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SYFY WIRE Bad Astronomy

CFBDSIR 2149-0403: A mystery lurks 1800 trillion kilometers from Earth

By Phil Plait

[Artwork credit: SwRI]

I do love a good coincidence.

For a recent article I wrote about whether exoplanets are considered planets, I needed a particular graphic of an exoplanet, a pretty piece of artwork to help get the idea across. I knew I had posted one on the blog years ago, so I searched my site and found it (it’s the image above, in fact). It was a lovely depiction of an alien world, one I had used as the header for an article I wrote about a very interesting object called CFBDSIR 2149-0403.

The name is clunky, but the object is mysterious. When it was discovered in 2012, it looked very much like it was a rogue planet, literally a planetary-mass body freely floating in space, not orbiting a star. Other rogue planets have been discovered, but not many, and this one was close enough to Earth that observing it over time could reveal quite a bit about it.

I reread the article I wrote about it back in 2012, and at the time CFBDSIR 2149-0403 hadn’t been confirmed to be a planet yet. It could be a more massive object masquerading as a planet, for example, especially if it were farther away then expected. So, out of curiosity, I did a quick professional journal search for it to see if anyone had followed up after its discovery ... and found to my delight a paper about it that was published on March 3, 2017, just a couple of weeks ago*!

So, what’s what with this object? Is it a planet or not? Well, that’s actually still not clear. But we’re closer to knowing …

First, some background.

While I’m pretty clear and firm in my stance that it makes no sense to define what a planet is, one thing we do know is they aren’t stars. A star is an object large enough that the pressure and temperature in its core can fuse hydrogen atoms together in a sustainable way. It turns out that, to do this, you need an object with about 0.08 times the Sun’s mass, which is about 80 times the mass of Jupiter. Less than that, and the object lacks the oomph needed to squeeze those atoms together enough to fuse them.

It turns out there’s an analogous limit at lower masses, too. Objects with about 13 times Jupiter’s mass can fuse lithium atoms, and above 65 times Jupiter’s mass fuse deuterium (an isotope of hydrogen). Planets, by common agreement, don’t fuse atoms, so the upper limit to a planet’s mass is 13 times Jupiter’s.

Objects in this 13 – 80 times Jupiter’s mass planet/star neutral zone are called brown dwarfs. We actually know a lot about them. I did an episode of Crash Course Astronomy about brown dwarfs and, if you watch it, it’ll help you grok what I’m talking about here.

Now comes the fun part. When a planet is young, it’s hot. Forming a planet means taking lots of small chunks of rock and metal and dust and ice and slamming then together at many kilometers per second, which generates a lot of heat. A planet can form in only a few million years, but it can glow visibly from its own heat for tens of millions of years.

The planets in our solar system are billions of years old, and we see them due to the light they reflect from the Sun**. But we can also see young planets around nearby stars that glow on their own. Sometimes, we can even estimate their age using various methods, and that helps us understand how long it takes a planet to cool off.

We see lots of objects that are glowing like this, but it can be hard to estimate their mass. Are you looking at a brown dwarf that’s 50 million years old, or a Jupiter-mass planet that’s only 5 million? It can be difficult to know.

Enter CFBDSIR 2149-0403. Glowing in the infrared, it’s a special object, indeed: It doesn’t appear to be orbiting a star. If it did, the star would be far brighter, blasting out light, but there’s none to be seen.

Possible rogue planet CFBDSIR-J21494

But it’s even more special than that: It looks like it’s part of a small cluster of stars called the AB Doradus group. This is critical: Getting the distance to isolated objects can be difficult, but there are several ways to get cluster distances. If it’s part of AB Doradus, then it’s about 75 light years away, which is pretty close. And not only that, using stellar dating methods, it’s been found that AB Doradus stars are no more than 200 million years old, and probably much younger.

Knowing the distance is really important! Coupling that with the brightness, the true luminosity can be found (think of it this way: It could be a bright object far away, or a fainter one). Given the distance, CFBDSIR 2149-0403 appears to be a 4-7 Jupiter-mass object. A true rogue planet.

But wait! Remember, I mentioned a new paper about it came out. And what they found is...irritating. CFBDSIR 2149-0403 is too far away to be a member of the AB Dor group.

Well, nuts.

They used parallax, a very reliable method of determining distance (and, guess what: I have a Crash Course Astronomy episode on that, too). They found it’s about 180 light years from Earth, over twice as far as AB Doradus. It’s very unlikely to be part of the cluster, so we can’t be sure of its age.

Worse, the greater distance means it’s actually more luminous than we first thought, and therefore possibly more massive.

How massive? Here’s the first irritating part. It could be anywhere from 2 - 13 times Jupiter’s mass! On the low end, it’s definitely still a planet, but on the high end it might be able to fuse lithium, which would make it a brown dwarf. The good news is that it’s likely to be on the lower end, but at the moment there’s no way to be sure.

That’s the second irritating part. While it’s likely to be a massive planet, the astronomers cannot with certainty rule out it being a more massive brown dwarf. The details are complex, but it has to do with the amount of heavier elements in CFBDSIR 2149-0403’s atmosphere; they can confuse the issue when trying to determine the nature of the object. It could be a brown dwarf with lots of heavy elements (basically anything in the periodic table past helium), or it might be a planet with fewer.

So, what do we make of this object? We can’t say for sure it’s a planet, but all the data so far do seem to point that way. It’s less likely to be a brown dwarf, but we can’t be completely sure.

But here’s the fun bit: We get to study it more! That’s the only way to know what’s what. Astronomers have used a good number of telescopes and techniques to observe this beastie, but it still defies our understanding, at least a little. So, we need to keep looking at it, thinking of new things to look for, keep figuring out new ways to analyze the data, so that the nature of CFBDSIR 2149-0403 will finally reveal itself.

In some ways, the differences between a planet and a brown dwarf near the border aren’t that important; it is we who put labels in things nature is fuzzier about. But, in this case, it actually could be important: We think brown dwarfs form like stars, collecting themselves out of clouds of gas and dust, while planets form around stars, gathering material from the disk of junk orbiting the star. If CFBDSIR 2149-0403 is a brown dwarf, it probably formed by itself, but if it’s a planet, it may have formed around a star, then got itself ejected, probably as it interacted gravitationally with other planets in the system.

That makes a difference indeed! So, this object will continue to be studied, and our understanding of the Universe will continue to grow. We may not know precisely what it is yet, but we will eventually. Science is really good at that sort of thing.

* A second coincidence: I had already started taking notes on this object when I saw others notice it as well, including AstroAthens and Universe Today. So, I missed getting a scoop on this, but hey, I wrote about it back in 2012, so I don’t feel too badly.

** The outer planets are actually so massive that their interiors are still cooling, even after all these eons. They glow in the infrared, outside what our eyes can see, because of this warmth