’s more like an M7 V). I assumed a mass of 0.9 Msol for the ‘primary’, with a radius of 0.9 Sol (it could be about the same mass as Sol, but from the stellar evolution models it looks like it’d have to be very young to have a radius of 0.9 Sol). We’ve got the temperature of 5809K for that too.
If you put in mass=0.9, radius=0.9, and temp=5810 for Star 1, and mass=0.45, radius=0.42, temp=3800 for Star 2 and change the system inclination to 90 degrees then you get a pretty good match for the dip. We’re probably looking at the bigger dip here caused by the M V crossing in front of the G V, I’d expect to see a much smaller dip later on.