Lying back on a dark night looking at the sky, seeing millions of stars sometimes we get an existential crisis. We are on a tiny planet in the solar system, orbiting Sun, which is a tiny star among billions of other stars in Milky Way galaxy which itself is a tiny galaxy in the supercluster of billions of galaxies which is a tiny part of our universe which may be just one in the many multiverses.
Ohkay, that’s incomprehensible for me. But is there someone else in maybe different world, trying to figure out this mysterious universe? OR as most of us would love to hear, Are there aliens out there?

What does Alien or Extraterrestrial life mean?

From the Cambridge English dictionary, Alien means “coming from a different country, race or group” OR “strange and not familiar”. This implies that pop culture references of the Aliens (most of them) do not stand true to this definition.
 Like Neil DeGrasse Tyson pointed out in one of his articles that Aliens depicted by the movies are very similar to us. In his words

Given the diversity of life on Earth, one might expect a diversity of life exhibited among Hollywood aliens. But I am consistently amazed by the film industry’s lack of creativity. With a few notable exceptions such as life forms in The Blob (1958) and in 2001: A Space Odyssey (1968), Hollywood aliens look remarkably humanoid. No matter how ugly (or cute) they are, nearly all of them have two eyes, a nose, a mouth, two ears, a head, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet – and they can walk. From an anatomical view, these creatures are practically indistinguishable from humans, yet they are supposed to have come from another planet. If anything is certain, it is that life elsewhere in the universe, intelligent or otherwise, will look at least as exotic as some of Earth’s own life forms.”

Well as ‘Alien’ does not quite fit here let’s look the other phrase “Extraterrestrial life”.  The word ‘Extraterrestrial’ is adjective which means coming from outside the planet Earth; thus the above phrase means “the life coming from outside the planet Earth”.
Hmm, that sounds much better, ‘Extraterrestrial’ seems bulky let’s suppress it to ET. So as we played enough with words lets focus on the scientific aspect of ET life.
So now here a question arises if there is any other life (E-T life) out there how are we going to find them?

How life occurred on the Earth?

What are the factors which make life possible on the Earth? There is so much to talk about this topic but we have to stick to the main issue. (It can be topic of next post. *wink*) So, in short, I will tell you what are the conditions required for us to form in first place.
So beginning with our solar system, Is there any place in the solar system other than Earth which can be suitable for life or maybe hosts life? 
 First of all, let’s see what makes Earth suitable for life. Life on Earth is made mainly of organic compounds, but organic ingredient simply doesn’t react to form living things. The second main thing for life is water, as the organic molecules require a medium in which they can mix and interact. And the final requirement is the power source which on Earth is fulfilled by the Sun.
Small microbes in water
So how these ingredients arrived on the Earth?
 At the beginning of the solar system, when Earth was one of the hundreds of protoplanets, there was a collision between Earth and another protoplanet called Theia, this collision was so intense that surface of the Earth was melted. And the water trapped between the rocks vaporized. This vaporized water rained later on creating oceans of water which harbored early microscopic life. 
But how did organic compounds come here?
Kuiper belt
The answer lies far beyond the orbit of Neptune in the cold region called Kuiper belt, home to large numbers of chunks of icy debris left over from the formation of the solar system.  These are primarily composed of ices, a mixture of light hydrocarbons, ammonia, and water ice.
But how did these objects orbiting that far smash with Earth?
 Here come heavyweight members of the solar system into the play. Jupiter and Saturn, gas giants of our solar system. In the Early Solar system when the Earth and other rocky planets had formed and accreted most of their mass there occurs an event called the Late Heavy Bombardment. One of the possible cause for the late heavy bombardment is Giant Planet migration.
Before and after effect of Giant planet migration
When the solar system was young and flourishing but have formed the majority of planets as we see now, the Jupiter and Saturn’s orbits were in the 2:1 resonance. It means that for every single orbit of the Saturn, the Jupiter completed two orbits. When Jupiter and Saturn aligned they created some sort of gravitational resonance which pushed Neptune, one of the gas giants far into the Kuiper belt.
Late Heavy Bombardment on young Earth 
This movement of the planet created disturbance in the Kuiper belt objects hurling most of them inward on their collision path with planets. The aftermath of these events can still be seen on the Moon and other planets. One such of the asteroid could have collided with Earth bringing with them necessary organic molecule needed for life.
Stardust mission
Recent findings strongly support this theory. Stardust was a mission launched back in 1999. Its primary mission was to collect dust samples from the comet. On January 15, 2006, it returned samples to Earth. When samples were analyzed it had traces of glycine.
Glycine is an amino acid used by living organisms to make proteins. Basically, it is the fundamental building block of life.

