Planets Found With Crisscross Orbits—A First

Two planets orbit the star Upsilon Andromedae in an illustration.

A "super Jupiter" and its sibling world have been found circling their parent star with steeply tilted orbits—the first time such a configuration has ever been spotted, astronomers say.

All eight planets in our solar system orbit the sun in roughly the same plane, an imaginary disk that extends from the sun's equator.

But new data show that two of the three Jupiter-like planets known to circle the sunlike star Upsilon Andromedae have orbits that are tilted 30 degrees from each other.

The orbit of the third, innermost planet is still unknown. (Find out about a batch of planets recently found outside our solar system that have tilted, "wrong way" orbits.)

The unusual discovery suggests that astronomers can't assume all star systems with multiple worlds will always be able to keep planetary siblings in line. (See "Our Solar System May Have Millions of 'Twins.'")

The Upsilon Andromedae system probably formed the same way as other planetary systems, including our own, said study leader Barbara McArthur of the McDonald Observatory at the University of Texas at Austin.

When a cloud of material condenses and collapses to form a star, some leftover material can circle the star's middle in what's called a protoplanetary disk. This disk, in turn, can coalesce to form planets.

In Upsilon Andromedae's case, something knocked two of the outer planets into strange, crisscrossing orbits after the star system had been born.

The star has a binary companion, a dim red dwarf star that orbits Upsilon Andromedae at a distance of about 70 billion miles (112 billion kilometers). If that red dwarf has a very elongated orbit, it might have swooped in close to Upsilon Andromedae sometime in the past, disturbing the planets' orbits with its gravity.

There's also evidence that a "missing" planet might have been involved in a gravitational tug-of-war with its peers. The planets could have pushed each other around so much that the loser got thrown out of the system, while the remaining worlds were left askew.

"Planet-planet scattering is a very likely cause for what we see," McArthur said.

Massive Planet Almost a Dim Star?

The new information about the Upsilon Andromedae system, collected using the Hubble Space Telescope and a suite of ground-based observatories, also allowed the researchers to more precisely calculate the masses of the star's planets.

Planet c, which orbits just 76 million miles (122 million kilometers) from the star, has 14 times the mass of Jupiter, the scientists estimate. A gassy body that massive would normally qualify as a brown dwarf, a starlike object just shy of being able to start hydrogen fusion and ignite.

"But people's theories of the formation of brown dwarfs [assume] that they were formed like stars," said study co-author Fritz Benedict, also of the McDonald Observatory. (See a picture of two brown dwarfs said to be the dimmest "stars" in the universe.)

"This one was obviously formed like a planet," since it's part of a larger planetary system, Benedict said. Study leader McArthur coined the term "super Jupiter" to describe the ultramassive gas giant.

Meanwhile, planet d has ten times Jupiter's mass and lies about 237 million miles (381 million kilometers) from Upsilon Andromedae.

That puts the planet in its young star's habitable zone, the region where rocky, Earthlike worlds would theoretically get the right amount of heat for liquid water—and thus life—to exist, Benedict said.

Since planet d is a gas giant, it's unlikely to harbor life as we know it, he added—"but there could be [potentially habitable] moons."

The tilted planetary system was described yesterday at the 216th meeting of the American Astronomical Society in Miami, Florida. The study will appear in the June 1 issue of the Astrophysical Journal.