I'm currently writing up a world inhabited by tall, humanoid crow-like aliens, and I have all the specifics down, I'm just struggling to figure out how far said planet, with the characteristics listed below, will have to be for solar wind from a sun-like star to not strip said planet's atmosphere

Planet specs:

  • ~75% of the mass and the gravitational pull of Earth

  • Atmosphere is 65% N2, 34% O2, and 1% H2O, CO2, CH4, and other gases with 1.39 Earth atmospheres of pressure

  • moon about the same mass, size, and composition as Earth's moon, creating a Pluto-Charon kind of binary planet system

  • Iron core is proportionally larger than Earth's core, creating a decently strong magnetosphere

  • ~69% of the planet is covered in an ocean

  • terrestrial planet

Please help me


2 Answers 2


Short Answer:

Don't worry about the distance from the star necessary to protect the atmosphere from the solar wind, worry about the distance from the star necessary for the planet to have liquid water on the surface.

Part One: The Suface Gravity and Escape Velocity of your Planet.

You write:

~75% of the mass and the gravitational pull of Earth

And the phrase "the Gravitional pull" is rather ambiguous. Does it meant h surface gravity of the planet or does it mean the escape velocity of the planet?

Assume that the planet has the same overall density as the Earth 5.514 grams per cubic centimeer. In that assumed case it will have 0.75 of the volume of Earth in order to have 0.75 of the mass of Earth. According to my rough calculations, a planet with 0.0989 the radius ofearth will have about 0.750871572 the volume of Earth, which I guess is close enough.

Assuming that your planet has exaclty 0.0989 the mean radius of Earth's mean radius of 6.371 kilometers it will have a radius of about 5,789.693 kilometers.

If it has a mass of 0.750871572 Earth mass, the surface gravity and escape velocity of the planet can be calculated. Earth's surface gravity is 1 g, or 9.80665 meters per second per second.

according to this online calculator: https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc

The surface gravity of your planet should be about 0.91 g whch will not be much help for your flying people. Since the calculator uses a radius of 6.378 kilometers for Earth, I tried the calculation again using 0.9089 times 6,378 to get 5,796.9642 kilometers, but the result still came out 0.91 g.

What will be the escape velocity of your planet, vital for its ability to retain an atmosphere? Earth's escape velocity is 11.186 kilometers per second.

According to this online calculator the escape velocity will be 10.167 kilometers per second, or 0.9089 that of Earth.

Note that the surface gravity and the escape velocity do not change at the exact same rate.

Venus, Mars, Mercury,an the Moon are less massive and less dense than Earth.

Venus has 0.815 the mass of Earth and a density of 5.24 grams per cubic centimeter, 0.945411679 the density of Earth. It has a surface gravity of 8.87 meters per second per second, 0.90448828 that of Earth, and an escepe volocity of 10.36 kilometers per second, 0.926157697 that of Earth.

Mars has 0.107 the mass of Earth, and a density of 3.9335 grams per cubic centimeter, 0.713365977 that of Earth. It has a surface gravity of 3.72076 meters per second per second, 0.379411929 that of Earth and an escape velocity of 5.027 kilometers per second, 0.4449401037 that of Earth.

Mercury has 0.055 the mass of Earth and a density of 5.427 grams per cubic centimeter, 0.98422198 that of Earth. It has a surface gravity of 3.7 meters per second per second, 0.377294998 that of Earth, and an escape velocity of 4.25 kilometers per second, 0.379939205 that of Earth.

The Moon has 0.0123 the mass of Earth and a density of 3.334 grams per cubic centimeter, 0.604642727 that of Earth. It has a surface gravity of 1.622 meters per second per second, 0.165397969 that of Earth, and an escape velocity of 2.38 kilometers per second, 0.212765957 that of Earth.

Notice that none of those properties of those worlds have the same ratio to that of Earth as the other properties have. YOur idea that a planet with 0.75 the mass of Earth would automatically have 0.75 the surface gravity of Earth or 0.75 the escape velocity of Earth is naive.

