I have 2 planets, a hot Jupiter with a standard majority Hydrogen/Helium atmosphere (Ab) & an Earth-like planet(Ac) orbiting around an orange dwarf star (A) which is very similar to Epsilon Eridani [https://en.wikipedia.org/wiki/Epsilon_Eridani] with these parameters:


Stellar classification: K1.5V - K2V

Luminosity: 0.36 Stellar luminosities

Mass: 0.8 Stellar masses

Radius: 0.78 Stellar radii

Temperature: 5090 Kelvin


Mass: 0.5 Jupiter masses

Radius: 2 Jupiter radii

Density: 0.24 Jupiter densities

Escape velocity: 29.78 m/s2

Temperature:1000 Kelvin

Semi-Major axis: 0.04 AU

Inclination: 62.5 degrees (open to changes very easily if necessary for stability)

Ac most parameters don't really matter, only that:

Semi-Major axis: 0.7 AU Radius: 0.8 Earth radii Atmosphere: 120% Earth atmosphere

So the question really is if the temperature is enough for hydrogen to be blasted out of Ab's atmosphere to create a long comet like tail (and if its temperature would even be 1000 Kelvin to begin with at it's distance from A). And how would this tail appear in the sky of Ac?, would it look like a simple streak or cover the entire sky?, would it be visible during both day and night? what color would it have?, would the large amounts of hydrogen have any affect on Ac's aurora's? enter image description here

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  • $\begingroup$ 0.04AU from a star only a bit lighter than and much less bright Sol? Would gravity not dominate, making the escaped gases form into a ring/gas torus around the star? $\endgroup$
    – user79911
    Nov 8, 2020 at 16:17
  • $\begingroup$ i don't think so, since there are hot jupiters with tail orbiting closer to bigger and hotter stars without the gasses forming a real ring, Example: en.wikipedia.org/wiki/KELT-9b $\endgroup$ Nov 8, 2020 at 16:30

1 Answer 1


Planets can have tails. But I am skeptical about a gas giant.

Comets have 2 kinds of tails: a tail made of dust left in the wake of the comet, and a tail of ionized gas blown off by the solar wind.

It turns out that Venus (and theoretically also Mars) sometimes have ionized tails caused by the solar wind. See this page on the ESA website.

venus tail

For Earth, which has a strong magnetic field, the ionosphere is relatively stable under a range of solar wind conditions. By comparison, Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere... New results from Venus Express reveal for the first time the effect of a very low solar wind pressure on the ionosphere of an unmagnetised planet...

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.

I admit I am still unclear about why weaker wind = longer tail. I would think the opposite; explanations welcome.

But here is the problem - the reason this works for Venus and Mars and comets is that none of those bodies have magnetic fields to deflect the solar wind. Earth does, fortunately, and so the solar wind is less able to strip ions away to form the tail. I think any gas giant would also have a magnetic field, because the liquefied gas in the center conducts charge and performs the role that molten metal in the core does on Earth. All 4 of our systems gas giants have magnetic fields. I think maybe that is part of being a gas giant.

Now maybe there are other ways to get a tail. Maybe the magnetic field of the gas giant produces something analogous to the solar wind and strips ions from its moons? Maybe a long trailing aurora (which if I understand it glows for reasons similar to those causing comet ion tails to glow - particles charged by the solar wind). In any case I don't think your gas giant can use the same mechanism as a comet, but that does not mean you should give up on a sweet tail!

  • $\begingroup$ Note that those "tails" are waaaaay too rarefied to be visible. $\endgroup$
    – AlexP
    Nov 8, 2020 at 17:07
  • $\begingroup$ @Willk From my understanding, some hot jupiters are so close to their star (which they migrated to otherwise it wouldn't be a gas giant) that their temperature is too high for hydrogen to stay in the gas giant's atmosphere, so it experiences atmospheric escape. then thehydrogen is blown away by solar wind, forming a large tail behind the gas giant as depicted in the reference images in my question. $\endgroup$ Nov 8, 2020 at 17:08
  • $\begingroup$ Edit: the process of atmospheric escape to the point of no hydrogen remaining will take billions of years $\endgroup$ Nov 8, 2020 at 18:49
  • $\begingroup$ One of the figures in the OP's question (from Ehrenreich et al 2015) depicts the tail of a gas giant, Gliese 436 b. There are certainly examples of this out there. From what I can tell, that's the type of tail the OP is looking for - not cometary tails. $\endgroup$
    – HDE 226868
    Nov 11, 2020 at 14:33
  • $\begingroup$ @HDE226868 - thanks for the link. So this giant is so hot that hydrogen can escape but once it escapes, the weak solar wind allows it to form a tail. Cool. I think I will put a ? on planetary stack about why weak solar wind is more conducive to tail formation because I don't understand that. $\endgroup$
    – Willk
    Nov 11, 2020 at 16:04

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