# Planet with only plant-based life and very thick atmosphere [closed]

Working hypothesis is as follows:

• Solar system with an A class sun.

• Planet has a very thick atmosphere which does not allow much visible light in to the surface. (Like dusk in the day time and darkness at night.)

• Atmosphere appears static from the surface of the planet.

• Atmosphere has multiple layers, some of which move with the rotation of the planet.

• A few small meteors have become "stuck" in the atmosphere's lower layers, allowing them to be seen in the day time.

• Planet's oceans are small and few, made not of water but some liquid unique to this solar system which evaporates slowly.

• Life forms are all plant/fungus/microbe based. No mammal life.

What sort of planet size, distance from sun and rotation might be reasonable plausible in this fictional scenario? Many thanks to anyone and everyone who lends some of their expertise and brainpower in this thought exercise!

• Atmosphere appears static from the surface of the planet. - What do you mean by this? No winds or what? – Adrian Colomitchi Mar 5 '20 at 5:13
• A few small meteors have become "stuck" in the atmosphere's lower layers, allowing them to be seen in the day time. - impossible. Unless the atmosphere is so dense it has the same density as the rock of those meteors, which is to say "I have an atmosphere with the same density as the solid rocks and yet... is gaseous" That requires a very special wicked magic, science-based rejects it. – Adrian Colomitchi Mar 5 '20 at 5:18
• not of water but some liquid unique to this solar system which evaporates slowly.  even water evaporates slowly if there's not enough heat or the pressure is enormous. So, what temperature/pressures you have in mind? – Adrian Colomitchi Mar 5 '20 at 5:20
• I see where you are aiming with this, but we'd need more specific info about the materials involved in creating your planet before moving on to answer the question you tagged at the end. You could start by asking a question regarding what materials would allow this configuration with whatever specific conditions (at Earth conditions on the surface, temp, pressure etc. for example), then build up to asking about rotation, orbital distance etc.. Meantime voting to put on hold till you can edit to fit the science-based tag's criteria.. – Tantalus' touch. Mar 5 '20 at 6:49
• I'm thinking specifically of the water cycle we see in Earth. This planet does not have such a cycle and I'm imagining that on a hot planet with no polar caps, water would evaporate quickly. @AdrianColomitchi, I'm just diving into specifics here but I'm imagining that this planet would do very well at retaining heat and the lowest temperature would be about 50°F, potentially a bit lower at night. Would it be possible the very thick atmosphere "regulates" temperatures so there is no vast variation? – LadyIthae Mar 5 '20 at 12:31

Small meteors maybe can stuck in atmosphere, but need winds a lot faster in a dense atmosphere.

Winds have variable speed due their altitude, but the density will demand layers super thick below, also demand tons of energy to generate these winds. One Sirius like star can provide it to one near planet, but temperature in the ground will make unable to lifeforms. This planet would be like a 'hot neptune'.

Lifeforms also demand energy sources and when they "unlock" new sources provide evolutionary leaps like professor Olivia Judson explain here https://www.nature.com/articles/s41559-017-0138.epdf Then, the planet need be able to evolution of plants and after something happen changing the atmosphere and only some plants will adapt to survive or no way to anything multicelular in a thick atmosphere. Maybe this planet is a Venus-like one, after a cataclysm but still no dramatic hellish condition as real Venus. The static appareance of atmosphere in the ground is unreal, cause rotation of planet is slow, but no tidal locked.

Size of planet need to be big enough to keep the atmosphere and even the core of planet generate a magnetic field. Also need be a metal-rock planet, then, this planet need form near of the star, but also no so close, one A type star is a lot hot.

One other problem is time: A type stars are kinda young, Denebola have a estimated life in 100 million years, Altair is a bit more older with 'only' 1.2 billion years. To one scenario of a cataclysm is ok, with worlds still in formation, but is hard-hardest a multicelular lifeform arise, evolve and adapt.

The planet need enough distance of star to avoid extreme temperatures, probably orbiting one A like star the Mars orbit is enough, its 1.5 UA.

• "Small meteors maybe can stuck in atmosphere" do you care to explain how this can happen, please? Even the dust gets out of a storm and lets the air cleaner if the dust storm can't raise more of it (e.g. dust storm raised over desert then evolving over sea). I can't imagine 'meteors that one can see during daytime' persisting in the wind. – Adrian Colomitchi Mar 5 '20 at 10:25
• This planet is too warm for polar caps - wind will have a different role than on Earth. Is it plausible that the winds are too close to the surface to make an effect on the upper layers of the atmosphere? – LadyIthae Mar 5 '20 at 12:44
• Adrian, need a f***ing wind speed to it, like keep in orbit. I told about 'hot neptune' because its way those exoplanets are called, but a better comparation would be Uranus with strong speed winds. Although, of course, not will remain floating in the skies, like a baloon or kite or that mass lands of Avatar. – Rodolfo Penteado Mar 5 '20 at 16:35
• The atmosphere accumulates and distributes the heat received from the star, so the planet will not have large temperature differences, perhaps, at most, some variation by seasons if its orbit is quite eccentric, like Mars. Winds are also faster in the upper layers than in the lower ones because the air masses will be denser below. As I said before, your planet looks like a less hellish version of Venus. en.wikipedia.org/wiki/Atmosphere_of_Venus – Rodolfo Penteado Mar 5 '20 at 16:52