Planet 9 – is there something BIG in the outer Solar System?

There’s something strange going on with the planets on the outer edge of the Solar System….

For centuries, astronomers have been using slight oddities in the motion of  known planets to uncover other planets. Probably the most famous of these was the discovery of the outer gas giant, Neptune, which was found  in 1846 after its position in the sky was calculated by Urbain Le Verrier in Paris as well as being independently  by John Couch Adams in Cambridge, UK.  These calculations were based on slight variations in the orbit of the planet Uranus, itself only confirmed as a planet 1781.  While Uranus was discovered by William Hershel through repeated observations that picked up on the planet’s movement against the background of Milky Way stars, Neptune was hunted down based on the fact that the orbit of Uranus wasn’t quite right.  Urbain Le Verrier and others thought that there was something beyond  Uranus that was causing these orbital variations. As it turned out – they were right.

Using Le Verrier’s calculations, Neptune was found less than one degree from its predicted position by a telescope at the Berlin Observatory.  As anyone who has looked at Neptune through even a smallish telescope will realise it’s simply not that hard to see. Both Uranus and Neptune had been observed by astronomers going back as far as Galileo, however they were not recognised as planets. Their movement against the background stars wasn’t enough to draw attention.

From Galileo’s notebooks from the night of December 27th and 28th 1612 where he recorded Jupiter and its moons along with several background stars – one of which was Neptune. 

The discovery in 1930 of the first of the, what are now called, trans-Neptunian objects (TNO), Pluto, was more of a search, rather than predicting an object to be in a certain location.  It took until 1992 for the discovery of the next TNO , and now over 3,000 bodies, of a wide range of sizes, orbiting the Sun in the outer reaches of the Solar System have been found.  Astronomers estimate that there’s many more of these TNOs or “dwarf planets” to be discovered.  While Pluto will always be  “The People’s Planet” for lots of folks, the vast collection of the same type of rocky bodies extending into space hundreds of times the distance of the Earth to the Sun made it hard to continue to group Pluto with the planets comparatively closer to the Sun. After all, if Pluto is a planet, then why isn’t <insert the name of your favourite TNO> as well?

Please note that all the planets out to Neptune are visible in even small telescopes, and a larger amatuer telescope will show you Pluto (which appears simply as a very faint star and not that interesting), you won’t be able to spot these TNOs visually.

We need to have  bit of dive into the scale of the Solar System. One term you might read about is the Astronomical Unit or “AU”. It’s the average distance that the Earth to the Sun and it’s a smidge under 150 million km. We can say the Earth orbits at one AU, Mars is about 1.5 AU and Jupiter at around 5 AU. Saturn is at 9.5 AU, Uranus at 20 AU and Neptune at 30 AU. This might sound like a mind boggling distance – thirty times as far from the Sun as the Earth is –  but we’re only getting started. The first TNO, Pluto, orbits the Sun at between 30 and 49 AU. Makemake’s 306 year rotation around the Sun is between roughly 38 and 52 AU, and Eris between 38 and 90 AU. But again – we’re just getting started.

Astronomers are now looking at data around the orbits of a group of these TNOs, which have long, looping orbits, and are referred to as ETNOs (Extreme Trans-Neptunian Objects). They have long looping orbits, which bring some of them within 150 AU and then some  out to as far several hundred AU. The Earth, by comparison, has an almost circular orbit around the Sun.

A diagram showing some of the main ENTOs (Extreme Trans-Uranian Objects)

Why are these ETNOs of interest?

Apart from the discovery and cataloguing of these small, rocky bodies at vast distances, are they of any real interest?  After all, none of  of them will ever be visited by a spacecraft in our lifetimes, and possibly not for several generations!

First of all, they’re all too distant to have their orbits influenced in any real way by known solar system bodies. However, multiple observations point towards these remote bodies slightly clustering together, towards a larger distant body – in much the same way Neptune was discovered by how it influenced the movement of Uranus, which started with the discovery of Sedna in 2004.

Even more tantalising, the outer Solar System body that’s influencing the orbits of these ETNOs, is calculated to be really massive.

