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This question is prompted by one posted on the worldbuilding stack exchange: https://worldbuilding.stackexchange.com/questions/189701/could-this-planets-tail-be-visible

I understand that the ionized gas tails of comets are formed by the effect of solar radiation on the comet followed by a push from the solar wind.

In the article on Venus I found and then an article on a gas giant exoplanet linked in the comments, both planets formed a tail under conditions of low solar wind. Text and links to these sources below.

http://www.esa.int/Science_Exploration/Space_Science/When_a_planet_behaves_like_a_comet

The observations were made in August 2010 when NASA’s Stereo-B spacecraft measured a drop in solar wind density to 0.1 particles per cubic centimetre, around 50 times lower than normally observed; this persisted for about 18 hours. As this significantly reduced solar wind hit Venus, Venus Express saw the planet’s ionosphere balloon outwards on the planet’s ‘downwind’ nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions. “The teardrop-shaped ionosphere began forming within 30–60 minutes after the normal high pressure solar wind diminished. Over two Earth days, it had stretched to at least two Venus radii into space,” says Yong Wei of the Max Planck Institute for Solar System Research in Germany, lead author of the new findings.

A giant comet-like cloud of hydrogen escaping the warm Neptune mass exoplanet GJ 436

According to our simulation, the stellar radiation pressure counterbalances ≲70% of the star’s gravity pull on escaping atoms, which is much less than in hot Jupiter systems, in which radiation pressure takes over stellar gravity by factors of 3 to 5 (ref. 20). The low stellar radiation pressure at GJ 436b allows the formation of a large coma and tail of escaping atoms, co-moving with the planet although not gravitationally bounded to it (Fig. 4).

When I think of wind forming a tail I think of a smokestack on a windy day. More wind = longer tail. I do not understand why weaker solar wind would allow a planetary tail and stronger would prevent it.

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I think the first link that you sent has an interesting explanation that we can extrapolate from - here's the relevant section:

The new observations settle a debate about how the strength of the solar wind affects the way in which ionospheric plasma is transported from the dayside to the nightside of Venus.

Usually, this material flows along a thin channel in the ionosphere, but scientists were unsure what happens under low solar wind conditions. Does the flow of plasma particles increase as the channel widens due to the reduced confining pressure, or does it decrease because less force is available to push plasma through the channel?

“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions,” says Markus Fraenz, also of the Max Planck Institute and co-author on the paper.

What I interpret is this: while solar winds do cause plasma to form a tail, or stream, a very strong solar wind will limit how voluminous that tail can be. Thus, a slightly weaker solar wind would have less of a limiting effect on the volume of the flow of plasma.

This pressure-reducing effect outweighs the effect of there being less force to actually move the plasma - as said:

“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions"

So, this comes down to which effect is stronger - is it the expansion of the tail due to less confining pressure, or the decreased force pushing the plasma into a channel/tail? Observationally, it seems to be the first.

Though, from here, I would also assume that there is some sort of lower limit - or equilibrium - at which the size of the tail/channel reaches its maximum. If the solar wind were far too weak, there would likely not be a tail at all. So, rather than "lower solar wind causes a larger tail", I think this phenomenon would be better phrased as "excess solar wind can decrease the size of a comet-like tail, specifically around planets". It's not that lower solar wind causes a larger tail, it's that there is a sort of mid-point at which the solar wind creates optimal pressure and conditions for the tail to reach its maximum size.

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