Two scientists at the Australian National University (ANU) have found further evidence that far away planet systems fall in a flat plane like our Solar System.

The research found planetary systems were more likely to feature the orbits of all planets aligned together in a two-dimensional plane. This conclusion goes against traditional assumptions that planetary systems are flared like the open end of a pair of bell-bottom trousers.

Associate Professor Charley Lineweaver and PhD student Tim Bovaird (the lead author of the paper), both from the ANU Research School of Astronomy and Astrophysics, challenged the notion that the Solar System was exceptional in its structure.

Lineweaver and Bovaird used data from the Kepler space telescope which has detected over 4,000 planets orbiting 3,200 stars. Only 656 of these stars have multiple planets with the rest hosting a solitary lone planet.

“The wealth of the Kepler planet data allows for the first time detailed studies of planet systems outside the Solar System. We are now able to ask and answer questions like, how common are planet systems like our own?” Bovaird said.

Prior research simulations have only matched the observed data by assuming what has been called the ‘Kepler Dichotomy’ – a hypothesis that there are only two types of star: one with only one planet and another with multiple planets.

“Simulations with flared planet systems were slightly easier to perform and that is what researchers had assumed,” Bovaird said.

“But this is an odd assumption because the inner part of our Solar System is flat, not flared. When we dropped the assumption that planet systems are flared, simulations naturally matched the observed data without using the Kepler Dichotomy.”

Researchers found that the average number of planets per star was approximately two for orbital periods (the amount of time it takes to go once around a star) between three and 200 days. The radii of the planets in the model were between one and five Earth radii.

The paper, A Flat Inner Disk Model as an Alternative to the Kepler Dichotomy in the Q1 to Q16 Planet Population, will be published in The Monthly Notices of the Royal Astronomical Society.

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