  “The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare,” said Dr. Carl Pilcher, Director of the NASA Astrobiology Institute which co-funded the research. 

So if life forming material is in abundance in Universe, then there is strong possibility that if right conditions were met life can flourish.

What are we looking for?

Let’s start with our neighborhood, no I am not talking about your neighbor who plays loud music when you are trying to work and you want to take the life out of him before finding life elsewhere.
I am talking about our neighbor planets and moons. To being with mercury, it is a pretty harsh planet with barely any atmosphere left (thanks to the closeness with Sun). Being tidally locked with Sun its one face is heated up to 427 ˚C while another face is frozen by temperatures of -173 ˚C. So nope organic life as here on Earth will not survive there.
Our next stop is Venus since it is called sister planet of Earth. And has an approximately similar size as of Earth. So you may think it may have some hospitability. But it is a hostile world, with high atmospheric pressures and hottest temperatures enough to melt lead it is far from being habitable for any life.
Mars is in the Goldilocks zone was considered a planet suitable for life. Its surface show evidence that water once flowed on the Mars.  But when Mars lost its magnetic field, it lost most of its atmosphere. And the water which was flowing of the surface vaporized and blown away by the Solar winds. Making mars inhabitable for any organic life.
Surface features of mars 
On May 25th, 2008 Phoenix mission landed a robotic spacecraft in the polar region of Mars. It dug a few inches below the surface and analyzed the soil. The whitish material dug by it vaporized over the course of four days, implying that they were composed of water ice.
Phoenix lander
Presence of water ice under the surface of mars strongly suggest that water once flowed on the mars. Satisfying two necessary conditions for life, did life exist on the mars?
Site dug by Phoenix probe
The Late heavy bombardment would have planted organic material on every planet but only planet habitual could sustain life. If water once flowed on the Mars then there should have been life on it.
But how would we find out that there was life on the Mars? OR Is there still life on the mars?
Here back on Earth life is present in extreme conditions. From the hot deserts to the cold one.  In the Antarctic region scientists have dug the ice covered with soil in the similar fashion as on the Mars, and surprisingly they have found moving microorganisms which are inactive in the colder times but when temperature increases during summer they come back to life.
Could this type of microorganisms still exist on the Martian surface? Scientists have found a possible trace amount of methane in the atmosphere of Mars. As methane is an unstable gas, its presence indicates that there must be an active source on the planet in order to keep such levels.
Methane sources on Mars
Sources of methane can be various like asteroids burning in the atmosphere, volcanic activities, or maybe microorganisms as methanogens. Since asteroids can only account for only 0.8% of the total methane production and inactive geological activities can’t produce this much methane ruling out these two factors.
Seasonal variation of Methane on Mars
Methanogens don’t require oxygen or organic nutrients and are non-photosynthetic. They use hydrogen as their energy source and carbon dioxide as their carbon source, so they could exist on the mars, far below the surface where it is warm enough for liquid water to exist.
Methanogens
But if once life existed on Mars is there any sign which will prove its existence?
Mars 2020 rover
Here comes NASA’s Mars 2020 rover mission. It’s one of the goals is to determine whether once life existed on Mars or not. It will land in the Jezero Crater. This crater contains deposit rich in clays and was supposed to be a lake before water evaporated from Mars.  This is considered a good site to detect any signs of ancient life on Mars.
Jezero Crater on Mars (landing site for mars 2020 rover)
We have only looked at the possibility of life on our neighboring planets only. In the next post, we will go interstellar distances to look for our Alien or I should say ET friends.

Till then Stay Curious and Keep asking.
Sources- wikipedia, NASA
Categories: LifeSolar System

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