Monthly Archive for December, 2010

Planethunters: Planetary transit depths

Is this a planet?

...or is this?

Which of the two lightcurves shown above contains a planetary transit? If you’ve been following my blog then you’ll know the answer is the one on the right, since that’s the Kepler-5b graph from my last post about planetary transits! The graph on the left shows a detached eclipsing binary, which is consists of two stars orbiting eachother at a great enough distance that they are two distinct objects – as they orbit their centre of mass, the stars pass in front of eachother and reduce the total light coming from the system, which manifests as the dips in the lightcurve. So in that case, the transit is caused by a star and not a planet.

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Planethunters: More examples of planetary transits

One of the questions that comes up quite frequently on the 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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Planethunters: Making sense of the lightcurves

Over the past week I’ve been having a whale of a time looking at lightcurves at http:/ (I’m on there as EDG) – well, actually I seem to be spending more time discussing them and trying to figure them out! I’ve learned a few things in the process that might be useful to other planet hunters out there:

Making sense of Eclipsing Binaries

I’ve learned a lot about the light curves of eclipsing binaries (EBs) this week, which consist of two stars orbiting eachother with one star passing in front of the other as seen from our perspective (which changes the light curve). If you’re looking through the data and want to see what the lightcurves for these EBs look like, then check out pages 18-22 of this Kepler paper (right-click the link and select “save as” to save it), which shows some typical examples of detached, semi-detached, over-contact, ellipsoid, and irregular binaries (there’s an explanation on page 17 for what these types mean – “detached” means that the stars are far enough away to be distinct from eachother, “semi-detached” means that one star has overflowed its roche lobe and is distorted, and “over-contact” means that the two stars are close enough to share a common envelope (i.e. both have overflowed their roche lobes). A dead giveaway for detached eclipsing binaries is that there may be two dips on the lightcurve, but one is shorter than the other.
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Planethunters: Want to find an extrasolar planet? Now you can!

Sample transit chart from (Simulated planet)

Planet Hunters is a brand new “citizen science” site, like Galaxy Zoo (where participants identify galaxies from images) and Moon Zoo (where participants identify and catalogue craters on the moon) – it literally just went online a few hours ago. This time, your task (should you choose to accept it!) is to look through Kepler science data looking for tell-tale dips in brightness caused by planets transiting across the face of the target stars! (the Kepler spacecraft basically stares at a fixed point in the sky containing hundreds of thousands of stars, and it monitors the brightness of all of them, looking out for dips caused by transiting planets).

It does take a while to get used to the brightness graphs, but you get the hang of it eventually. I’ve already found a couple of known transiting planets (it tells you whether it’s a “Kepler Favourite” or not, which presumably means it’s a known planetary system), correctly identified a simulated planet around a giant star (they throw some simulated data in occasionally to check that you’re finding things properly!).

There might still be a few bugs and issues in the discussion forums, but they’re rapidly getting them sorted out. It’s a brillianty idea and it’s all very exciting – I’ve had a look at 50 systems so far and found a few interesting ones (I’ve collected them here, and I’ll be adding more as time goes on). They also have a Facebook page and a Twitter feed to follow as well!

So here’s a perfect opportunity to join in and contribute to scientific discovery – and who knows, you might even find a planet! Head over to now and start looking! 🙂

[Review] Fun with Gravity Simulator

Lately I’ve been playing around (again) with a very interesting program called Gravity Simulator. I’ve been using it on and off for the past four years or so, and it’s proved to be a very useful tool for worldbuilding.

Gravity Simulator is a Windows-based program that allows you to create celestial objects orbiting eachother and see what happens to their orbits under the influence of gravity. You can create planets orbiting stars, satellites orbiting planets, and even asteroid belts – if it can orbit something, it can be made to work here. The algorithms used in the program don’t quite account for everything (for example, the change in orbit caused the transfer of angular momentum between two bodies by tidal forces is not calculated), but the results are still very accurate.

Planetary orbits evolving (spiralling outwards) while a star loses mass

The good points are that it’s a very powerful orbital modelling tool, and known phenomena such as orbital resonances and the Kozai mechanism (where a planet’s eccentricity can be increased by interactions with a nearby massive object in an inclined orbit) have been known to naturally come out of the simulations. It can also output to a data file that you can then use to plot graphs of parameters using Excel (e.g. semimajor axis vs time), and can output screenshots too so that you can make animations if you have movie-making software.

To create a system, you just enter the mass and orbital parameters for all the objects and then set it going – you can even create entire asteroid belts by getting it to create many objects with a range of parameters that you specify (though the more objects you have, the more processing is required which obviously slows things down). The program uses a ‘timestep’ system, in which it recalculates everything once per timestep – a smaller timestep means that the resolution of the interactions is higher and they are more accurate as a result, but the downside is that it takes longer to do the calculations. If the timestep is set too large however then the accuracy can be compromised – so the trick is to find a value that is a balance between processing speed and accuracy, which varies depending on what you’re looking at. If you do it right though, you can run a simulation for hundreds of thousands (or millions) of years of simulated time if your system is left running for long enough. This literally brings stuff that formerly was done on supercomputers into the hands of desktop users!

