How does earth maintain a constant level of oxygen?

It doesn’t! the oxygen level of the planet has varied quite dramatically in the last 500 million years ago; as the climate cooled and the land plants died off, oxygen level fell to as low as 12 percent by the beginning of the triassic. Back then, the air at sea level would have felt thinner than at the top of the Alps today.

Burning fossil fuels has reduced oxygen levels very slightly  - about 0.057 percent over the last 30 years. Deforestation only has a small effect because when rainforest is cut down, other plants are usually grown in its place. But it’s marine plankton, rather than trees, that produces about 70 percent of atmospheric oxygen. Global warming will have a significant impact on plankton, which is a much more serious threat to oxygen levels.

Could we exist on earth under a red giant star?

About five billion years from now, the sun’s usual source of nuclear energy will be depleted. It will begin to expand and cool significantly, becoming a ‘red giant’. Its outer layers will be thrown off into space. As the sun loses mass, its gravitational pull will weaken and the planets’ orbit will widen. We know that mercury and venus will not be able to outrun the expanding sun, and will be engulfed and incinerated.

Earth may just outrun the swelling red giant but its proximity, and the resulting rise in temperature, will probably destroy all life on Earth, and possibly the planet itself. However, there’s no reason that life could not survive on another planet (or moon) sufficiently far out from the sun, as long as it lies within the sun’s expanded ‘habitable zone’. Life could also survive on suitably hospitable planets around other Red Giants.

COULD LIFE HAVE ORIGINATED DEEP INSIDE EARTH?

The idea that life could thrive deep below earth’s surface was once regarded as heretical. Lacking any obvious source of energy, such as sunlight, and subjected to intense heat and pressure, subterranean organism would seem to have little chance of survival. Yet since the 1980s, bacteria, fungi and worm-like creatures have been found lurking kilometres down in mine boreholes and deep sea sediments. These organisms have extraordinary source of energy. For example, some bacteria rely on the reactions between water and rocks to get there energy.

Dating techniques suggest bacteria have existed at depths of several kilometres for at least 30 million years. What isn’t clear is where they fit into the history of life on earth: were they washed down, or are they progenitors of life on the surface? Either way, their existence has boosted hopes for life on Mars. While none has been found on the surface, NASA’s curiosity rover recently detected methane coming from within the planet – which may be the result of subterranean organisms.