Astronomers looking for the elusive Planet 9 in the far reaches of the solar system might want to widen their search to include something much smaller but incredibly dense - a primordial black hole.

The idea that the feature influencing some of the most distant objects in the solar system could be the size of a bowling ball is being floated by two physicists in a paper published on the arXiv preprint server this week.

Four years ago scientists from the California Institute of Technology in the US said they had good evidence for the existence of Planet 9, based on mathematical and computer modelling. They expected it to be a gas giant about as big as Neptune in a far distant orbit that meant it took 10,000 to 20,000 years to circle the Sun. 

Science website Gizmodo is reporting the scientists behind the black hole proposal - Jakub Scholtz and James Unwin - still think a planet is the more likely explanation, but they made the black hole proposal because they wanted astronomers to be more creative in their search.

No one has seen any primordial black holes yet but some astronomers think they could exist.

Primordial black holes could only have formed in the first second following the Big Bang, according to Astronomy.com. For that brief period some areas of space were denser and hotter than others, and those areas could have collapsed into black holes.

Planet 9 is thought to have a mass of something between five and 15 times that of Earth. A five-Earth mass black hole could fit into a person's hand, while one with the mass of 10 Earths would be the size of a bowling ball.

Unwin told Gizmodo that he and Scholtz wanted astronomers to widen their search for Planet 9. "We advocate that rather than just looking for it in visible light, maybe look for it in gamma rays. Or cosmic rays," he said.

A report on the paper in Science said it was thought a black hole with the mass of a planet would likely be surrounded by a halo of dark matter that could stretch up to 1 billion kilometres on every side. Interactions between dark matter in that halo - particularly between dark matter and dark antimatter - could release a flash of gamma rays.

Unwin and Scholtz were planning to search data from the Earth-orbiting Fermi gamma ray space telescope, looking in particular for groups of sporadic gamma ray flashes moving slowly across the sky.