So, i want to make a planet with these requirements:
Also my question is if a Low G world would favor these requirements than a High G world, being the gravity range between 0.5 and 1.4.
Thanks
Short Answer:
It is probable that a high surface gravity world would better for floating objects or lifeforms, increasing bouyancy.
But a low surface gravity world would probably be better for trees to grow tall and for animals to fly.
In my long answer I discuss several different claim aobut he smallest mass of worlds that could retain dense atmospheres for geologic eras of time. And I also discuss the densest atmosphere that humans beings, or beings with similar environmental requirements, could endure.
Long Answer:
Part One: A Low Gravity and High Escape Velocity World
Since you want large flying animals and large trees if possible, I guess that a lower surface gravity planet would be better than a higher surface gravity planet.
A world will have a dense atmosphere if it produces or acquires atmosphere faster than it loses it, and will have a thin atmosphere if it loses atmosphere faster than it produces or acquires it.
The feeling of weight, and how easily large animals can move, on a planet, is based on its surface gravity. But the ability of a planet to retain an atmosphere is based largely on a different property, its escape velocity.
The surface gravity and escape velocity of a world are not calculated the same way and do not increase or decrease at the same rate with various masses and radii of worlds.
Thus it is possible for a world to have a relatively high surface gravity and a relatively low escape velocity, which would be worse for your purpose. Or a world could have a relatively low surface gravity and a relatively high escape velocity, which would be better for your purpose.
A world will have a dense atmosphere if it produces or acquires atmosphere faster than it loses it, and will have a thin atmosphere if it loses atmosphere faster than it produces or aquires it.
There are many factors which influence the production and loss of atmospheres, so the density of the atmmosphere a world has is not directly proportional to its escape velocity.
But for any atmospheric gas at any temperature in the exosphere (temperatures in the exosphere tend to be much higher than temperatures at the surface) a relatively small change in the escape velocity of the world could change whether the world can retain that gas for seconds or for billions of years. Having a sufficiently high escape velocity is not sufficient to have a dense atmosphere for flying or floating, but it is absolutely necessary.
Here is a link to surface gravity formulas:
https://en.wikipedia.org/wiki/Surface_gravity
And here is a link to escape velocity formulas:
https://en.wikipedia.org/wiki/Escape_velocity
I, of course, prefer to use online calculators to calculate escape velocity and surface gravity.
[https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc][3]
[https://www.omnicalculator.com/physics/escape-velocity][4]
Part Two: A Dole-full Lower Mass LImit for Worlds with Dense Atmospheres
Stephen H. Dole, in Habitable Planets for Man, 1964, discussed the minimum mass of a world capable of retaining oxygen in its atmosphere for geologic eras of time. Dole wrote that the temperature in the exosphere of Earth's atmosphere varies between 1000 degress K and 2000 degrees K.
https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf
If a world could have an Earth like temperature at its surface, and a maximum exosphere temperature of only 1000 degrees k, the root-mean-square speed of oxygen atoms in the exosphere would be only 1.25 kilometers per second. So if the world had an escape velocity five times that, 6.25 kilometers per second, it could retain an oxygen rich atmosphere for about 100 million years.
Dole calculated that with an average density Dole considered reasonable, a planet with an escape velocity of 6.25 kilometers per second would have 0.195 the mass of Earth, aradius of 0.63 Earth radius, and a surface gravity of 0.49 g. Dole believed that such a small world could not produce an oxygen rich atmosphere, but maybe Dole was wrong, or maybe your planet has been given an artificial oxygen rich atmosphere by an advanced civilization.
If a world with 0.195 of Earth's mass had the same density as Earth, it would have 0.195 of the volume of Earth. Actually a less massive world would have a less dense core and a lower overall density. The cube root of 0.195 is about 0.5799.
The mean radius of Earth is 6,371 kilometers, so 0.5799 of that is 3,694.5429 kilometers.
A planet with 0.195 Earth mass and a radius of 3,694.5429 kilometers would have a surface gravity of 0.58 g and and an escape velocity of 6.487 kilometers per second.
If we give the planet a slightly larger radius, 0.59 of Earth or 3,758.89 kilometers, we get a surface gravity of 0.56 g and an escape velocity of 6.431 kilometers per second.
If we give the planet a slightly larger radius, 0.6 of Earth or 3,822.6 kilometers, we get a surface gravity of 0.54 g and an escape velocity of 6.377 kilometers per second.
If we give the planet a slightly larger radius, 0.65 of Earth or 4,141.15 kilometers, we get a surface gravity of 0.46 g and an escape velocity of 6.127 kilometers per second.
And that is about as far as I would want to go in lowering the density and thus the escape velocity of a world with 0.195 the mass of Earth.
Part Three: An Even Lower Mass Limit for Worlds with Atmospheres
However, in "Exomoon Habitability Constrained by Illumination and Tidal Heating" Heller and Barnes suggest on page 20 that a somewhat less massive world than 0.195 the mass of Earth could retain an atmosphere for geologic ages.
