New planets for old stars

Conventional wisdom says that planets form with stars. Following the formation of a star, a collection of gravitationally bound material in orbit around that star will form a protoplanetary disk, which will coalesce into planets. So deeply ingrained in our collective subconscious is this idea that frequently, it goes completely unquestioned. It’s easy to forget that newly formed stars aren’t the only objects which host gravitationally bound disks of material. The same can be found when you look at the opposite end of the stellar lifecycle too.

Every now and again, I uncover a paper which is old but fascinating. This is just such a paper. Spurred by the pulsar planets detected around PSR 1257+12, Jura and Turner consider the formation of planets around an evolved star. In this particular case, that star is a binary known as HD 44179. It lies at the centre of a preplanetary nebula more commonly known as the Red Rectangle.

Anyone who’s seen me present my actual research will know that the Red Rectangle is one object which holds rather a special place in my heart. In the process of forming as a dying star casts off its outer layers, this is a fascinating little nebula for a number of reasons. But perhaps most interestingly it does actually show a hint that this, a several billion year old dying star system, might just be forming planets.

At least, that’s one possibility. In truth, I’m not entirely sure what they’ve found, and from reading the paper, neither are they! The Red Rectangle is a dusty old thing. It holds a mammoth torus of dust and debris around it, extending as far out as 43000 AU by some calculations. That makes the full extent of the dusty region around this nebula on the same scale as the hypothesised Oort cloud thought to encircle our solar system. In brief, in the midst of that big dusty torus, Jura and Turner found a surprise. A big clump of dust.

This dust clump was completely unexpected. It was detected at radio frequencies (shown here, the contour lines give continuum emission at 230 GHz overlaid on an optical Hubble image). That is a huge dust cloud, around 1600 AU from the central star, clearly in the same plane as the circumstellar disk. It contains about one Jupiter mass of material and it has a diameter larger than our entire solar system. It was a surprise because it’s only visible in the radio. At optical and infrared wavelengths, it’s completely invisible. But exactly what the hell is it?

Well, it’s a distinct cold source. Quite possibly gravitationally bound to the system. It might be an early stage in the gravitational collapse of material into a planet. Or it could be a dust cloud surrounding a hitherto unknown dim third star in the system (the authors speculate that this dust clump could still be condensing into a disk around that star, if it exists). Such a star would need to be a red dwarf with a luminosity less than 0.04 times that of the Sun, which isn’t actually unreasonable. Or it could, I suppose, be a dust cloud which is at least partially bound to an existing giant planet that far out. Whatever it is, the inner circumstellar disk around HD 44179 – the same one which funnels the nebula into its characteristic X shape – is doing a good job of protecting this dusty oddity from any ionising stellar radiation. It may well be free to continue forming into a planet (or whatever) as the main star evolves into a planetary nebula.

Unfortunately, there isn’t really any way for us to know what this bizarre object might eventually form into. For now, this mysterious dust clump will have to remain, simply, a mysterious dust clump.

Note that despite the paper’s title and terminology, this shouldn’t really be confused with the circumstellar disk also found in the system. The Red Rectangle’s circumstellar disk is pretty small by comparison, even though it would still dwarf our entire solar system. The difference between the disk and the torus here is a bit like the difference between Saturn’s rings and this.

ResearchBlogging.orgJura, M., & Turner, J. (1998). A mysterious dust clump in a disk around an evolved binary star system Nature, 395 (6698), 144-145 : 10.1038/25938

Image Credits:
Upper – Red Rectangle (Nebula ESA/Hubble/NASA)/Planet Resource (thiagochackal @ deviantART)
Lower – Figure 1 from Jura and Turner (1998), reproduced here for reporting, research, and review in accordance with fair use policies.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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