Habitability and Gravity

The gravity of a planet has many effects, and some of them affect strongly the habitability of the planet. By habitability we mean the utility of the planet as a colonization destination for star traveling aliens. Habitability is an easier thing to have than being a solo planet, one on which life can originate and evolve.

Gravity affects habitability by making the surface tolerable to aliens for walking, standing, or whatever motion and positions they use. For Earthlings, we can survive up to about seven or so times as much gravity as is on the planet, but only for a short time, and not while performing normal actions. Somewhere between ten and twenty-five percent change in gravity, from the aliens’ home planet, might be tolerable, provided they were land creatures. If they were water creatures, it could be substantially more.

Human beings range in weight, for a given height, by a factor of more than three. A lightweight human being, put on a 3g planet, might be able to move if he was athletic, but the energy burn would be much higher and exhaustion would soon arrive.

Bones respond to the amount of stress they are put under, and a young alien borne on a higher gravity planet would grow thicker bones, if they had bones, as well as larger muscles. Just how much adaptation is possible is not clear.

The gravity that matters is the gravity on the surface of the planet, and this is determined by the mass of the planet and its density. A planet with more silicate material than Earth would be larger, if of the same mass, and would have lower gravity.

The atmosphere is also affected by gravity. Different gases escape at different combinations of gravity and temperature, although the interaction is not as simple as it might seem. Water vapor is a lighter molecule than neon, yet there are only a few parts per million of neon in the atmosphere, while there is a great amount of water vapor. Neon is a noble gas and non-reactive, and can move through the atmosphere to the exosphere and escape. Water vapor, if it passes through the stratosphere into the mesosphere, which ranges from 50 to 80 km above the surface, will deposit out into the thin clouds at higher parts of the mesosphere, thus inhibiting it from escaping into space. Deposition is exothermic, but there is so much energy loss by radiative emission in the mesosphere by CO2 molecules that it maintains its extremely cold temperature profile. The atmosphere is sufficiently thin there, that the photons from vibrational relaxation of the CO2 molecules, which have been excited by thermal collisions, escape into space, cooling the gases. This keeps water in the atmosphere and on the planet. Neon, a heavier gas than water vapor, is mostly gone.

What this means is that if gravity were less on a planet, water vapor would be escaping more readily, and the planet would not have oceans or rain or rivers. So there is a minimum value of surface gravity for water to continue to exist, whether or not it is in the habitable zone. The exact value depends on the thermal flux from the star, and there are several confounding factors, such as the solar wind and the magnetic field of the planet.

On the higher side of gravity, by the time a planet, with the same density as Earth, gets to twice its surface gravity, it will be retaining helium. Helium, like neon, is a noble gas and will not interfere with life processes chemically. But if there is a significant percentage of helium in the atmosphere, it will make it harder for birds to fly, and they might not evolve. This has an ecological effect, but evolution being what it is, it will outflank any problems and figure out other ways to do the things birds do on earth, such as spread seeds.

Helium in large quantities might serve to absorb the vibrational energy of the CO2 molecules in the mesosphere, and possibly raise the temperature there, thus interfering with this mode of water vapor retention. However, the increased gravity should more than compensate for this. Perhaps the most serious change would be the alteration of cloud level and amount from the reduced density of the atmosphere. Condensed water vapor will be suspended less in a helium atmosphere, meaning that they will remain in the atmosphere for a shorter time, and descend more rapidly. This means more rain nearer sources of water vapor, and less elsewhere. It also means less cloud cover at higher altitudes, changing the thermal profile of the atmosphere.

Helium may also result in a reduction of ozone content by facilitating the reverse reaction that creates it. With less ozone, there will be a larger UV-C and UV-B flux onto the surface. UV-C is fatal to living cells, and UV-B is damaging. There may be some positive benefits to having large quantities of helium in the planet’s atmosphere, but the negative ones outweigh them. Thus, from an atmospheric point of view, for a planet to be desirable as a habitable colony for land-dwelling creatures, gravity should be in the range of about 0.9 g to 1.5 g.

Gravity also has other changes on a planet. With lower gravity, topography can be more extreme, with larger mountains and deeper valleys and rifts. With higher gravity, the opposite is true: lower mountains and shallower valleys and rifts. There would be more stratification of the mantle, and lower gravity might allow more volcanic activity, assuming the planet had not cooled far enough to banish this. Rivers would run faster with higher gravity and avalanches would occur with lesser outcropping or less steep slopes. The oceans would be more stratified, and possibly for some amount of increased gravity, geostrophic circulation would cease, meaning nothing like the Gulf Stream and the Japanese current to transport heat from the tropics to the polar zones. This might lead to more ice over larger areas.

To be forthright, not much work has been done on planets with differing gravities, but indications are that gravity is a strong determiner of habitability. To take a tentative position, colonizable planets might have between 0.9 and 1.25 g as their surface gravity. For solo planets, gravity is not such a barrier for the first steps of life, but it becomes more significant at the transition to land organisms.