London: Planetary scientists have claimed that the majority of 'elliptical' galaxies are not spherical but disc-shaped, resembling spiral galaxies such as our Milky Way with the gas and dust removed.

The results come from Atlas3D, a survey of all 260 early type galaxies in a well-defined volume of the nearby universe. It shows a much closer link between "elliptical" galaxies and spiral galaxies than previously thought.

The findings are likely to change our ideas of how galaxies form and see astronomy text-books rewritten, say the findings to be published in an upcoming issue of the 'Monthly Notices of the Royal Astronomical Society'.

"Because we rely on optical images, up until now it has been very difficult to separate discs of stars seen face-on from rounder, spherical balls of stars seen edge-on," said Dr Michele Cappellari of Oxford University, a Royal Society Research Fellow who is the UK lead of the Atlas3D project.

"But because stars in a thin disc rotate much faster than those in a spheroid, obtaining maps of stellar motions for all elliptical galaxies in the sample, we have shown that out of these 66 percent are disc-like," Cappellari added.

The findings suggest that the idea that galaxies can be clearly separated into two different "families", spiral galaxies and elliptical galaxies, reflecting two distinct paths to galaxy formation, is inaccurate. This "two families" approach was famously visualized in Edwin Hubble's "tuning fork" diagram of 1936 in which elliptical nebulae, which consist of more spherical groups of stars, split off into two prongs of spiral galaxies.

The Atlas3D results suggest that this tuning fork should be replaced with a "comb-like" diagram where elliptical galaxies are parallel to spirals and linked to them along the teeth of the comb while only a few true elliptical are separated into the handle.

"According to our survey only a small fraction of elliptical galaxies, the 'slow rotators', are genuine spheroids. It reveals a strong family resemblance between elliptical and spiral galaxies once we can adjust for whether we are seeing them face-on or from the side.

"This close relationship will need to be considered in any future models of how galaxies form. It's an exciting moment, after four years of work in the project, we have the final piece of the puzzle which enables us to say that text-books used to teach astronomy for over 70 years now need to be revised," Cappellari said.