海角大神

Scientists have found a multi-planet system in which the planets are arranged at a tilt to their home star. The finding, based on Kepler telescope data, offers an unusual level of precise detail about a system of worlds thousands of light years afield from our own, while also highlighting just how much is still unknown about how planetary systems form.

鈥淎s we discover more planetary systems, we're learning about the variety of 'architectures' that they display,鈥� says Steven Kawaler, the leader of the Kepler Asteroseismic Investigation and an author on the paper, .

鈥淓very system has a different story to tell,鈥� he said.

The new system is orbiting Kepler-56, a red giant that is four times the mass of the sun and some 3,000 light years afield from Earth. Two of the star鈥檚 planets are in close orbit around it. Another, larger object is orbiting the star farther out 鈥� that object could be a planet, too, but scientists aren鈥檛 yet sure.

The Kepler-56 system, one of hundreds so far discovered in our galaxy, is normal so far as exoplanet systems go 鈥� which is to say that it is 鈥渘ormal鈥� in that, like most of these systems, it is unfolding to look little like what it had been expected it to look like and to upend what we thought we knew about the universe.

In this system, the perplexing detail that has thrown astronomers for a loop is its off-kilter arrangement: the star鈥檚 equatorial plane is at a 45-degree tilt to the three objects鈥� orbits. Though tilted orbits have been found before, they have appeared just in single planet systems where a hot Jupiter 鈥� an enormous planet similar in size to our Jupiter but orbiting near to its star and piping hot 鈥� was the lone companion to a star. Based on prevailing evidence, scientists had though that this tilt-a-whirl architecture must be unique to systems with a 鈥渉ot Jupiter.鈥澛�

And that had made sense, since, unless a traumatic event pulls things out of whack, planets tend to develop in neat alignment with their star, forming out of the thin disk of dust and gas swirling around the central star. In our own solar system, the eight planets all orbit the sun at no more than about a seven-degree tilt from the plane of the sun鈥檚 equator.

Put a hot Jupiter into that system, though, and everything rearranges. That鈥檚 because the formation of a hot Jupiter is believed to be a traumatic event in itself. It鈥檚 not possible to make a Brobdignagian planet close to a host star, since sufficient cold is required for enough ice and gas to fuse together to build an enormous planet. So, much as our Jupiter formed, hot Jupiters must begin in space鈥檚 cold, dark reaches, far out from a star鈥檚 warmth.

But, unlike our Jupiter, these grown-big planets end up in orbit near to their star, after gravitational interactions with nearby objects push them inward. There, the giant planet will become a 鈥渉ot鈥� Jupiter in the sizzling glow of its star.聽 It will also, in all likelihood, have an unusual alignment.

鈥淭his process is like a complicated 3-D billiard shot, often resulting in an orbit well out of the original plane of formation of the planet,鈥� says Dr. Kawaler.

It now appears, though, that tilted planetary orbits are possible in systems that don't have a hot Jupiter. That鈥檚 a find that could revise the basics of planet formation theory, suggesting that it鈥檚 possible that other, intervening forces must be accounted for in how systems develop. In this case, the researchers propose that the gravitational force of the third outer object is responsible for the tilt, pulling the two inner planets in line with its own tilt. Just what that object is, though, is still under investigation, the team reported.

鈥淭he final configuration of a planetary system tells the story of the formation process,鈥� says Kawaler.

鈥淚f all systems form through a quiet accretion process, then all systems should have planar orbits around their host stars,鈥� he said. 鈥淪ystems with tilted orbits 鈥� and the degree to which such systems are rare or common 鈥� tell us that the planet formation process itself is highly variable.鈥�

The Kepler telescope鈥檚 hunt for exoplanets has been hobbled since one of its reaction wheels was declared un-repairable this summer. But astronomers still have heaps of data from the telescope鈥檚 four-year-long gaze at the solar systems beyond our own. On Wednesday, NASA to its catalogue of far-flung worlds, four of them dug out of Kepler data.

To date, Kepler has helped astronomers find some 3,589 confirmed exoplanets and exoplanet candidates. As of this week, NASA has confirmed the existence of 919 exoplanets.