In the last post, we have seen what it takes for life to form out of non-living things, what conditions are necessary for sustaining life and what are signs of life.
As we have seen that building blocks of life are pretty common in our universe, it may be possible that seeds of life would have been planted on many planets. And as there are innumerable planets out there, there is strong possibility that life also developed elsewhere.
Today we will look if there is interstellar life or not. We will also discuss difficulties which can stop us from finding life elsewhere.
So let’s begin.

Finding Exoplanet

According to Wikipedia, Exoplanet or Extrasolar planet is a planet outside the Sun’s solar system. We have discovered thousands of exoplanets in the past two decades. Most of the discoveries were made with NASA’s Kepler Space Telescope.

The question arises that is it compulsory for other Stars to have planets around them?
The answer was given by slow rotation of Sun. According to the origin story of our solar system, a spinning cloud of gas and dust collapsed under its own gravity and formed the sun and planets. By conservation of angular momentum, the sun should have spun faster and faster. Sun has 99.8% mass of solar system, but planets have 96% of angular momentum.
So what accounts for slow rotation of Sun?
The young-sun would have had a very strong magnetic field, reaching far into a disk of swirling gas. These field lines connected with charged particles acted like anchors, slowing spin of forming the sun and spinning gas which will soon be planets.
Most stars like Sun rotate slowly so astronomers applied the same theory to them, meaning planet formation must occur for them.

Methods to search Exoplanet

Stars near to us are actually pretty far. The nearest star to us Alpha Centauri is at 4.3 light years from us. They are just pinpoint of light in our night sky. So how to spot planet-sized object around these pinpoints of light?
Here brilliance of astronomer can be seen. They said if we can’t observe planets directly, then we can look for minute effects that orbiting planets can have upon them.
Methods involved in the search of exoplanets are:

Radial velocity

Also known as Doppler spectroscopy, it is based on fact that a star doesn’t remain completely stationary when it is orbited by a planet.
Star moves slightly in small circle or ellipse in response to the gravitational tug of a smaller companion. From Earth, these slight movements appear as a shift in star’s color signature. If the star is moving toward the observer, then its spectrum is slightly shifted toward blue; if it is moving away, it will be shifted toward red.

It was a first successful method for detection of exoplanets and is responsible for identifying hundreds of them.
A major drawback of this method is that it is most likely to find the types of planets that are least likely to be habitable.
Most planets discovered by this method are “hot Jupiters”. As cool they sound, they are far from being habitable. These are giants planets similar to our neighbor, Jupiter but orbiting at the dizzying speed at a very short distance from their star.

First confirmed the discovery of a world orbiting a sun-like star in 1995, was 51 Pegasi b – a Jupiter-mass planet 20 times closer to its sun than we are to ours.
While “hot Jupiters” are relatively easy to find, they can’t host any organic life as we know. But they make conditions even worse, their presence at the center of a planetary system makes it less likely for Earth-like planets to survive in their neighborhood.

Transit Photometry

In this method, the slight dimming of the star is measured as an orbiting planet passes between it and the Earth. If such dimming is detected at regular intervals and lasts a fixed length of time, then it is very probable that a planet is orbiting the star.
It gives a fair estimate of the size of the planet. A large planet causes bigger dimming while a small planet causes slight dimming. As the size of the star is determined by its spectrum, photometry gives astronomers a good estimate of orbiting planet’s size, but not its mass.
This makes photometry an excellent complement to the spectroscopic method, which provides an estimate of a planet’s mass, but not its size.
The determination of size and mass enables us to determine its density.
This method has been the most effective and sensitive method for detecting extrasolar planets. Kepler mission was very helpful in searching exoplanets before retirement this year it has discovered 2,682 exoplanets and more than 2,900 candidates awaiting confirmation.
And its benefit doesn’t stop here, it can provide “absorption spectrum” of planet’s atmosphere. The light from the star passing through the planet’s atmosphere is absorbed to different degrees at a different wavelength.
Its absorption spectrum can give important details of constituents of the atmosphere.


It is a method in which Einstein General relativity comes in handy.
According to Einstein, when the light emanating from a star passes very close to another star on its way to an observer on Earth, the gravity of intermediary star will slightly bend the light rays from the source star.
This method helped in finding furthest and smallest planets of any currently available method.
In 2006 we discovered a planet only five Earth masses orbiting a star near the center of our galaxy.

