Nine – The Elusive Planet in Our Solar System

A hypothetical Ninth planet has been lurking on the outskirts of our Solar System.  But Planet Nine may not be a planet at all… 

A new study, published September 24 on the arXiv pre-print server, suggests that the mysterious object may be a primordial black hole.

Search for an elusive Ninth Planet

Over a century ago, astronomers noticed a discrepancy in the gravitational field that exist beyond Pluto.  They suggested that the weird gravitational pull observed to be acting on distant bodies in the Solar system were being caused by another planetary body.

And the existence of Planet Nine was speculated.

Planet X

Percival Lowell was one of the first astronomers to search for the hypothetical Planet Nine.

Following the discovery of Neptune in 1846, there was much speculation that another planet might exist beyond its orbit.

The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of both Uranus and Neptune.

After extensive calculations Percival Lowell (1855 – 1916) predicted the possible orbit and location of the hypothetical trans-Neptunian planet.

He began an extensive search for it in 1906.

He called it Planet X.

400 to 800 times as far from the Sun as Earth


Our Solar System still harbours some secrets…

The Solar system is a big place, with many tiny worlds in its outer regions yet to be found. 

Despite rapid advances in astrophysical technology, and what a team of US and UK astronomers calls the “growing body of observational anomalies” connected to distant objects in our Solar system, Planet Nine was never directly observed.

So the possibility of finding another large planet orbiting the Sun is nothing short of fascinating.

As yet undiscovered, this super Earth-sized planet would have a predicted mass

5 to 10 times bigger than the Earth


But try as they might, scientists could not find Nine.

The extra planet remains a mystery to this day.

However, they did not give up.

In 2017, astronomers enlisted the help of amateur stargazers to help look out for Planet Nine.

“If this planet exists, it’s already in one of our thousands and thousands of images in existing databases.”

Now, astronomers are exploring another idea.

Gravitational Anomalies

The recent paper, by Jakub Sholtz at Durham University’s Institute for Particle Physics, and James Unwin, assistant professor at the University of Illinois, is available on the arXiv preprint service.

Their research ponders the possibility that the elusive Planet Nine, theorised to be orbiting the Sun at a distance between 300 and 1,000 AU, could be an old compact black hole.

Their intriguing hypothesis focus on two unsolved gravitational anomalies of similar mass:

    • anomalous orbits of trans-Neptunian objects (TNOs) and
    • an excess in micro-lensing events.

What is interesting is that both events are due to objects with masses estimated to be between 0.5 and 20 Earth masses.

Extreme Trans-Neptunian Objects

The elongated orbit of Planet Nine is predicted to be 400 to 800 times as far from the Sun as the Earth.

Its gravitational effects could explain the unusual clustering of orbits for a group of extreme trans-Neptunian objects (eTNOs) – bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth.

These eTNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted.

The anomalies of TNO orbits are assumed to be triggered by a new gravitational source in the outer solar system.

While it is widely accepted that this source could be a free-floating planet, Scholtz and Unwin chose instead to “focus on a more exciting possibility.”

A primordial black hole is causing the anomalies.


The researchers argue that their PBH (primordial black hole) scenario is not unreasonable and should be taken into account:

“Capture of a free-floating planet is a leading explanation for the origin of Planet Nine, and we show that the probability of capturing a PBH instead is comparable.”


Primordial Black Holes

Primordial black holes, spawned shortly after the Big Bang, have the ability to either remain tiny or quickly gain mass. In the former case, they are candidates for dark matter. In the latter case, they could serve as seeds for supermassive black holes.  Source:

Planet Nine could actually be a Primordial black hole (PBH) – a hypothetical type of black hole thought to have formed  as a result of density fluctuations in the very early Universe.

Small primordial black holes are non-baryonic and plausible candidates for dark matter.  They belong to the class of massive compact halo objects (MACHOs).

Scholtz and Unwin assume that PBHs could be even closer to us than we think.

Relatively small PBHs would have emerged soon after the Big Bang, and those with the lowest mass would have by now likely evaporated.

However, primordial black holes with larger masses may still exist, evaporating at the present epoch, even though they have never been directly observed.

According to the paper, such a hidden black hole on the edge of our Solar system could be the missing link behind this mysterious ninth planet. 

However, it could prove difficult to confirm this theory, as such a hypothetical PBH, with a mass of around five Earth masses and a radius of about five centimetres, would have a Hawking temperature of approximately 0.004 K, making it colder than the cosmic microwave background (CMB)

Therefore, the power radiated by a typical PBH alone is minuscule, which makes it hard to detect.

Looking for Annihilation Signals

In order to overcome this obstacle, the authors of the paper propose to search for “annihilation signals” emitted from the dark matter micro-halo around a PBH.

Using micro-lensing to study a Dark Matter Halo. Source:

Such a dark matter halo, if annihilating, is thought to be able to provide a powerful signal that could be identified by observations.

Hence, astronomers suggest dedicated searches for moving sources in X-rays, gamma-rays and also other high-energy cosmic rays, which could provide more evidence supporting the PBH hypothesis.

What if Planet 9 is a Primordial Black Hole?

We know “something” is out there because it affects other astronomical objects in its vicinity.  It’s really big, really far away from Earth, and it takes a really long time to orbit the Sun.

So, it almost certainly exists…

The gravitational effects of a black hole could explain the atypical orbits of planetary objects located beyond Neptune.

It could also explain why Planet Nine has never been directly observed.

“Perhaps the most natural explanation [for the gravitational anomalies] is that they are caused by the existence of an unknown population of planets.

“However, this would imply that our models for planet formation may need updating to account for this new population of free floating planets.”

The race is on to find it.


And you never know who might discover it.

It could be you.