What makes the Earth habitable? There are multiple factors, but most of them are related to a single aspect of our world: it orbits the sun at the right distance. Each star is surrounded by a region called the habitable zone. This region is around a star where water can potentially exist in liquid form on the surface of a planet. Water is used to define the habitable zone because it is the main ingredient used by life on Earth. One of NASA’s mottos is “Follow the water.” It should be noted, however, that not all worlds in the habitable zone are guaranteed to have liquid water flowing on their surface. In our solar system, the habitable zone extends from the orbit of Venus to Mars, but none of these worlds is currently home to oceans, lakes, or rivers. Instead, the habitable zone is simply a way of determining the potential habitability of a world. Not all planets in the habitable zone will be habitable. The habitable zone defines the potential for a world to become habitable.
Different Living Areas
Representation of planets orbiting a red dwarf star. NASA
The living area varies from star to star. The more massive and hot a star is, the farther away from the star is the living area. For smaller stars, the living area is much closer to the star. Mass stars are scorching and emit huge amounts of deadly radiation, making it even more difficult for a planet in the habitable zone to form an atmosphere. In addition, high-mass stars do not tend to stay long. If our sun had been a star of mass mass, it might have become a supernova before life began. Red dwarf stars have shown many promise simply because more Earth-sized and super-Earth-sized planets have been discovered around them than any other class of star. However, these also have their problems. First, despite being smaller and colder than most stars, red dwarfs are extremely active, erupting solar flares and solar storms at a much higher rate than most stars.
Solar storms would pose a considerable risk to habitability because a planet must orbit very close to a red dwarf to be in the habitable zone. Second, the proximity needed to be in the habitable zone of a red dwarf also means that all the planets in this region will be blocked by the tide, which means that one side of the planet will always face the ‘star. Instead, the other side always looks back. Half the planet would experience an eternal day, while the other half would experience an endless night. Unfortunately, this dramatically affects the surface temperature, so the daytime side is usually too hot and the night side is too cold. Interestingly, the region that separates night from day could serve as an oasis to develop life, a region that would experience an eternal sunset (or sunrise). In addition, some models have predicted that, depending on the planet’s atmosphere, the heat of the day could circulate to the side of the night, maintaining temperatures and allowing the formation of liquid water on the surface. At the moment, stars like the sun seem to be the most promising for life. The sun is active, but in general, solar storms are not extreme enough to pose any threat to life on Earth. The temperature of the sun is neither too hot nor too cold, which allows us to orbit at such a distance that our planet is not blocked by the tide. In addition, the sun burns its hydrogen constantly, which allows it to exist long enough for life to evolve and bloom long before it goes extinct.
Habitability outside the living area
Europe is a potentially habitable moon orbiting Jupiter. NASA
A planet can orbit the habitable zone and still never become habitable, but is it possible for a planet outside the habitable zone to become habitable? In recent years, scientists have had to rethink the terms of habitability. Only in our solar system is it known that a handful of moons orbiting gas giants possess habitable conditions such as liquid water. These moons exist far outside the habitable zone, but the conditions have allowed the formation of liquid water. For example, Jupiter’s Europa moon and Saturn’s Enceladus moon have subsurface oceans of liquid water. A world can become habitable outside the habitable zone. This finding allows scientists to expand their search for habitable worlds around other stars.
Aidan Remple June 16, 2022 in Science