Of course with some effort such a planet could be designed with 0.75 the mass of EArth and either 0.75 the surface gravity of Earth or 0.75 the escpe velocity, but not both.

Notice that these worlds which are all both less massive and less dense than Earth have lesser surface gravity than Earth and lesser escape velocity than Earth.

And notice that these worlds which have are both less massive and less dense than Earth have escape velocities, vital for retaining gases in their atmospheres, which are higher relative to Earth than their surface gravities.

You want a planet which has as low as a surface gravity as possible to make flying easier for your species. But you also want a planet which has as high an escape velocity as possible to retain a dense enough planetary atmosphere to make it easier for your flying people to fly, and also for the oxygen which they will need to breathe to have the energy to fly.

Thus you apparently need a planet with less mass and lower desnity than earth.

Part Two: A Titanic Solution?

I note that Titan, the largest moon of Saturn, has a surface gravity of 1.352 meters per second per second, 0.137865632 that of Earth and an escape velocity of 2.369 kilometers per second, 0.211782585 that of Earth. And it has an atmosphere even denser than Earth's. It has been claimed that humans on Titan could strap artifical wings on their arms and fly by flapping their arms.

Of course they would have to use bottled oxygen and very warm clothing, since the atmoshere of Titan is unbreathable and very, very cold.

Titan's amtosphere is partially protected from being slowly knocked away by the solar wind by the facts that Tital is within the magneto sphee of saturn during part of its orbit and by the fact that the Solar wind is only about 0.01 times as strong at the orbit of Titan as it is at Earth's orbit.

If Titan was brought as close to the Sun as Earth is, Saturn would have to be brought with it, and the orbit of Titan would have to be closer to saturn to stay sithin Saturn's magnetosphere at all times to protect from the Solar Wind.

But that wouldn't help much. As Titan warmed up the gas particles in its exosphere would become fast enought o rapidly escape from Titan.

So any story about flying people on a world like Titan would be stuck having a very, very, cold world with the lifeforms drinking liquid methane instead of water, and human visitors needing environmental protection.

Part Three: Could There be Flying People on Earth?

Not all intelligent beings need to have exasctly the same mass as Earth humans. Possibly some intelligent beings could have the same adult mass as young human children have, for example.

What were the heaviest flying animals on Earth?

Argentavis magnificens was among the largest flying birds ever to exist. While it is still considered the heaviest flying bird of all time, Argentavis was likely surpassed in wingspan by Pelagornis sandersi which is estimated to have possessed wings some 20% longer than Argentavis and which was described in 2014. A. magnificens, sometimes called the Giant Teratorn, is an extinct species known from three sites in the Epecuén and Andalhualá Formations in central and northwestern Argentina dating to the Late Miocene (Huayquerian), where a good sample of fossils has been obtained.1

Prior published weights gave Argentavis a body mass of 80 kg (180 lb), but more refined techniques show a more typical mass would likely have been 70 to 72 kg (154 to 159 lb), although weights could have varied depending on conditions.[8][4][9] Argentavis retains the title of the heaviest flying bird known still by a considerable margin, for example Pelagornis weighed no more than 22 to 40 kg (49 to 88 lb).[6] For comparison, the living bird with the largest wingspan is the wandering albatross, averaging 3 m (9 ft 10 in) and spanning up to 3.7 m (12 ft 2 in).


So argentavis wighed about as much an adult human, while Pelagornis weighed about as much as human child. So there were flying birds which weighed about as much as humans.

What about the largest flying reptiles?

Quetzalcoatlus /kɛtsəlkoʊˈætləs/ is a pterosaur known from the Late Cretaceous period of North America (Maastrichtian stage); it was one of the largest known flying animals of all time. Quetzalcoatlus is a member of the family Azhdarchidae, a family of advanced toothless pterosaurs with unusually long, stiffened necks. Its name comes from the Aztec feathered serpent god Quetzalcoatl. The type species is Q. northropi, named by Douglas Lawson in 1975; the genus also includes the smaller species Q. lawsoni, which was known for many years as an unnamed species before being named by Brian Andres and Wann Langston Jr. (posthumously) in 2021.