[https://faculty.washington.edu/rkb9/publications/hb13.pdf][5]
A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (MsT0.1M4; Tachinami et al., 2011); to sustain a substantial, long-lived atmosphere (MsT0.12M4; Williams et al., 1997; Kaltenegger, 2000); and to drive tectonic activity (MsT0.23M4; Williams et al., 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Gurnett et al., 1996; Kivelson et al., 1996), suggesting that satellite masses > 0.25M4 will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy—such as radiogenic and tidal heating, and the effect of a moon’s composition and structure—can alter the limit in either direction.
So they cite papers which suggest that 0.12 the mass of Earth would be sufficient to sustain a substantial, long-lived atmosphere.
They cite:
Kaltenegger, L. (2000) What does it take for a moon to support life? In Proceedings of the Fourth International Conference on Exploration and Utilisation of the Moon: ICEUM 4, ESA SP-462, edited by B.H. Foing and M. Perry, European Space Agency, ESTEC, Noordwijk, the Netherlands, pp 199–201.
And:
[Williams, D.M., Kasting, J.F., and Wade, R.A. (1997) Habitable moons around extrasolar giant planets. Nature 385:234–236.][6]
Assuming for the moment that their calculations ae correct that the minimum mass for a world to retain an atmosphere for a long time would be only 0.12 the mass of the Earth, and assuming that such a world would have the same average density as Earth (which seems unlikely to me), it would have a volume 0.12 that of Earth.
The cube root of 0.12 is about 0.49325. 0.49325 the radius of Earth is 3,142.4957 kilometers.
A world with 0.12 the mass of Earth and the same density, and thus a radius of 3,142.4957 kilometers would have a surface gravity of 0.46 g and an escape velocity of 5.517 kilometers per second.
Of course a world with such a low mass would probably have a lower average density than Earth and thus a larger radius, and thus a lower surface gravity (good), and also a lower escape velocity (bad).
Part Four: Really Small Worlds With Floating Lifeforms
There is one way that a world much smaller than that could have lifeforms that float easily.
There are some worlds in the outer solar system coverd with thick layers of ice, which are believed to contain internal oceans beneath the ice. There is much speculation that there might possibly be lifeforms in those internal subsurface oceans. Any hyptheticla multicelled animals in such subsurface oceans would float or swim in the water.
I think that the smallest such world with asubsurface ocean and possible life floating inside is Encladus, a moon of Saturn, which has a mass of about 0.00018 Earths and a radius of 252 kilometers.
But of course you want life forms to float or fly in an atmosphere instead of floating and swimming in liquid.
Part Five: The Smallest Worlds with Atmospheres
And here is a link to an article from 2019 discussing the possibility that worlds much smaller than 0.12 Earth mass could retain enough atmosphere for water to be liquid on their surfaces. The article claims that the lower mass limit under certain conditions would be about 2.7 percent of the mass of Earth, which is 0.027 times the mass of Earth, considerabley lower than 0.195 or 0.12 times the mass of Earth.
[https://earthsky.org/space/small-rocky-exoplanets-can-still-be-habitable/][7]
However, the article deals with water planets, whose atmospheres are entirely water vapor and which are largely made out of water.
[https://iopscience.iop.org/article/10.3847/1538-4357/ab2bf2/pdf][8]
Thus there would not be land based trees on such a planet, nor land based animals. Any intelligent life would live either in the water or in the air (sometimes landing on the water).
And without oxygen in the atmosphere, multicelled animals wouldn't be able to get enough energy to live unless they could utilize some other matabolic process.
But water vapor in the atmosphere of a world would have a tendency to be split by ultra violet light into hydrgen and oxygen. Thus any world with water vapor inits atmosphere would produce some oxygen. So unless that world lost oxygen at least as fast as it produced it, oxygen would accumulate in its atmosphere.
The article doesn't discuss whether oxygen might accumulate in the atmosphere of a small water world with a water vapor atmosphere.
Anyway, if a water world had 0.027 times the mass of Earth and the same average density as Earth, it would have 0.027 times the volume of Earth. 0.3 is the cube root of 0.027, so that world would have a radius of 1,911.3 kilometers, 0.3 the radius of Earth. That would give a surface gravity of 0.3 g and an escape velocity of 3.356 kilometers per second.
But such a low mass world, made largely out of water, would be less dense that Earth. If it had twice 0.027 the volume of Earth, it would have 0.054 the volume of Earth and half the density of Earth. 0.3 times 1.2599 (the square root of two) times Earth's radius equals 2,408.0468 kilometers or 0.377977 of Earth's radius. Such a world would have a surface gravity of 0.19 g and an escape velocity of 2.9897 kilometers per second.
Part Six: Flying on a Titan Sized World
Titan, the largest moon of Saturn, is rather similar. It has a mass of 0.0225 Earth mass; a radius of 2,574.73 kilometers, 0.4041327 of Earth's; a surface gravity 1.352 meters per second per second, 0.1361525 that of Earth; and an escape velocity of 2.6394 kilometers per second.
