In the very briefest of descriptions, a Sub-Earth is an exoplanet that orbits a star somewhere else in the universe that is smaller than our own planet, Earth. A Super-Earth is a planet that is larger than our planet but not as big as a sub-Jupiter. The final group is a Hospitable-Earth which one that supports life which is one that can also be a sub or super Earth. Mars and Mercury are both described as Sub-Earths because they are rocky planets that are smaller than our planet
The gravitational strength of a planet is related to the size of the planet. The bigger the planet is, the more mass it has, the stronger the planet's gravity would be. If we were to leave Earth and find another planet, the ideal planet would be of equivalent size to our own otherwise would struggle. Astronauts onboard the International Space Station have to do two hours everyday to ensure their muscles don't waste away whilst up in space as there is no gravity onboard the space station
We would be able to do things on a Sub-Earth that we wouldn't be able to do on Earth, we'd also be able to jump over a house because we'd be strong enough to overcome gravity. How small a planet would have to be to allow someone to jump a house is hard to say. According to Ref: Business Insider, you could easily jump more than 12 feet off the ground on the moon or 6 foot if you're on Mars.
At the other extreme on a Super-Earth, we would struggle to some of things we take for granted on Earth. Jumping on a Super-Earth would be difficult, breathing would be difficult. In the same article that I linked in the previous paragraph, it describes a Jupiter jump as being sad, not very descriptive. On Jupiter, you'd probably have extreme difficulty standing and you can forget about even lifting a foot even without a sock on because the gravity is so big. Sub, Normal or Super-Jupiters are all no-nos and best to avoid.
An alien from a Super-Earth would be best avoided as they would be much harder and stronger than anyone on Earth owing to have to deal with his/her planets gravity. A Sub-Earth alien would want to avoid us. In the below picture, we would probably be able to survive on Kepler-20f without any difficulty and any alien be of similar strength to us.
As mentioned briefly before, a Sub-Earth is a planet that is smaller than our own Earth that may be possible to support Earth. An example of a sub-Earth are those that orbit the star Kepler-42. The three planets are all smaller than the Earth and orbit a red dwarf star in the constellation of Cygnus, the swan.
Sub-Earths compared to larger planets are the hardest to detect. The reason why they are the hardest to detect is because they don't exert as much influence on the host star compared to the larger version. The first exoplanets were actually found around a pulsar, Lich in the 1992 and those planets were all sub-Earths. As Lich is a Pulsar and therefore the planets would be unsuitable for life to ever exist.
In John Carter, the hero of the story gets teleported to Mars where the planet has a breathable atmosphere and John is able to run and jump longer and higher than any Barsoomians. Barsoom is what the natives or who we called Martians called Mars. In the film, John Carter picks up a massive stone and lobs it a monster which he would be able to do because gravity is weak and therefore his muscles are more than capable of doing it. A Barsoomian would not have been able to do that compared to John.
If Dejah Thoris, the Barsoomian princess and lover of John Carter were to come to Earth, she would experience what it would be for us humans to visit a planet that was bigger than our Earth, she would struggle. In the film, Dejah didn't visit Earth.
Super-Earths are Extrasolar Planet(Exoplanet) that orbit other stars in the universe that are more massive that our own Earth. Massive being the key word, massive shouldn't imply size as in radius or diameter but mass. Being called a Super-Earth should not be taken as being a planet that can support life. A planet that was smaller than the Earth would be known as a Sub-Earth or a Minor-Earth.
A Super-Earth would be great for humanity as we are currently 7 billion people and although some are saying the numbers are levelling off, its going to grow even further. With a large population comes the need for more and more resources until one day we will run out.
At the moment, we are banking on the hope that new supplies will be found and that the supplies we currently have will be sufficient until we are able to move to a sustainable renewable energy policy. At the moment, the move to using renewable energy is slow. China leads the way as being the lead investor in renewables with the U.S. being second. Although China is a lead investor, it is not the country which generates most of its energy in renewables, that title goes to Denmark. Ref: EcoWatch
The only planet in the Universe that we know of that currently has any life is our Earth, the third rock from the Sun. Hopefully one day we will discover that there are others out there hopefully not in the form of an alien attack. There are many candidates for hosting life on other planets in the galaxy.
The primary thing to look for in the case of a planet that can provide the right conditions for life is that it is within the Goldilocks Zone, a zone where the temperature on the planet is neither too hot nor too cold but just right.
All the pictures that we have of Earths outside the solar system are artists impressions, the planets might not look anything like what we are expecting them to look like.
In February 2017, N.A.S.A. announced they had found a star system with a number of possible habitable Earths in orbit. There was a lot about on the news at the time. Planets e, f and g are all within what is known as the Goldilocks zone where life could exist. The planets zip round the star far faster than we do so "you'd age" a lot more as your birthday would come round much quicker than on Earth. Its not an ideal planet to live on but its a start.
The star in the centre is Trappist-1 and is a red dwarf star, a star that is cooler and smaller than our Sun. Red Dwarves are easier to spot planets orbiting because the bigger a star is the harder it is notice any change in dimming or moving as a planet orbits. If there was a planet around the giants of the galaxy such as UY Scuti, any affect the planet has on it is negligible as to not be seen. Even our Earth has an affect on the Sun that aliens from another world could spot.