A 9th planet?

Just a quick post to summarize my thoughts on the possibility of there being a large, almost Neptune-sized 9th planet in the outer reaches of our solar system. You can read the full story here, and if you are very interested or one of my students then I recommend Mike Brown’s blog. Mike is one of the co-authors of the paper proposing the existence of this planet. His twitter handle is @plutokiller because he’s the man who, in effect, was responsible for demoting Pluto from planet status by finding quite a few bodies around Pluto’s size also lurking in the outer solar system.

Also, if you can’t be bothered with the long read, I did an interview with an Austrian science radio show this morning which you can listen to half-way down this page.

To cut a long story short, this announcement is not a discovery, but a prediction based on the orbital properties of half a dozen small rocky bodies in a region of the outer solar system called the Kuiper Belt. Similarities and co-incidences between the orbits of these “dwarf planets” convinced Mike Brown and his colleague Konstantin Batygin that something large and as yet unseen is tugging at and influencing them. Extensive mathematical modelling suggests a planet about 10 times the mass of Earth could be responsible (so, getting on for the size of Neptune). But it has to be far away, and probably in an elliptical orbit that may only come as close as around 200 times the distance of the Earth from the Sun, and maybe as distant as a 1000 times or more.

Such a planet would be faint (18th-24th magnitude), although still bright enough that it probably would have been detected by one of the large sky surveys undertaken in recent years from ground and space based telescopes. The problem is that it won’t be moving very fast (not even a few arcseconds per year), and since no-one has been looking for it (until now) then it could be lost among millions of stars of similar brightness.

In his blog Mike Brown says that he has now searched for it in some surveys (specifically, for the astronomers reading this, Catalina and Pan-Starrs), but without success. However, the most likely direction that it lies in is co-incident with the plane of our galaxy, i.e. the Milky Way. As anyone who has seen the Milky Way realises, finding something faint and slow moving against a bright fuzzy background of billions of stars will not be easy. Indeed, most sky surveys routinely avoid the Milky Way precisely for that reason.

Of course, the race is now on to find Planet 9. I’m sure lots of astronomers will now pour  over old data and surveys, and be scheduling new observations for the most likely directions to search.

Will we find it? Well, astronomy has been here before. Back in the 18th and 19th centuries, discrepancies in the orbits of the known planets convinced mathematicians and astronomers of the existence of more planets, and that’s how Uranus and Neptune were discovered. Later, perceived discrepancies in their orbits were used to predict the existence of a so-called Planet X. Pluto was found because astronomers were looking for Planet X. But Pluto was never massive enough to be that planet, and its discovery was just a remarkable coincidence.

More recently, a large infrared sky survey by NASA’s WISE satellite was used to look for a Jupiter or Saturn sized planet lurking way out at the edge of our solar system, that might be Planet X, but nothing was found. WISE wasn’t sensitive enough to detect this newly predicted Planet 9 though.

If Planet 9 is found, it would be a remarkable discovery and achievement. If it is smaller than Neptune but still several times the mass of Earth then it would be what’s called a “Super-Earth”. Such planets are being found around other stars and are of great interest because we don’t (currently) have anything in our own solar system between the Earth and Neptune in size. What is such a body made of? Is it rocky, or a gas giant?

And if Planet 9 really is wandering so far from the Sun, how did it form and how did it get there?

But let’s not get too far ahead of ourselves. Observations and data will confirm, or refute, this planet. Right now, astronomers are quite excited because the evidence and modelling seems quite compelling and Mike Brown is a highly respected authority on the outer solar system. But such predictions have been made before, and found wanting. Time will tell.

[For a more skeptical assessment, I like this blog: Not So Fast: Why There Likely Isn’t A Large Planet Beyond Pluto]