January 22, 2016, by Lindsay Brooke

Would the REAL ninth planet please stand up?

Blog by Dr Julian Onions, Post Graduate Research Student, School of Physics and Astronomy

You may remember back in 2006 there was a big furore as Pluto was demoted from planet-hood to be an also ran or dwarf planet. There were good reasons for doing this. Lots of small objects had been discovered around where Pluto lives and when Mike Brown Professor of Planetary Astronomy at Caltech and his team started to discover first one, then a number of large bodies in a similar orbit to Pluto, life really did become complicated.

At least one of these objects, Eris, turned out to be even more massive than Pluto itself, so if Pluto was a planet, surely Eris must be too, and also maybe, Makemake, Haumea, Quaoar, Orcus and so on, all of which weren’t far behind. Either they were all planets, and it looked like there were still more to be found, or none of them were.

The International Astronomy Union decided in the end that none of them were – a decision that is still disputed today.

This is not to say Pluto is uninteresting.  The stream of images from the New Horizons probe that visited it briefly last summer show it has a fascinating surface, and scientists will be studying the data for years. It’s just that it is not unique enough to be called a planet.


So Mike Brown killed Pluto, he even wrote a book about it: “How I Killed Pluto and Why It Had It Coming.” and even goes under the name @plutokiller on twitter.

Now though, Mike Brown is back – working together with Konstantin Batygin, and in a rather interesting twist of fate, they claim to have found good evidence for a real 9th planet. So – the Pluto killer is now not only responsible for killing the original 9th planet, he is suggesting it’s replacement – which comes pretty close to dancing on Pluto’s grave.

He is suggesting there exists a true 9th planet, that is of considerable size, maybe 10 times the mass of the Earth, so similar in size to Neptune. Pluto for comparison is much smaller than the Earth, and smaller indeed than the Moon.


It orbits way beyond where Pluto does too – which is why no one has spotted it.

So does a 9th planet really exist?

Planetary distances are usually measured in astronomical units (AU) where 1 AU is the distance from the Earth to the Sun. So Earth orbits at 1 AU. Pluto orbits at around about 40 AU, so 40 times further out. This possible planet 9, if it does exist, would at it’s closest approach come to 200 AU, so way way beyond Pluto, and it would also have a very elliptical orbit. At its furthest point it could reach maybe up to 1200 AU – so 300 times further out than Pluto. It would orbit the Sun once every 10-20,000 years, so birthdays would be few and far between on that cold dimly lit planet.

To be clear from the outset, neither Mike Brown or anyone else has seen this planet. What they have found is a suspicious alignment of objects in the outer solar system, which is extremely unusual. However, if you throw into the mix a large planet, you can explain why these objects are all lined up as they are, and this is what they have done through computer simulations. This large 9th planet is pulling them into line every time it comes close.

So why hasn’t anyone seen it yet?

There are several reasons – including the fact it may not exist at all. Despite the putative planet being predicted to be quite large, it is also very distant. It has no light of it’s own, so only shines by the Suns reflected light – and at the distance it is thought to orbit, there is really very little light reaching it, and even less reflected back to Earth into telescopes.

However, it should certainly be within the ability of our larger telescopes to make out, so why can’t we see it? One reason is that the modelling Mike and Konstantin have done only gives an approximate region where it might be. So they are basically saying its probably somewhere in that region of sky. Therefore, you can’t just point a telescope to a location and look, you would have to examine a fairly large chunk of the sky to see if you can find it.

How do you spot a planet?

Well it looks a bit like any other dim object in the sky, and at the magnifications needed to see this planet billions of stars are visible. So you have to use the fact that most stars are moving so slowly they appear nearly stationary, whereas planets move against the background of the stars. So with several images of the same bit of sky, you are looking for something that is moving in the sequence against the stationary stars to detect such things.

There have been several attempts over the years to do this, so why hasn’t it been found yet?

Mike has been back through the data to look for possible candidates, but not found a match as yet. Given they know about how big they think it is, they also know about how much light it will reflect, so they know whether it would be detectable in each sweep. However, there are some places it is easier to look than others.

Some parts of the possible orbit cross the galactic plane, which means you are looking straight into the heart of the Milky Way. There you are trying to pick out the object against a huge number of bright background stars, which makes the task much harder, so there are some parts of the sky that haven’t been examined in the requisite detail.

If you want to go hunting for it yourself Mike himself explains this in more detail in a blog post:


Going on a planet hunt

Planets tend to glow in the infra red, so perhaps the best way to look is with infra red telescopes. It’s somewhat easier to separate the wheat from the chaff in the infra red. So using infra red telescopes to survey the likely area of sky might be the best option. However, this amounts to quite a lot of precious telescope time, time that is also in demand for other investigations, so it may be a while before such a survey is carried out.

How did it get there?

It’s difficult to form large planets at big distances from the Sun, at least with our current understanding of planet formation. They need to be built up where there is lots of material ready to hand, and that tends to be closer into the star. However, the early solar system was probably a very turbulent place, with collisions and near misses very common place – we think the moon for instance was created when a Mars sized planet crashed into the recently formed Earth.

There is also a good chance that neither Jupiter, Saturn or any of the outer planets formed where they are today, but danced around each other early on until finally settling to where they are. In amongst all this it is quite possible that a close encounter with Jupiter and Saturn for instance, could toss a planet far out of the solar system. We know this happens elsewhere as we have found planets wandering on their own through space, not attached to any star. So with the right conditions this planet could have been formed in the inner part of the system, and then kicked out as things settled down.

So are there really 9 planets?

While we are not quite ready to reconstitute our number of planets back up to nine at this point, there is at least some hope that we may get back to that number someday, or who knows if there are more even further out still to be discovered. If so, the next thorny question is what would you call it, and I am sure that would provoke much heated discussion!

Dr Onions is an astronomer in The Nottingham University School of Physics and Astronomy, in the  Astronomy group http://www.nottingham.ac.uk/astronomy/

He works mostly on large scale computer simulations of the universe and recently passed his Doctorate in Astronomy. He has an interest in all aspects of astronomy and is the local organiser for the National Astronomy Meeting when 500 astronomers will visit Nottingham this summer. 

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