‘Dark exoplanets’: Planets made of dark matter might be orbiting stars

There are believed to be planets where it rains diamonds, and planets so dark they absorb 94% of visible light - but could there be planets made of ‘invisible’ dark matter?

A new pre-print study suggests that “dark exoplanets” could exist - and if they do, we could spot them.

Many scientists believe that dark matter - matter that we cannot see - is everywhere in the universe, and helps explain how galaxies hold together.

Ordinary matter (or ‘baryonic’ matter) isn’t enough to account for gravity, so scientists believe there must be something else out there.

The researchers write in their pre-print paper, “Dark matter is still one of the biggest mysteries of the universe, even though astrophysicist Fritz Zwicky pointed out its existence close to a century ago.”

Read more: Mysterious “rogue planet” could be even weirder than we thought

The researchers believe that it’s possible that a large ‘blob’ of dark matter could behave like a planet orbiting a star.

They write, "A macroscopic dark matter state with its mass and/or radius similar to those of a planet will behave as a dark exoplanet if it is bounded to a star system, even if the object's underlying physics resembles something else entirely.”

The researchers say that these huge blobs of mysterious dark matter might be captured if they form near a star - or even if they pass through one.

If so, they could be detected when they ‘transit’ in front of a star - a technique currently used to spot more normal planets.

The researchers write, “To see a transiting dark exoplanet , it must become gravitationally bound to an ordinary star. If stars tend to be born in regions of enhanced dark matter density (in analogy to early stars, though it is unclear if this is true for later-forming stars), then the dark exoplanet may become bound in situ. Alternatively, the dark exoplanet may be captured in a similar way to free-floating planets.

Another possibility is if a free-floating dark exoplanet passes through a star, its interactions with the star could slow it enough to allow it to become bound.”

Read more: Astronomers find closest black hole to Earth

Earlier this year, researchers suggested that a collision between two mysterious ‘dark matter stars’ may have been behind a gravity wave ‘ripple’ that reached Earth last year, researchers have suggested.

Gravity waves are ripples in the fabric of spacetime that travel at the speed of light - and are produced by incredibly violent events like collisions between black holes or neutron stars.

But a signal picked up last year may have had a far more mysterious origin, researchers at the Galician Institute of High Energy Physics (IGFAE) and the University of Aveiro believe.

Instead of being produced by black holes, they suggest it might have been produced by a collision between boson stars - hypothetical objects made of ‘dark matter’, which scientists believe makes up 27% of the Universe.

The research was published in Physical Review Letters.

Boson stars have never been detected, but the gravitational wave could have been created by a collision between two of the mysterious objects.

Watch: What is dark matter?