1. Why is there life on Earth?

2. The 5 Ingredients Needed for Life Beyond Earth
Most Earth life is microbial. And microbes have been here for at least 3.5 billion years. This is a very long time considering the first rocks formed about 3.8 billion years ago.
Astrobiologists estimate that the Milky Way has 500 habitable planets, so there is a good chance that microbial life exists elsewhere. Any suitable planet must fit the following criteria:
1. Temperature. The temperature that allows water to be in liquid form. This is what defines the "habitable zone." But is water actually present? How do you know a planet has water? To do this, we need to look not at a planet’s oceans, but at its atmosphere.

3. 2. Energy sources. If you're in the habitable zone of a star, you need to have enough sunlight to support photosynthesis.
3. Make sure there's nothing that will kill you, such as radiation. The planet Kepler 186-f orbits a red dwarf star which has a lot of solar flares. This creates radiation causing sunburns and skin cancer. A suitable planet must be a comfortable distance away from a star, so that they are far enough away to be out of the heavy heat and radiation, but not so far that they're really cold. This just-right distance is called Goldilocks or
the habitable zone. A suitable planet also needs a protective atmosphere holding carbon dioxide and other gases that keep the planet warm and protect its surface from radiation.
4. Nitrogen. We need nitrogen to create amino acids. These are the building blocks of life.
5. Oxygen. It is important to build complex life, plants and
animals.

4. Just as conditions on Earth have changed over time, so have conditions on other planets. When astrobiologists look at a planet, they look not just at its current conditions, but for signs that it could have been habitable in the past.
How can we check for these things? Not with a telescope. We need to use other methods….
Liquid water leaves behind a kind of footprint. Mars, for example, has what looks like dry river and lake beds, making us think if the cold, dry planet could have once been covered with life. If it was, there may be enough remaining liquid water to sustain microbial life deep underground or under the frozen ice caps.
But is it still worth looking for life on Mars? Because Earth and Mars are so close, any life found on Mars might have come from Earth and been carried over on a meteorite—or vice versa, that life on Earth might have been carried over from Mars. If we find it 500 light-years away, we know it's not related.

5. Whether more-complex life forms like plants and insects exist elsewhere is a bigger question. For 2.9 billion years or so, all life on Earth was microscopic. Multicellular, algae and animals came onto the scene about 600 million years ago. That's about 17% of the total time that life has existed on Earth. It's very hard to say how likely we are to find other intelligent life forms. The oldest fossils of our species, Homo sapiens, are less than 200,000 years old. If, like on Earth, intelligent life takes much longer to evolve than microbial life, it might be rare. But some galaxies and solar systems are much older than ours, so maybe other intelligent life has formed; however, it may be long dead.