EXOPLANETARY systems are like peas in a pod, whatever type of star the planets orbit. This challenges our ideas about how such systems form.
A team led by Lauren Weiss at the University of Montreal in Canada has looked at 909 planets discovered by the Kepler space telescope in 355 systems. All planets in a given system seem to be close in size and similarly spaced in their orbits when compared with planets in other systems. 鈥淲e see this pattern happening again and again,鈥� says Weiss 鈥� regardless of what kind of star these planets are orbiting ().
That鈥檚 not what we鈥檇 expect, given how we think star systems are born: that stars form from a cloud of gas and dust, pulling it into a thick disc as they rotate. Denser clusters of gas and dust within the disc condense into planets, suggesting there should be a link between planets and their star.
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The team thinks something other than stellar mass must influence how protoplanetary discs give rise to planets, such as the total mass of the disc, the solid mass within the disc or what happens to the disc after a planet鈥檚 initial formation.
鈥淭here鈥檚 probably something related to the physics of the disc that the planets are forming in that is determining how big the planets grow and how far apart from each other they end up,鈥� says Weiss. 鈥淏ut this idea has yet to be tested.鈥�
It鈥檚 also possible that these patterns are just a fluke created by our limited data. Kepler can only find planets with short orbital periods 鈥� those that crossed in front of their star during the four years of the spacecraft鈥檚 mission. That鈥檚 like only looking at Mercury, Venus, Earth and Mars in our own system.
So can we really build theories on Kepler鈥檚 limited observations? 鈥淭hat鈥檚 the question that keeps me and many other people up at night!鈥� says Weiss.
This article appeared in print under the headline 鈥淧lanets around other stars fit a strange pattern鈥�
