why we only live in this planet
Answers
Bcoz the climate is favourable for us on earth to live.
Answer:
The closest star system to our own made headlines on Tuesday (Oct. 16) with the announcement that it hosts a planet about the mass of Earth — a tantalizing discovery so close, astronomically speaking, to us.
While the newfound planet may be Earth-sized, researchers say it is almost certainly barren.
Astronomers detected the alien world around the sunlike star Alpha Centauri B, which is a member of a three-star system only 4.3 light-years away from our solar system. This planet, known as Alpha Centauri Bb,is about as massive as Earth, but its hot surface may be covered with molten rock — its orbit takes it about 25 times closer to its star than Earth is from the sun.
"We're pretty sure there's no chance of life on this planet," said MIT planetary scientist Sara Seager.
So what makes a world such as ours able to host life? Why is Earth so special?
There are a few key ingredients that scientists often agree are needed for life to exist — but much debate remains as to what limits there actually might be on life. Even Earth hosts some strange creatures that live in extreme environments. [Strangest Places Where Life Is Found on Earth]
This artist's concept shows the newfound alien planet Alpha Centauri Bb, found in a three-star system just 4.3 light-years from Earth. (Image credit: ESO/L. Calçada)
Here's what makes life able to thrive on our home planet (and likely for alien life to arise on other worlds):
Water
"First, you'd need some kind of liquid, any place where molecules can go react," Seager told OurAmazingPlanet. In such a soup, the ingredients for life as we know it, such as DNA and proteins, can swim around and interact with each other to carry out the reactions needed for life to happen.
The most common contender brought up for this solvent is the one life uses on Earth: water. Water is an excellent solvent, capable of dissolving many substances. It also floats when it is frozen, unlike many liquids, meaning that ice can insulate the underlying fluid from freezing further. If water instead sunk when frozen, this would allow another layer of water to freeze and sink, and eventually all the water would get frozen, making the chemical reactions behind life impossible.
Astronomers looking for extraterrestrial life most often focus on planets in the so-called habitable zones of their stars — orbits that are neither too hot nor too cold for liquid water to persist on the surfaces of those worlds. Earth happened to hit the Goldilocks mark, forming within the sun's habitable zone. Mars and Venus lie outside it; if Earth's orbit had been just a bit further inside or outside of where it is, life may likely never have arisen and the planet would be a cold desert like Mars or a cloudy furnace like Venus.
Of course, alien life may not play by the rules we're used to on Earth.
Astrobiologists increasingly suggest looking beyond conventional habitable zones. For instance, while liquid water might not currently persist on the surface of Mars or Venus, there may have been a time when it did. Life might have evolved on their surfaces in that time, and then either fled to safer locales on those planets, such as underground, or adapted to the environment when it became harsh, much as so-called extremophile organisms have on Earth, or both.
In addition, other solvents might host life. "Saturn's moon Titan has liquid methane and ethane." Seager said.