Most Planetary Systems are ‘Flatter than Pancakes’

UCLA graduate student Julia Fang. Credit: Julia Fang / UCLA

Our solar system looks like many others, "flatter than pancakes," report UCLA astronomers who were able to statistically determine the properties of planetary systems using the latest data from NASA’s Kepler space telescope.

The number of planet candidates around other stars discovered so far by Kepler is more than three times the current number of such planets found by other means, notes UCLA graduate student Julia Fang, lead author of a new study that uses data from Kepler as a laboratory to study the typical number of planets in each planetary system and the degree of flatness of planetary systems.

In new research submitted to the Astrophysical Journal, Fang and UCLA professor Jean-Luc Margot developed detailed computer models of planetary systems and compared them to the properties of Kepler data. Their results reveal very flat orbits: more than 85 percent of planets have inclinations of less than three degrees.

The scientists examined the trajectories of planets around their host star and found that the trajectories are very closely aligned in a pancake-like geometry, much like the planets in our own solar system. These very flat orbits imply low relative inclinations with planets all orbiting near the same plane, Fang said.

"Next time you eat a thin-crust pizza, you can get a sense of the flatness of a typical planetary system," Margot said.

"We find it thrilling how flat and aligned these planetary systems are," Fang said.

UCLA professor Jean-Luc Margot. Credit: UCLA

An important motivation for the study was to compare the properties of these Kepler planetary systems to the solar system and to determine how typical the solar system is, said Margot, UCLA associate professor in the departments of Earth and Space Sciences, and of Physics and Astronomy. Seven out of the eight planets in our solar system have inclinations less than three degrees, with Mercury as the exception.

"It looks like our results are consistent with the flatness also evident in the planetary orbits in our solar system," Fang said. "Our solar system may be common compared to other planetary systems in this regard. Perhaps we’re not that special."

"I made pancakes this weekend to verify our analogy," Margot said. "I measured a mean thickness of 7.3 mm (a little under 1/4 inch) and a mean radius of 65 mm (about 2.5 inches). This corresponds to inclinations of six degrees. So most planetary systems are flatter than pancakes, by about a factor of two. The best mental image for the geometry of planetary systems is somewhere between a crepe and a pancake."

The team’s intricate models of planetary systems also yielded the typical numbers of planets per planetary system.

For orbital periods out to 200 days, about 75 percent of systems have one or two planets, Fang said.

As additional data from the Kepler mission streams in, the scientists will be able to extend their study to longer orbital periods.

Artist impression of NASA’s Kepler Space Telescope. Image Credit: NASA

For planetary astronomers, the launch of Kepler and its ground-breaking discoveries has ushered in a golden era of exoplanet science.

"Kepler is an amazing telescope in space; so far it has discovered a treasure trove of planets totaling more than 2,300 candidates," Fang said.

The Kepler space telescope stares at more than 100,000 stars for glimpses of planets crossing in front of the stars, thus blocking off some of the starlight. This is akin to staring at more than 100,000 car headlights a few miles away to look for the dimming due to a mosquito crawling across the headlight, Fang said.

"Our study has begun finding answers to fundamentally important questions in planetary astronomy," Fang said. "We’ll be presenting exciting results at upcoming conferences."

Fang, a doctoral candidate, and Margot will continue to study the haul of planets discovered by Kepler to learn more about their interesting dynamical properties.