When it was first named as a new species in 1975, scientists estimated that the largest Quetzalcoatlus fossils came from an individual with a wingspan as large as 15.9 m (52 ft). Choosing the middle of three extrapolations from the proportions of other pterosaurs gave an estimate of 11 m, 15.5 m, and 21 m, respectively (36 ft, 50.85 ft, 68.9 ft). In 1981, further advanced studies lowered these estimates to 11–12 m (36–39 ft).[10]

More recent estimates based on greater knowledge of azhdarchid proportions place its wingspan at 10–11 m (33–36 ft).[8] Remains found in Texas in 1971 indicate that this pterosaur had a minimum wingspan of about 11 m (36 ft).[11] Generalized height in a bipedal stance, based on its wingspan, would have been at least 3 m (9.8 ft) high at the shoulder.[3]

Body mass estimates for giant azhdarchids are extremely problematic because no existing species share a similar size or body plan, and in consequence, published results vary widely.[3] Generalized weight, based on some studies that have historically found extremely low weight estimates for Quetzalcoatlus, was as low as 70 kg (150 lb) for a 10 m (32 ft 10 in) individual. A majority of estimates published since the 2000s have been substantially higher, around 200–250 kg (440–550 lb).[12][13]

n 2022, Gregory S. Paul estimated that Q. lawsoni had a wingspan of 5 m (16 ft), body length of 3.5 m (11 ft), and body mass of 65 kg (143 lb).[14]

So Quetzelcoatlus seems ot have been able to fly with a weight two to three times that of an adult human.

Of course such flying creatures, heavy enough to have brains equal to human brains, would have very large wings, possibly larger than you want, which is why you want a palnet with lower gravity and a denser atmosphere to make flying equal.

Part Four: Your planet has to be Within the Habitable Zone of Your Star.

You need to put the planet within the habitable zone of your star so it has the prober surface tempertures for liliquid water using life forms, unless they have a totally alien biochemestry like the hypothetical liqud methane using lifeforms mentioned on Titan.

And as long as your planet has a reasonably high escape velocity to retain atmospheric gases for a long time, and as long as it generates a decent magnetosphere to protect agains tthe solr wind the planet should retain its atmosphere for billions and billions of years.

PLanetary scientists still do not totally understand the generation of planetary magnetic fields and and can't really predict how strong the magnetosphere of a planet will be. So there is no particular need to design a planet with features you think will make its magnetoshphere even stronger than Earth's.

I note that the planet Venus has a semi-major axis of 0.728 AU, meaning that the solar wind is about 1.886852 tiems as strong as as it is at the orbit of Earth.

Venus has a very weak magnetosphere which is not, repeat not, generated by its core. Thus the solar wind is slowly stripping away lighter atoms from its atmosphere.

But Venus has a very dense atmopshere over 90 times as dense as that of Earth. the escape velocity of Venus is high enough to retain many gases for billions of years.


One Astronomical Unit

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This planet is similar enough to Earth, there is no reason to change anything else.

Perhaps a larger iron core will mean a stronger magnetic field that is better at deflecting stuff than Earth. So you could put it a bit closer and have the same amount of particle showers hitting the surface.

But a closer planet brings its own problems like being hotter. This might make the crow people unhappy. Crows don't sweat after all.

  • 1
    $\begingroup$ Wouldn't the lower force of gravity affect how much atmosphere the planet retains? $\endgroup$ Nov 3, 2022 at 18:31
  • 1
    $\begingroup$ @redfrogcrab You have already decided how much atmosphere there is by deciding the atmospheric pressure. $\endgroup$
    – Daron
    Nov 3, 2022 at 18:32

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