To show off a bit, here’s a relatively basic example of a sim I made – 10 closely spaced planets the same size and mass as Earth, separated by 0.1 AU between 1 and 2 AU from the sun. This is what happens when the system is left to run for 175000 years (every second of video corresponds to the passage of 747 years of simulated time) – all of the action is in the first 2:50 mins of the video, after that nothing much happens other than a bit of precession of the remaining orbits. The planets start off in circular orbits but then they start to get unstable and individual worlds eventually start making close approaches to eachother, which really disrupts their orbits. This one has it all – orbital precession, collisions, and planets thrown into very eccentric orbits! At the end of the run, only four planets are left, and I suspect that if I’d left it running for longer one or two of those might eventually be lost too.

Orbital Evolution of 10 close planets, simulated over 175,000 years

There’s a good discussion forum for it too, and the author of the program is there quite often and is very helpful. Being a rather specialised program, only a handful of people post to the forums on a regular basis (I am one of them – I post there as “EDG”) but there’s a lot of interesting material posted there (especially by frankuitaalst, who posts a lot of very interesting animations and graphs of resonances). I’ve done some investigations myself of the Kozai mechanism, and used the program to track the evolution of asteroid orbits while a star loses mass as it changes from red giant to white dwarf.

This is why I think Gravity Simulator is so great – it’s an excellent tool for curiosity-driven science (the best kind of science, I think!). I know that more often than not I didn’t have a clue what the result would be when I started running my simulations, and it’s really fun to see how a complex system turns out. As a result, it’s fantastically educational too.

The downside is that the program is a little fiddly to use, and it’s probably going to be a bit scary at first if you haven’t had any previous experience with orbital dynamics. There are example simulations that you can download from the gravity simulator website though, and you can find the Tutorial/Help File there too which explains how everything works (you can also access this page through the Help menu in the program). Plus you can always ask for help on the forums if you’re stuck!

Another thing to be aware of is that the version of the program that you can download from the website via the download page there is somewhat old – once you’ve installed it from there, you should grab the latest beta of the executable from the forums, copy that into the folder you installed it to, and use that as the executable instead. This adds some very handy functionality, including the ability to create new objects with a range of values (handy for asteroid belts) and to dynamically vary the timestep so that it slows down when objects get close enough to gravitationally interact.

Overall, Gravity Simulator is a great educational tool and produces some fascinating results. It’s pretty much unsurpassed as an general orbital modelling tool (I’m sure orbital dynamicists use their own custom programs that are way more technical, but this is great for us non-professionals!), and there’s a lot of support for it (many sample simulations can be found on the rest of website as well as on the forums). It’s well worth checking out and playing around with anyway, and if you have any interest in orbital dynamics then it’s a must-have!

Behold, Hyperion!

The Cassini spacecraft orbiting Saturn recently had everyone worried when it unexpectedly went into safe mode, but fortunately it was brought back to life in time for a targeted flyby of Enceladus on Nov 30th. Images from that flyby are being released as I type (here’s a particularly nice one of a crescent Enceladus, with the plumes at the top of the image), but today I’m going to show you another interesting moon called Hyperion, that was also imaged as part of the flyby.

Hyperion’s was discovered by earth-based astronomers in 1848, but like all of saturn’s moons we didn’t get a good look at it until Voyager 2 flew past Saturn in 1981. Unfortunately Hyperion was pretty far from Voyager 2 at the time and it couldn’t get very high-resolution images, but the images did reveal that Hyperion was irregularly shaped. Fortunately, Cassini got some better images – here’s a sequence of images taken at the end of November 2010, showing Hyperion in all its glory (click to expand, and then magnify it – it’s a big image!):

Hyperion flyby sequence

Hyperion flyby sequence, November 2010 (click to view)

Hyperion is a grand-sounding name, but the satellite itself is only a few hundred kilometres across (since it’s non-spherical, it’s actually 328 km × 260 km × 214 km) – but that’s pretty big for an asteroidal body. In fact, if it was in the asteroid belt, Hyperion would be the 11th largest asteroid! As you can see, it looks a bit like a giant sponge, suspended in space! So could it be a captured asteroid perhaps? Or is a fragment of a formerly spherical icy moon that was broken up by some cataclysmic impact?



All of these images are false colour, constructed using images taken in infrared, green, and ultraviolet filters (see Postcards from a Distant Moon for more details about this process). Apart from some cleaning of the UV images (which had lots of bright speckles in them), they are otherwise unaltered. Before I continue though, I’m going to digress a little and discuss another important aspect of image interpretation.

