Lonely planets can blame big, pushy bullies. Giant planets may bump off most of their smaller brethren, partly explaining why the Kepler space telescope has seen so many single-planet systems.
Of the thousands of planetary systems Kepler has discovered, about 80 per cent appear as single planets passing in front of their stars. The rest feature as many as seven planets 鈥� a distinction dubbed the Kepler dichotomy.
Recent studies suggest even starker differences. While multiple-planet systems tend to have circular orbits that all lie in the same plane 鈥� like our solar system 鈥� the orbits of singletons tend to be and are with the spins of their stars.
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Now, a pair of computer simulations suggest that hidden giants may lurk in these single systems. We wouldn鈥檛 be able to see them; big, Jupiter-like planets in wide orbits would take too long for Kepler to catch, and they may not have orbits that cause them to pass in front of their stars in our line of sight. But if these unseen bullies are there, they may have removed many of the smaller planets in closer orbits, leaving behind the solitary worlds that Kepler sees.
The simulations show that gravitational interactions involving giants in outer orbits can eject smaller planets from the system, nudge them into their stars or send them crashing into each other.
Pushy planets
鈥淭here are bigger things out there trying to pull you around,鈥� says at the University of Toronto, Canada. She and her team also showed the giants pull the few remaining inner planets into more elliptical and inclined orbits 鈥� the same kind seen in many of the single systems Kepler has spotted.
at Lund Observatory in Sweden and his colleagues mimicked more general scenarios, including planets orbiting a binary star system, and got similar results. The studies complement each other, say Huang and Mustill.
鈥淲e know these configurations have to occur in some fraction of exoplanet systems,鈥� Mustill says.
But that doesn鈥檛 mean they鈥檙e universal. 鈥淭hey don鈥檛 occur all the time, and this is one reason why you can鈥檛 explain the large number of single planets purely through this mechanism,鈥� Mustill says. According to his analysis, bullying giants can only account for about 18 per cent of Kepler鈥檚 singles.
To confirm their proposed mechanism, the researchers must wait until next year for the launch of the Transiting Exoplanet Survey Satellite (TESS), which will target closer and brighter systems 鈥� and thus be easier for follow-up observations to uncover the bully planets.
Journal references: arXiv; ,听
