Most known planets orbit in the same plane, one that corresponds to a star's equator. In our solar system, Pluto (now demoted) is a diminutive exception.
So astronomers were surprised to find a world much larger than Jupiter in an off-kilter orbit around a faraway star.
The orbit of planet XO-3b is tilted about 37 degrees from the star's equator.
Theory holds that such a misalignment must have occurred as a result of a disturbance sometime after the planet's formation, according to a statement released today by MIT. Close encounters with other planets could greatly amplify a slight initial tilt, the astronomers note, but they don't yet know what caused the orbit of XO-3b.
The planet was found in 2007, but determining its orbit was tricky.
XO-3b was discovered by a method that depends on a chance alignment of the planet's orbit with the line-of-sight between its star and the Earth. Because of that alignment, the planet sometimes passes directly in front of the star as seen from here -- an event called a transit -- thus causing a slight dimming of the star's light. It dimmed the star's light by about 1 percent.
But to measure the angle of its orbit means "we have to be sneaky about it," said MIT physicist Joshua Winn who led the effort. It turns out that if a planet crosses the star's disk at an angle to the star's own rotation, it causes a distinctive pattern of change in the overall color of the star, as measured by a highly sensitive spectrograph, because of the Doppler shifts caused by the star's rotation.
The Doppler shift is experienced when an ambulance heads toward you: Its sound waves are compressed, then the pitch changes as the ambulance goes the other way and the sound waves are stretched out.
Like many of the 350 or so other known extrasolar planets, XO-3b is large and close to its host star — those are the ones findable by present technology. This world is about 13 times as massive as Jupiter and orbits its star once every 3.5 days.
The orbit of XO-3b was hinted at by another team last year. New observations, carried out by Winn and his team in February at the Keck I Observatory in Hawaii, provided a clear, solid measurement of the planet's distinctive tilt. The results are detailed in the Astrophysical Journal.