Posted on Thursday 17th October 2013
Scientists at the University of Birmingham have discovered a stellar system in our Galaxy where the spin of its ‘red giant’ star and the orbits of its planets are misaligned, according to research published in the journal Science today (17 October 2013).
In our own solar system the rotation of the Sun and the orbits of the planets are perfectly aligned, with the spin axis of the Sun at right angles to the orbits of the planets. However the spin axis of Kepler-56, an old star four times bigger than our Sun, is strongly tilted with respect to the orbits of its planets.
Strange alignments like this have been seen before in systems with single ‘close-in’ planets, so-called ‘hot-Jupiters’, but never before in a system with several planets.
The scientists were able to identify the two planets in orbit around the star as these were revealed by tiny miniscule dimming of the starlight as the planets transited, or passed across, the host star. They were also able to detect that the star was tilted on its axis by detecting the natural oscillations of the star, which make it resonate or vibrate like a musical instrument. These ‘asteroseismic’ signatures shift ever so slightly, depending on the orientation of the spin-axis of the star relative to the observing Kepler spacecraft. This makes it possible to compare the orientation of the star’s spin with the planetary orbits.
Observations that the scientists made using the huge Keck telescope on the summit of Mauna Kea, in Hawaii, solved the puzzle of Kepler-56. They revealed the presence of a previously unseen third planet that does not transit, gently tugging on the host star. The orbit of the third planet is also tipped relative to the two transiting planets, and in fact it is the pull it exerts on the other planets that explains the strange misalignment.
Dr Andrea Miglio from the University of Birmingham’s School of Physics and Astronomy, who led the asteroseismic modeling work on Kepler 56, said: ‘Our results provide further evidence to help explain how the weird and wonderfully strange alignments discovered in some planetary systems came to be. We are also getting a better handle on how common systems like our own solar system are, with its well ordered alignment.’
Bill Chaplin, Professor of Astrophysics from the University of Birmingham said: ‘This work has implications for understanding stability in planetary systems, which can have relevance for development of life on potentially habitable planets.’
Notes to Editors
An animation showing how the scientists measured the tilt of the star:
Here is an explanation of the video;
The observed strengths of different natural oscillations of the star change depending on how the spin-axis of the star is tipped relative to the observing Kepler spacecraft. The top row shows three different oscillation patterns on a star. As the animation runs, the tilt of the spin-axis of the star changes: initially, it points at right angles to the line-of-sight of the observing Kepler spacecraft, but by the end of the animation it is tipped directly towards Kepler. The graphs on the bottom row represent the "seismograph"-like traces of changing brightness as a function of time that each oscillation gives rise to. As the tilt of the spin-axis changes, so too do the amplitudes of the traces given by the different oscillations. The way the traces change depends on which oscillation one is considering. By measuring the ratios of the strengths of different oscillations, we obtain information on the tilt of the spin axis of the star.
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