rfmcdonald | [LINK] On Earth-like worlds orbiting white dwarf stars (Reply)

I posted posted here last year about the possibility that Earth-sized planets with habitable environments could orbit white dwarf stars--briefly, stars that have reached their end of their fusion-burning lifespan, shedding their outer layers into space and evolving into a compact ball of superdense cooling matter. The very idea seems to be one of those ideas that came as an initial surprise--"Could such a thing be possible?"--but then, after some consideration of the possibilities, became bemused acceptance--"Well, why not?"

A recent Supernova Condensate post fetchingly titled "Undead Suns" amplifies on the theme of a recent Centauri Dreams post that examines the idea in greater detail.

First, Centauri Dreams on the detectability and potential for habitability for such hypothetical--and as yet undiscovered--worlds.

The conditions on planets orbiting close to a cool white dwarf might be relatively benign. What Fossati and team show is that the cooling process in these stars slows down as their effective temperature approaches 6000 K, producing a habitable zone that can endure up to eight billion years. And it turns out that white dwarfs offer advantages M-dwarfs do not, providing a stable luminosity source without the flare activity we associate with younger M-class stars. As you would expect, a cool white dwarf has a habitable zone close to the star, ten times closer than for M-dwarfs. One recent study has used this to argue that a Mars-sized planet in the white dwarf CHZ would be detectable with today’s ground-based observatories even for faint stars.

But there are other options including polarized light that may be used to detect a planet with an atmosphere around a white dwarf. Normally, starlight is unpolarized, but when light reflects off a planetary atmosphere, the interactions between the light waves and the molecules in the atmosphere cause the light to become polarized. The paper notes that the polarization due to a terrestrial planet in the CHZ of a cool white dwarf would be larger than the polarization signal of a comparable planet in the habitable zone of any other type of star except brown dwarfs. Analyzing polarization is thus a viable way to detect close-in rocky planets around white dwarfs.

Next, Supernova Condensate on the possibility of yet another type of Earth-like planet, this one the exposed core of a former gas (Jupiter-type) or ice (Neptune-type) giant. The two planets found so far which most closely resemble the type of world being speculated about may be the cores of massive planets, not having started as predominantly rocky worlds. Is this such a problem for life?

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.