Undead Suns

A recent article which I’ve stumbled upon comes from the quite delightful Centauri Dreams, concerning the possibility of habitable worlds around white dwarf stars. Frankly, I find the whole concept rather enthralling (and I touched on the idea briefly once before). I’m not the only one who’s mused at great length about the possibility of life sustaining planets existing around red dwarfs, but… White dwarfs? Well, a white dwarf is a very different beast, with considerations wholly different to any other type of star. For one thing, white dwarfs are technically dead.

Sirius B, compared with Earth. It really is that small.

The article is all about a paper referring to two planets, KOI 55.01 and KOI 55.02, which have seemingly been engulfed by their parent star during its red giant phase and lived to tell the tale. Apparently, the star KOI 55 ended its red giant phase 18.4 million years ago (sufficiently long ago for any hot stellar material it shed to have long since cooled enough to fade from view), leaving a couple of Mars-sized planets hurtling around it at breakneck speed – their orbital periods are only a matter of hours. Less of a problem, considering a white dwarf is roughly the same size as planet Earth.

Following a rather interesting conversation this morning, I should point out that the star KOI 55 is not a white dwarf, but a hot B-type subdwarf star which will evolve into a white dwarf. No planets around actual white dwarfs have been discovered yet.

Interestingly, while a white dwarf starts off as an intensely hot little object, after they’ve cooled down somewhat their habitable zones are expected to be ten times closer than that of a red dwarf. Assuming it isn’t one of the handful of white dwarfs with a lethal magnetic field, a planet could hug one of these stellar cinders as an energy source. Its conditions may even be relatively benign. While a hot, freshly cooked white dwarf puts out a huge amount of ultraviolet, once they’ve cooled down, a planet in such a white dwarf’s habitable zone actually receives a daily dose of UV comparable with what you might receive from the Sun here on Earth.

Also, I feel compelled to respond to the first commenter on that post too…

The “elephant in the room” here is that we don’t yet know of a particularly plausible mechanism to get a terrestrial planet into a stable orbit close to the white dwarf. The planets KOI 55.01 and KOI 55.02 are probably the cores of Jovian planets that survived “common envelope” evolution with the host star. As such, they won’t be much like Earth.

I’m not convinced I agree with this view. Certainly, they may well be the cores of former gas giants, and surviving the common envelope evolution is a given. But I’m not convinced the final assertion holds. Being the cores of former jovian planets, all that’s left may well be a big ball of iron and silicate, not all that dissimilar to an Earth-like planet. When it first formed, Earth wasn’t all that Earth-like either, and it’s entirely possible that some amount of material may have been captured gravitationally by these planetary cores as the red giant lost its mass. Carbon and oxygen rich material may well have been accreted. If anything, these planets would likely be enriched in heavier elements. They would likely contain more carbon and oxygen than Earth does. But then, this is merely my own speculation on the matter.

Food for thought. It’s an interesting topic to muse upon, it has to be said…

Centauri Dreams, incidentally, is without doubt one of my favourite blogs. If you enjoy reading my ramblings here on Supernova Condensate, you really should go and spend a while there. You certainly won’t be disappointed!

Images:
Upper – Unsourced (please help me if you know the original source of this image!)
Lower – composite – originals from ineedfire @ deviantART, gileryd @ deviantART, NASA

About Invader Xan

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