And Titan has a dense atmosphere, mostly of nitrogen, which is actually about 1.48 times as dense as Earth's atmosphere.
One reason why Titan has been able to retain a dense atmosphere is its much lower temperature than any world where water is liquid. The surface temperature of Titan is about 94 degrees K. The temperature that is important is the temperature of the exosphere of Titan's atmosphere, where gases escape into space. The temperature of Titan's exosphere should be much higher than its surface temperature, and may be proportional to the distance of Titan and Saturn from the Sun.
The example of Titan shows that such a small world could have a dense and oxygen rich atmopshere like Earth, temperatures high enough for liquid water on the surface, and a low surface gravity, if is has temperatures in the exosphere as low as the exopshere temperatures on Titan.
That would be a problem to solve, to have Earth like temperatures at the surface, but exosphere temperatures as low as in the exosphere of Titan.
One way to solve it would be to have the world as far from its star as Saturn is from the Sun, so the world would naturally have exosphere temperatures as low as Titan's, while giving the surface of the world another source of heat instead of the radiatin from the star. If that world was a large exomoon orbiting a giant exoplanet, it is possible that tidal heating form the planet and other large moons might heat the moon to Earth like surface tempuratures. And if that heat from within could avoid heating up the moon's exosphere, it could keep a dense atmosphere for geological eras.
It is the only moon in our solar system to have a dense atmosphere – so dense that, in combination with its limited gravity, humans on Titan could fly by just flapping their arms.
https://bigthink.com/strange-maps/259-unnamed-methane-sea-on-titan/
Of course if a world the size of Titan had Earth like temperatures and a lot of oxygen in its atmosphere, humans could fly better without being weighed down by warm clothing and by breathing apparatus.
It is utterly impossible for humans to attach wings to their arms and fly by flapping their arms on Earth, but many Earth animals, including insects, bats, birds, and flying reptiles flew well on Earth. And the largest flying birds and reptiles were very impressive.
Thus the largest possible flying creatures on a world the size of Titan but with a dense oxygen rich atmosphere and Earthlike temperatures, would be awesome indded.
Part Seven: Maximum Atmospheric Density
I note that the atmospheric density of such a world would not necessarily be limited to the density of Earth's or Titan's Atmosphere. Various factors on that world might possibly produce a much denser atmosphere.
But there would be an upper limit to the density of an atmosphere that humans from Earth could breathe.
The atmosphere at sea level on Earth has a total pressue of 760 millimeters of mercury (mmHG)
According to Dole on page 19 humans need between about 60 and 400 mmHG of oxygen and about 0.05 to 7 mmHG of carbon dioxide. And some nitrogen is necessary for plant life, and there should be some water vapor in the atmosphere. There should be only tiny trace amounts of toxic gases in the atmosphere, not enough to contribute noticably to increasinge the air pressure to the actual air pressure on Earth, let alone to several times that much pressure.
So it is up to more or less unnecessary but more or less harmless gases to increase the air pressure as much as possible tomake flying easier.
According to Dole on page 16, the gas that humans can tolerate the most of is Helium, possibly up to 61,000 mmHG.
Heliox is a mixture of helium and oxygen, it is used in medical treatments and by deep divers. Heliox is usually 21 percent oxygen and 79 percent helium, but is sometimes in a 70/30 or 60/40 mix. It is quite possible that a lower upper safe concentration of Helium isnow known than in Dole's era.
Anyway, helium could be a major component of the atmosphere of a planet wthi high gravityand high escape velocity, but it would be only a minor component of the atmosphere of a world with low gravity and a low escape velocity.
Dole suggests that humans might be able to tolerate as much as 3,900 mmHG of neon. 3,900 mmHG of neon and 400 mmHG of oxygen would produce a total pressure of about 4.300 mmHG of total pressure, or about 5.657 atmospheres. I am not certain how accurate Dole's estimate is. How a world would acquire a large amount of neon in its atmosphere is not obvious.
Humans can stand a pressure of 2,330 mmHG of nitrogen according to Dole. So an atmosphere of nitrogen and oxygen could have as much as 2,730 mmHG total pressure, or 3.592 standard atmopsheres.
So I expect that an atmosphere nitrogen, oxygen, neon, & argon could reach 4, 5, or 6 standard atmospheres pressure without being dangerous to humans.
It is possible that alien animals evolved on a different planet might evolve to be able to breathe much denser atmospheres than humans can survive.
Combining a denser atmosphere with a much lower gravity should be able to make flying much easier for large animals and enable trees to grow much taller than on Earth.
Since the thing that you want is larger organisms, lower gravity is always going to be desirable. For the same reason as aquatic animals grow larger than terrestrial ones, having less of your own mass pulling you down, or more buoyancy holding you up, makes being big a lot easier. There's no question that a higher gravity planet would make megafauna more of a challenge.
As to whether that planet could sustain a breathable atmosphere, I'd point you at questions such as this one on the physics stackexchange: https://physics.stackexchange.com/questions/52527/can-low-gravity-planets-sustain-a-breathable-atmosphere.