Digression: Incidence, Emergence, and Phase angles

There are three important angles to be aware of in imaging – the incidence angle, emergence angle, and phase angle (I’m not sure that there’s a collective name for all three!):

  • The incidence angle (i) is the angle between the surface and the light source (usually the sun) – if i=0° then the sun is directly overhead, if it is at 90° then the sun is at the horizon. High incidence angle enhances the visibility of topoghaphy since that means the sun is low to the horizon and casting a lot of shadows.
  • The emergence angle (e) is the angle between the surface and the viewer (usually a spacecraft camera) – if e=0° then the viewer is looking straight down at the surface, and if e=90° then the viewer is looking parallel to the surface. As e increases, the view becomes more and more foreshortened due to perspective (you can see this by looking straight down at your computer keyboard from above (e=0°) and comparing the view to looking across at it from the same vertical level as the keys (e=90°). e is sometimes known as the “emission angle”.
  • The phase angle (p) is the angle between the light source and the viewer, and ranges between 0° and 180°. If p=0°, the sun is behind the viewer, and if p=180° then the sun is in front of the viewer (usually the object being targeted is between the two, so we’d be in its shadow at this point).

Putting this all together, high phase angles are the best way to see topography on the surface, since the sun is low to the ground and casting lots of shadows (this is why you can see lots of mountains and craters when you look at the boundary between the light and dark sides of the moon). A low phase angle combination is the ideal way to see differences in albedo on the surface because then all of the variation in brightness that you see are due to differences in the colour, material or reflectivity of the surface. When the phase angle is exactly zero, you get an opposition surge where the object becomes significantly brighter than at other angles because of the complete lack of shadows being cast by grains on its surface (see this page for more info), but I won’t dwell on that here since that’s getting into complicated stuff about photometry ;).

High emergence angles give you nice oblique views if you want an impressive perspective view of something, and have been used to good effect in Apollo images of the moon (like this classic Apollo 17 image of Copernicus crater). You might also notice that the emergence angle is independent of phase angle – the sun can be behind the viewer (low phase angle) but the viewer (and sun) can be looking obliquely at the ground (high emergence angle) or directly overhead relative to the surface (low emergence angle). In both of these cases, the albedo variations of the surface will be dominant because of the low phase angle, but the view will be affected by perspective differently.

OK, now that’s out of the way, you can look again at the imaging sequence at the top of the post and see that we’re going from high phase angle (crescent Hyperion – I’d say the phase angle is about 150° there) to low phase angle (full Hyperion – phase angle is close to 0°) as the image sequence progresses in time from left to right. You can see in the first couple of images of the sequence (on the left) that the topography dominates what you see – you can’t tell much about the actual brightness of the material that Hyperion is made of, or the colour of its surface – and that’s typical of high phase angle images. In the last couple of images (on the right), the incidence angle is lower, which means there are less shadows and we can see a lot more of the surface. Here’s one of those images, in which we can see that many of Hyperion’s craters are filled with dark material.

Hyperion, low phase angle

Hyperion (low phase)

Interestingly, this dark material is reddish, and suspiciously similar to the dark material coating an entire hemisphere of Iapetus, the next moon out from Saturn – so it seems that some of that stuff is getting onto Hyperion too. This leads me to the other interesting feature of Hyperion – the dark material is found all over satellite and is not concentrated on one hemisphere, because unlike pretty much every other major satellite in the solar system it is not tidally locked to its primary (Saturn’s outer moon Phoebe and Neptune’s satellite Nereid being the other exceptions). Tides generally act to rapidly slow a satellite’s rotation so that it matches its orbital period, so that one face permanently points toward its primary planet – this is what happened in the case of our own moon for example, and the satellites of Jupiter. If an object is small enough and far enough away from its primary (as is the case with Nereid and Phoebe) then the timescale in which this happens is longer than the age of the solar system, which means that today they have rotation periods independent of their orbital periods – but this isn’t what has happened in Hyperion’s case.

The reason for this is Titan, Saturn’s largest moon and the next satellite in from Hyperion. Hyperion is in a 3:4 orbital resonance with Titan, which means that Hyperion completes three orbits around Saturn in the same time that Titan takes to complete four orbits – so they reguarly get back to the same configuration relative to eachother. This means that Hyperion receives repeated, regular gravitational “tugs” from the much more massive Titan, which may explain why Hyperion’s orbit is eccentric too. All of these factors combine to prevent Hyperion from tidally locking to Saturn – but it gets even crazier than that. Hyperion actually has chaotic rotation as a result of this – not only is it not tidally locked, but its rotation period also varied, and what’s more the orientation of its axis of rotation varies too! Hyperion is literally tumbling through space as it orbits Saturn, with no two rotations being the same length or even around the same axis! This explains why the dark material is found in craters all over the satellite, since it lands on a random surface when it falls onto Hyperion from the outer Saturnian system.

All in all, Hyperion is a rather fascinating little satellite. There are still at least five more flybys of Hyperion scheduled for the rest of Cassini’s (very) extended mission, so hopefully we’ll be seeing more interesting images in the coming years! I’ll leave you with a gallery of (false-colour) images from the flyby sequence at the top of this post, and a rather nice animation that I found showing the whole sequence!

Hyperion Gallery (November 2010, Image credit: NASA/JPL)

Hyperion, high phase angle Hyperion, high phase angle Hyperion
Hyperion Hyperion, low phase angle Hyperion, low phase angle

(colour!) Hyperion flyby animation by IanR at