Professor Mike Brown (yes, the same Mike Brown who lead the charge to boot Pluto from the roster of planets) and Konstantin Batygin put forward the idea of Planet 9, a massive body with a mass somewhere between the Earth and Uranus orbiting. Estimates of distance put Planet 9 around 500 and 600 AU from the Sun.

How massive?

Current estimate of the mass needed to cause the observed strangeness of the Solar System bodies out past Neptune, that of a large planet, possibly at least 7 times the mass of the Earth. This is not another planet in the TNO community, the largest found so far being  Pluto, which is just  0.2% of the Earth’s mass. Planet 9 is something very different.

How did it form?

There’s a number of ways a large body that far out, on the outer edges of the Solar System, could end up being there.  First of all, it could have formed there. This is unlikely as there is simply not enough matter on the fringes of the Solar System to clump together under gravity, or “accrete” and form a planet of this size.  The Solar System we see today is relatively stable but could have come under the influence of a passing star, that gravitationally ripped a planet from the orbit where it formed, and flung it to the edges of the Solar System. However, we haven’t, to date, found any other distortions in the orbits of other major planets to confirm this has happened.

Another possibility is Planet 9 is an interstellar body that was captured by the Solar System, as it was passing on its own through interstellar space.  We’ve touch on rogue planets before, and they’re thought to exist in vast numbers.  These are planets that do not orbit a host star like the Earth, Mars or Saturn does, but instead orbit the Milky Way on their own.

Illustration of what Planet 9 could look like. Robin Dienel/ Carnegie Institution of Washington

Where is it?

Astronomers including Brown and Batygin along with Matthew Holman, earlier in 2024, released results from a long survey taken with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) observatory in Hawaii.

The team scanned 78% of the likely region of the sky where Planet 9 might be found with no success. To test their techniques, they also seeded 50,000 “fake” Planet 9 data points and their processes spotted 99.99% of them. The remaining region of approx 22% is further away, and will need more powerful observing tools. Brown , Batygin and Holman are currently working with data from larger telescopes in Hawaii. Other searches including citizen science projects have not found Planet  9 either.

What else could it be?

Astronomers Jakub Scholtz and James Unwin have another idea about Planet 9.

Along with others, they theorised that a large numbers of micro black holes were formed early on in the history of the Universe, called primordial black holes or PBH, and Planet 9 could be a small black hole.  For it to be the mass that’s causing the anomalies observed  – i.e. around 7 times the mass of the Earth – a small PBH would only need to be around the size of a cricket ball.  (It’s not the size of the object disturbing orbits – it’s the mass.)

With no matter nearby to fall into or accrete into a  PBH, and emit energy as it does, we’d be extremely hard pressed to spot it apart.  Microlensing events – light from behind massive objects being bent  – might be a way to determine if this is the case.

Astronomers Ann-Marie Madigan and Michael McCourt suggested Planet 9 might be an unseen ring of material, similar to that seen in other system, and there’s also been theories about Planet 9 being a collection of dark matter.

When will we know

If the current data survey of the remaining 22% of the Solar System where Planet 9 could be located draws a blank, the final confirmation might be made using one of the upcoming new generation “mega” telescopes such as the Vera C. Rubin Observatory, which is due to start scientific operation in 2025, as we talked about in a previous blog article.

“Within a year of that telescope operating, I think we’ll find it,” Professor Mike Brown has said.

“We have spent centuries studying the giant planets that we have. Imagine we get a new one all of a sudden. All the things we’ve done for studying the giant planets, we get to do all over again for the first time.”

Wrapping it up….

While not accepted by every  astronomer, the concept of a large body on the outside of the Solar System influencing the orbits of ETNOs has wide support and the searches will continue to find out what exactly what it is. It’s one of the many mysteries still remaining in astronomy.  When it’s finally tracked down you won’t have to worry about where to find out more – it’ll be front page news around the world!


Earl White


2nd July 2024

PS: As usual, this is an introduction to a complex and developing topic. Drop me a message at BINTEL if you’d like more information or comment.

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