Astronomers on Wednesday said they had found possibly the biggest black hole ever observed, a leviathan with a mass 17 billion times that of the Sun, brooding at the heart of a distant galaxy.

The black hole is as unexpected as it is vast, for it accounts for nearly a seventh of its galaxy's mass, a finding that may rewrite theories of cosmic formation, they said.

Named NGC 1277, the monster lies 220 million light years away in a small galaxy just a tenth the size of our Milky Way.

The hole's maw is more than 11 times wider than Neptune's orbit around the Sun.

It accounts for a whopping 14 percent of the galaxy's mass, compared with the 0.1 percent that is the norm for galactic black holes.

"This is a really oddball galaxy," said Karl Gebhardt of the University of Texas at Austin in a press release.

"It's almost all black hole. This could be the first object in a new class of galaxy black hole systems."

NGC 1277 is already the second biggest black hole ever observed, and it is a strong contender for the top spot, for the current record holder, spotted in 2011, has still not been precisely calculated. It is somewhere between six and 37 billion solar masses.

Black holes are the most powerful known forces in the Universe, creating a gravitational field that is so strong that even light cannot escape from it.

A black hole of stellar mass is formed when a very big star collapses in on itself at the end of its life.

It may then grow by gobbling up other stars and merging with other black holes, sometimes creating "supermassive" black holes which scientists say inhabit the centres of galaxies.

NGC 1277 challenges part of the galactic black hole theory because of its size relative to its galaxy.

In addition, it sits at the centre of a small disc-shaped galaxy, whereas a black hole of this size would have been expected in a far bigger blob-like, or "elliptical," galaxy.

Further work is needed to confirm whether NGC 1277 is a one-off or part of a hitherto-overlooked process of black hole creation.

"The galaxy hosting the new black hole appears to have formed more than eight billion years ago, and does not appear to have changed much since then," the Max Planck Institute said.

"Whatever created this giant black hole must have happened a long time ago."