A high gravity rocky planet is a planet with breathable atmosphere (after terraforming), with accessible ground and with a gravity over 1.75 times stronger then Earth. In majority of cases, the planet is a Super-Earth, but in some particular cases, it can be smaller then Earth. All human activities will be different there from what we see on an Earth - like planet.
Usually, if gravity is higher, the atmosphere must also be denser, up to a limit, where the planet becomes a gas giant. However, it is possible that a large enough planet exists without too much air circling it. If we take a gas giant (like Neptune) very close (0.05 AU) to the Sun, its atmosphere will be slowly blown away. Then, if at some point we take the planet back to its orbit, it will be far smaller, because almost only the rocky core has survived. Another way of forming a high gravity planet is by colliding a few smaller planets very close to the hosting sun (for example, by colliding 4 Earths at 1/2 the orbit of Mercury). If the angle and speed are correct, it is possible to blow away much of the crust, while the heavy cores will merge. Finally, a third possibility to find a high gravity - low radius planet, is around White dwarfs, Neutron stars and Black holes. The nebula resulting from a supernova can sometimes be just enough to create a few small planets, only that they will have a different composition, with higher amounts of heavy elements. The ultraviolet and X radiation will blow away water and atmosphere from the planet. In all cases, it is required that the planet has moved back to somewhere closer to Habitable Zone.
As seen here, there are many ways habitable high gravity planets can be formed. And also, by terraforming, some small gas giants can become high gravity rocky planets. Also, many Super Earths can be transferred into this category once the excess of atmosphere is removed.
Since random chance affects atmospheric density, as evidenced by the differences in atmospheric density between worlds with similar gravity, some high gravity habitable planets may be naturally formed.
It all depends on major conditions found on the target planet.
- A gas giant or a super Earth can become a high gravity rocky planet once the excess of air is removed. The process requires technology beyond our current achievements.
- Native high gravity rocky planets might be hard to terraform. If the planet's radius is similar to Earth but its gravity is over twice as high, it means that the planet contains high amounts of heavy elements. This is good news for miners and industrial corporations, but bad news for biologists. Heavy elements are usually associated with toxins. Certainly, it will be a hard life on a planet with lead, arsenic or cadmium in mostly all the rocks. Natural mitigation could come in the form of higher gravity causing stronger differentiation, collecting almost all heavy metals into the planetary core. Also, some Peruvian villages are already inhabited by genetically arsenic resistant people and some Russian families from western Siberia are genetically resistant to mercury. These genes could be transferred to many colonists by mild genetic engineering.
- Some heavy gravity planets may be more easy to terraform. A good example would be a planet with a radius twice that of Earth, with similar internal composition, but with nearly the same atmospheric pressure.
A high gravity will have strong effects on geography and biology.
First thing to be noted is a stronger erosion. Rivers will flow much faster, eroding their valleys. In time, it will probably lead to a world without mountains, only with plains and smooth hills. If there were no liquids to flow on surface before terraforming, the new formed rivers will create deep canyons in short time. New settlers will probably build towns where they say it's safe, but their grandchildren will have to relocate. Even rains will be more different. With higher gravity, rain drops will be like little falling rocks, with harsh effect to any exposed ground.
Volcanoes might be found, but will be different from what we see on Earth. They will not throw their lava far into the air. Volcanic cones will be larger and lower. Earthquakes are also expected to be found if the planet has plaque tectonics.
The sea will also be different. Because of higher gravity, strong winds will not blow big waves. Each wave will be small and will move faster on the surface. Even so, waves will still have the same erosion to the coasts. Tides will also be smaller.
Gravity will play an important role. All plants will need more strength to sustain their bodies. Trees will grow smaller and wider. Grains will also need to spend more energy to strength and will have less productivity. But this will not be the only difference. Also, animals will change. Their bones and muscle tissue will increase in size and strength. The first thing to change is the behavior. On a planet with twice the gravity of Earth, even simple things are risky. Gravity acceleration grows exponentially, so it is four times higher. On a planet with 3 times more gravity, gravitational acceleration is 9 times higher. Basic things, like sitting on a chair, are different. If you throw yourself to an armchair or a bed, because of the strong acceleration, you risk to break your bones. Walking on the stairs downwards, is also strange. Jumping from one stair to the other will be like jumping steps of a meter down. So, the first thing to change is animal and human behavior. Both will try to avoid jumping and falling.
Human life Edit
Not many people will like to go on a world with high gravity. Only the toughest pioneers will enjoy such a world. Their children will be more adapted (see Future races for details). Still, there might be planets with gravity over 3 times higher then Earth. Earth humans will adapt very hard to this environment, but settlers from a planet with gravity 1.8 times stronger then Earth might be able to adapt.
It is not plausible that humans will be able to live on a planet with gravity over 5 times higher then Earth without genetic manipulation. Even if we can survive on short term, on long term, our bones, joints and blood vessels will be damaged.
The highest problems will be for builders. Engineers will face a lot of problems. Each building will need more and stronger building materials. And this is not all. More materials means that each floor will be heavier and even more weight will push on lower floors. Building bridges will also be a hard task. Building a tunnel will cost a lot, because rock weight will push harder on tunnel walls, making it to collapse. Creating an airplane is an even harder task, since it will require stronger engines. Overall, technological level will be far lower then on an Earth-like planet.
The hardest task will be to get into outer space. Each spaceship will request a much higher amount of fuel. Since space transport becomes too expensive, goods from the planet cannot be exported, even if imports are possible with the help of a parachute.
A theory of everything (quantum gravity) is a solution to many of these problems by manipulating gravity. Also helpful in reaching the exoplanet in the first place.
Without many new settlers, population will grow slowly, thanks to natural growth.
On a world with high gravity, there will be far less connections to the exterior world. Shipping good will cost a lot more then in case of Earth, meaning that industrial centers will not be able to export. In fact, they will have to sell their goods on internal market. Without much money income from exports, there will not be much money for imports.
The direct result is that the economy will go into isolation, since it will not be competitive to other planets. Doing so, all factories will remain outdated. Technological level will remain low and will grow very slow. Without any chance to stand the competition with other states, local governments will focus on isolation.