Rouge Planet.

This is CFBDSIR2149-0403, which I’m sure you’ll agree just rolls off the tongue. CFBDSIR2149-0403 made the news yesterday as one of the more recently-discovered rogue planets. Contrary to popular belief, a rogue planet is not:

  • A planet which backstabs other planets.
  • A planet which drains the powers of other planets.
  • A planet all done up in makeup designed to emphasise the cheekbones.

Rogue planets are instead planets which wander the universe all on their lonesome rather than being gravitationally bound to any star, orbiting the galaxy independently instead of as part of a solar system. There are two ways this can happen: either extreme gravitational interactions with other planets eject the planet from the solar system that formed it at an early stage in its lifetime, or else the planet forms on its own out of a molecular cloud collapse just like a star.

My first reaction when reading the headline to this one was “How the hell did they see that?” Extrasolar planets are mainly detected through their effects on their parent star; either they directly block light from the star at fixed intervals, resulting in a regular dip in the intensity of the light we see emitted from that star, or else the back-and-forth wobble they cause in the star’s motion due to mutual gravitational interaction can be spotted by analysing the wavelength of the star’s light and how it changes over time. If you have a hobo planet going around on its own with no parent star than both of these methods are ruled out and you’re reduced to attempting to detect the planet through directly observing it.

Fortunately a direct detection is much easier when there’s no parent star. It’s usually very tricky because stars are vast sources of electromagnetic radiation which swamp the feeble emissions we might see from a planet in another solar system – kind of like trying to spot a candle when it’s placed next to a high-powered searchlight several kilometres away. With no star drowning out the planet’s emissions, though, we can do a proper direct search; planets out on their own obviously do not emit or reflect light, but they do emit a visible quantity of infra-red radiation, and so IR surveys are the best bet for spotting any potential rogue planets1.

So we’ve found this (potential) new rogue planet through an infra-red survey, but it’s not even the first (potential) rogue planet we’ve found. Why is it making news headlines? The answer basically has to do with nobody knowing what a “planet” is, really; according to the paper CFBDSIR2149-0403 shares common motion with a star cluster called AB Doradus, implying that it formed at the same time in the same way out of the same molecular cloud as the stars in AB Doradus. It’s one of these rogue planets that (probably) did not form as part of a solar system’s planetary disk, but which instead formed on its own just like a star would. The age of the rest of the stars in the cluster – 50-120 million years – implies that CFBDSIR2149-0403 is also 50-120 million years old, and since we know its age we can compare it to computer models of planet formation in molecular clouds to get a very rough idea of what its composition and mass might be like. It’s important to stress that – as with so much in astronomy – the composition and mass details are essentially an educated guess. Nobody has directly seen any of this stuff and nobody ever will, so guessing is all we can really do. If the guess is correct, though, CFBDSIR2149-0403 has between four and seven times the mass of Jupiter.

This is the first time we’ve been able to pin an age and a mass estimate onto a planet/brown dwarf like this, and it’s a little bit inconvenient for the Astronomical Union, who defined a planet as anything below deuterium-burning mass ranges (about 13 Jupiter masses) and classed anything above that as a brown dwarf. It was assumed up until now that anything forming out of a molecular cloud would have more than enough mass to count as a brown dwarf, with the previous rogue planet possibles having mass estimates that were far too uncertain to come conclusively under the brown dwarf limit. CFBDSIR2149-0403 changes things, though; it implies that planets can and do form out of molecular clouds, and that a significant percentage of the currently known brown dwarf population could also be classed as rogue planets if they came it at the lower end of their mass estimates.

This changes our view of what a molecular cloud can spit out by a fair bit. Of course it’s also possible that CFBDSIR2149-0403 was ejected from a solar system, but I’d consider that extremely unlikely because of how big it is. It’s big enough that it’d be the one chucking other planets out of solar systems, not the other way around. I don’t know what the scientist at the end of the article is blathering about when he raises the possibility of lots of other orphaned worlds floating in space; somebody should point him towards the Nice model research that implies that that sort of thing is a practical certainty given what we know about solar system formation, and that CFBDSIR2149-0403 would merely raise the upper limit on the size of planet that loses the gravitational tug of war. Of course he probably knows that already and is just trying to wax poetic, but this is something scientists should never ever be allowed to do because they’re bloody awful at it.


  1. CFBDSIR stands for Canada France Brown Dwarf Survey Infra-Red.
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6 thoughts on “Rouge Planet.

  1. Gap says:

    So I found this epic review paper on the IMF (, and one of the things that it brings up is that it’s quite difficult for very small stars/brown dwarfs to form without a host star, because it requires that the core of the protostar is ejected almost instantly from the molecular cloud before it can accrete anything. Not that they don’t exist, of course. Also, yeah, the attempt to reproduce the IMF in simulations of molecular clouds are pretty neat, and mainly serves to underline how messy and complicated astrophysics can be.

    • Gap says:

      Also your bad spelling mars an otherwise excellent post or whatever.

    • Hentzau says:

      Oh, now that is interesting. So it’s basically saying that if this rogue planet/brown dwarf/whatever formed on its own its very much an outlier rather than the status quo? That makes sense given that they only found one in the entire survey, although obviously what you can see in astronomy isn’t necessarily the same as what is there.

      • Gap says:

        Here’s a paper on simulations of star formation in molecular clouds, which has quite a nice model for the origins of the initial mass function:

        I think the bottom of the IMF is pretty open, still; small things tend to be harder to resolve than big things. But I’m sure that Kroupa review has all the information you could possibly ever not want to read.

  2. Smurf says:

    Can a rogue planet be captured by another system and then not be a rogue planet anymore? Like, would it be possible for our Sun to capture a rogue planet and give us another planet in our solar system?

    • Hentzau says:

      Possible, but unlikely. Planets eventually migrate to stable orbits thanks to resonances etc, but a new planet that took up even temporary residence in the solar system — especially one four time the size of Jupiter — would have a dramatically destabilising effect on the rest of them. While it might be theoretically possible for a rogue planet to be captured and assume a stable orbit, the solar system would undoubtedly lose others, very probably including the Earth since it’s the smaller planets that get chucked first.

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