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.
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.
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.