11/13/2012 11:23 am ET Updated Jan 13, 2013

Other Solar Systems: Flatter Than Pancakes but Thicker Than Crepes

Located in an Earth-trailing orbit around the Sun, a nifty spacecraft called Kepler is part of NASA's mission to look for Earth-sized planets around other stars. The Kepler spacecraft has just one main task: it stares at hundreds of thousands of stars, nearly continuously, without pausing to blink.

Since the launch of the spacecraft in 2009, researchers have used data sent back from the Kepler spacecraft to discover over 2,300 planet candidates. That is a lot of planets -- prior to the Kepler era, we only knew of about 500 planets total. So how is Kepler so efficient at finding planets? By simultaneously staring at hundreds of thousands of stars nearly constantly without pausing to take a break, Kepler measures each star's brightness over time. If the light coming from a star drops off periodically, it can be due to a planet crossing in front of its host star on its orbit. Looking for such glimpses of a planet is not trivial; this task allotted to Kepler is akin to staring at hundreds of thousands of car headlights a few miles away to look for the dimming due to a mosquito crawling across the headlight.

So where do pancakes and crepes come in? One of my research goals is to investigate the flatness of planetary systems in general. A flat planetary system means that the orbits of the planets in the system are pretty well-aligned with each other, meaning low inclinations and so the system appears flatter. On the other hand, a thick planetary system has planets with orbits that are more misaligned, meaning larger inclinations and so the planetary system would appear thicker. You can begin to see the analogy with pancakes and crepes.

To investigate just how flat or thick planetary systems are, I used the planet detections from Kepler as my data sample. I developed computer models of artificial planetary systems: some had flatter configurations (low inclinations) and some had thicker configurations (higher inclinations). Then I compared the results and properties of my simulated planetary systems with the actual properties of Kepler planets -- this key step allowed me to determine which models were better matches to the data. Based on best-fitting models, I discovered that in general, planetary systems have low inclinations (most have less than 3 degrees) and are therefore quite flat! My paper (Fang & Margot 2012) on these findings has been accepted by The Astrophysical Journal and is currently in press; you can find a free version here. To put this result into perspective, I declared that planetary systems are as flat as pancakes.

To determine the veracity of this analogy, Jean-Luc Margot, co-author of my study and UCLA professor (and accomplished chef), held a series of cooking trials. He cooked up a batch of pancakes and measured their thickness and radii. After discarding the first and last pancakes as obvious outliers, he found a mean thickness of 7.3 mm and a mean radius of 65 mm. This corresponds to an inclination angle of about 6 degrees, which is larger than the 3 degrees I found for planetary systems, meaning that planetary systems are generally thicker than pancakes! On the other hand, crepes were simply too thin. So the best visualization for the flatness of planetary systems is that they are somewhere in between that of a pancake and that of a crepe. Gives food for thought next time you eat at your local IHOP.

One reason we care about how flat or thick planetary systems are is because such knowledge can provide important constraints on planet formation and evolution models. For instance, flatter and well-aligned planetary systems are consistent with a standard formation model of planets forming in a protoplanetary disk. Thicker and more misaligned planetary systems can be indicative of past perturbative events that increased their inclinations. Consequently, information on the flatness or thickness of planetary systems can reveal clues about important planet formation and evolution processes.

Another interesting result of my investigation is that we can compare the flatness of planetary systems to our best known planetary system: the Solar System. The orbits of the planets in the Solar System generally have low inclinations. With the exception of Mercury, their inclinations are all low at less than 3 degrees, which is consistent with my findings of planetary systems in general. So perhaps our Solar System is not that different from other planetary systems in this regard. Mercury's orbit is more inclined at about 6 degrees (so thicker and more like a pancake!).

In the meantime, Kepler is still diligently working away by staring at stars to find more planets. Earlier this year, the mission was approved by NASA to be extended to 2016, given Kepler's revolutionary impact in planetary astronomy and studies of stellar variability. As more data streams in, researchers will be able to use a longer baseline of observations to make exciting discoveries.