Atmospheres are key to pulsar-proofing exoplanets

For the first time, astronomers have determined what it takes for a habitable exoplanet to survive around a pulsar.

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Tau.Neutrino said:

Atmospheres are key to pulsar-proofing exoplanets

For the first time, astronomers have determined what it takes for a habitable exoplanet to survive around a pulsar.

more…

My immediate reaction to this is that it’s impossible to pulsar-proof a habitable planet. The radiation from pulsars is deadly stuff, not just X-rays but also extreme energy electrons and protons and subatomic nuclei. And quite possibly even more penetrating high intensity radiation in the form of neutrons and muons. Even then, once you block out the deadly stuff you block out all the visible light as well.

But let’s look at the paper.

> We discuss the survival time of planet atmospheres and the planetary surface conditions around different classes of neutron stars, and define a neutron star habitable zone based on the presence of liquid water and retention of an atmosphere.

Hmm, “habitable zone” is something of a crock anyway. Subsurface living would be better. I suppose, given this definition, the “habitable zone” could extend outward to an infinity, with liquid water in the atmosphere of a planet warmed from within nowhere near a pulsar or any other star.

> If we choose as an illustrative example an Earth-like atmosphere, then the height z will occur at around 50–70 km from the surface. Since the atmospheric density scales exponentially …

That gives a surface density of what? Quick calc suggests 200 atmospheres. But that is twice the atmospheric pressure of Venus. So the surface would have to be much hotter than that of Venus.

That actually rules out the existence of liquid water, the surface temperature would have to be under 370 degrees celsius for liquid water to exist but the surface temperature would be much hotter than 470 degrees Celsius.

Unless I’ve got my back of envelope calculation wrong.

mollwollfumble said:

Hmm, “habitable zone” is something of a crock anyway. Subsurface living would be better. I suppose, given this definition, the “habitable zone” could extend outward to an infinity, with liquid water in the atmosphere of a planet warmed from within nowhere near a pulsar or any other star.

> If we choose as an illustrative example an Earth-like atmosphere, then the height z will occur at around 50–70 km from the surface. Since the atmospheric density scales exponentially …

That gives a surface density of what? Quick calc suggests 200 atmospheres. But that is twice the atmospheric pressure of Venus. So the surface would have to be much hotter than that of Venus.

That actually rules out the existence of liquid water, the surface temperature would have to be under 370 degrees celsius for liquid water to exist but the surface temperature would be much hotter than 470 degrees Celsius.

Unless I’ve got my back of envelope calculation wrong.

Which makes me wonder. What would be the surface temperature of Venus if Venus was not in orbit around a star? The heat would then be generated by radioactivity within the core, and insulated against the cold of space by the thick atmosphere.

mollwollfumble said:

mollwollfumble said:

Hmm, “habitable zone” is something of a crock anyway. Subsurface living would be better. I suppose, given this definition, the “habitable zone” could extend outward to an infinity, with liquid water in the atmosphere of a planet warmed from within nowhere near a pulsar or any other star.

> If we choose as an illustrative example an Earth-like atmosphere, then the height z will occur at around 50–70 km from the surface. Since the atmospheric density scales exponentially …

That gives a surface density of what? Quick calc suggests 200 atmospheres. But that is twice the atmospheric pressure of Venus. So the surface would have to be much hotter than that of Venus.

That actually rules out the existence of liquid water, the surface temperature would have to be under 370 degrees celsius for liquid water to exist but the surface temperature would be much hotter than 470 degrees Celsius.

Unless I’ve got my back of envelope calculation wrong.

Which makes me wonder. What would be the surface temperature of Venus if Venus was not in orbit around a star? The heat would then be generated by radioactivity within the core, and insulated against the cold of space by the thick atmosphere.

In the absence of the sun, Venus’s atmosphere would freeze out, so the top of solid surface would be dry ice. The surface temperature would be something below -200C.

dv said:

Which makes me wonder. What would be the surface temperature of Venus if Venus was not in orbit around a star? The heat would then be generated by radioactivity within the core, and insulated against the cold of space by the thick atmosphere.

In the absence of the sun, Venus’s atmosphere would freeze out, so the top of solid surface would be dry ice. The surface temperature would be something below -200C.

Let’s see. It could be. The atmosphere would only start to freeze at -78C. It would start to freeze from the top. But … the top of Venus atmospere, at altitude 75 to 90 km, already at the moment has a temperature below -100C. Does that make sense to you? How can Venus’ atmospere be already frozen?

mollwollfumble said:

dv said:

Which makes me wonder. What would be the surface temperature of Venus if Venus was not in orbit around a star? The heat would then be generated by radioactivity within the core, and insulated against the cold of space by the thick atmosphere.

In the absence of the sun, Venus’s atmosphere would freeze out, so the top of solid surface would be dry ice. The surface temperature would be something below -200C.

Let’s see. It could be. The atmosphere would only start to freeze at -78C. It would start to freeze from the top. But … the top of Venus atmospere, at altitude 75 to 90 km, already at the moment has a temperature below -100C. Does that make sense to you? How can Venus’ atmospere be already frozen?

Oh, I see, freezing point is as low as -135C at low pressure.

I still have no idea how Venus atmosphere would freeze, though. How could an atmospere possibly freeze from the top down?