While such tilted orbits have been found in planetary systems featuring a so-called hot Jupiter -- a giant planet in a close orbit to its host star -- until now such a thing had not been seen in multiplanetary systems without such a big "interloping" planet, they reported.
"This is a new level of detail about the architecture of a planetary system outside our solar system," Iowa State University physics and astronomy Professor Steve Kawaler, a co-author of the study, said. "These studies allow us to draw a detailed picture of a distant system that provides a new and critical test of our understanding of how these very alien solar systems are structured."
Two planets in close orbits and a massive third planet in a distant orbit circle Kepler-56, an aging red giant star about 3,000 light years from Earth, the researchers said.
Generally the simplest way for a planetary system to develop is with orbits in the same plane as the host star's equator, they said, as they coalesce from a thin disk of dust and gas surrounding the host star.
As an example, all the planets in our solar system orbit within 7 degrees of the sun's equator.
A tilted orbit can mean a planet had a traumatic youth, pulled into a different plane after encountering another planet, the researchers said.
That's generally the case with migrating hot Jupiter that encounter other planets and material, resulting in a higher chance of tilted orbits.
In the case of Kepler-56, however, the more massive outer planet -- in a fairly normal orbit -- seems to be maintaining the tilted orbits of the two inner planets.
"It issues a continuous tug on the orbit of the smaller ones, pulling them into their inclined orbits," Kawaler said.
The finding is strong evidence tilted planetary orbits are possible even in systems that don't contain a hot Jupiter, the astronomers said.
The study by Kawaler and lead author Daniel Huber of NASA's Ames Research Center in Mountain View, Calif., has been published in the journal Science.