But microlensing method is a one-shot, means planets detected by microlensing will never be observed again. Because these events are unique and non-repeatable.
So far using these methods, we have discovered 3,903 confirmed planets in 2,909 systems, with 647 systems having more than one planet.
Out of these planets, there are 16 planets discovered in the habitual zone around their parent star.
But we have observed only a fraction of our night-sky, there is a large fraction of night-sky unobserved.

Drake Equation

In 1961, astrophysicist Frank Drake devised an equation to predict the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation summarizes all the various concepts which scientist must contemplate when considering the question of life elsewhere.
N = R* • fp • ne • fl • fi • fc • L
N = the number of civilizations in the Milky Way Galaxy whose electromagnetic emissions are detectable.
R* = the rate of a fraction of those stars with planetary systems.
ne = the average number of planets that can potentially support life
fl = the fraction of planets that actually support life
fi = the fraction of planets with life that evolves to become intelligent life
fc = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space
L = the length of time over which such civilization broadcast detectable signals into space
The challenge is that astronomers don’t have firm numbers on any of those variables so that any calculation of the Drake equation remains a rough estimate for now.
Based on observations from the Hubble Space Telescope, there are between 125 and 250 billion galaxies in the observable universe. At least ten percent of them have Sun-like stars having planets. It means that there are 6.25 x 1018 stars with planets orbiting them in the observable universe.
2013 study suggests that Milky Way contains 100-400 billion exoplanets.

Signs of Life

So how would we know that a planet has a life if not an intelligent one?
Means how will we verify sign of life on those planets?
Absorption Spectra is the answer. When we cross light through a prism, it shows a band of color from violet to red called the spectrum of white light. But it has one interesting property, different gases and chemicals absorb certain specific color. So every Chemical has a unique absorption spectrum, like its signature.
As we have seen in the transit photometry method that it can give us an absorption spectrum of the atmosphere of the transiting planet.
So we know which gases are in the atmosphere of those planets. But what it has to do with the life there?
The answer is every living thing changes the composition of its atmosphere by some extent. As on Earth, we have changed our atmosphere far more than its original state. Every organism on our planet expels waste gases as they live.
If we find similar chemical disequilibrium on exoplanets, then there is a strong possibility of life on that planet.
If conditions of life are this prominent then why don’t we witness any extra-terrestrial civilization?

Where are all the aliens?  – The Fermi Paradox

It is nature of life to reach out, explore and cover every niche it can. If we look from our perspective, we are on the verge to become an interplanetary civilization. Further steps would be to colonize our solar system, then reach to the stars and other planetary systems and the next obvious step would be to become a galaxy-wide civilization. This is very likely for life to do, no matter wherever they are. Thus we should be able to see something evident. But we see nothing.

The observable universe is about 90 billion light-years, contains 100 billion galaxies each with 100 billion to 1000 billion stars. Most of the stars have planets and a few percents of them must be habitable which makes billions of planets having the life. As most of the planets are born way before the Earth then there could be millions if not billions of Civilization that have developed far better technologies.
The universe should have been filled with Space-ships and multi-planetary civilizations, so where are they?
If the other galaxies have civilizations, it would be very difficult for us to ever visit them. Due to expansions of the universe, the superclusters of galaxies are separating at the rate of thousands of miles per seconds, with our fastest ships it would take billions of years to reach them.
Therefore we have to limit our scope to our home galaxy, Milky Way. Our milky way is thought to have 100 billion stars, out of which let only 10% have planets. It would be 10 billion stars with planets. If only 0.01% of them have a planet capable of hosting life, then it would be a hundred millions of planet with life. Only if a small proportion of them have developed into civilization, then there would be at least 1 million civilizations.
This means even our milky way had to be filled with spaceships visiting nooks and corners of the galaxy. But where are they?
This paradox is called Fermi Paradox.
One possible answer to this paradox is the Great Filter.

Great Filter

There should be a filter that prevents any species to become a galactic civilization, and if any species tries to cross it, they are eliminated.
There are two possibilities that filter lies behind us or in the second case, it lies in front of us.

Case-1: the filter is behind us.

In this case, we have crossed the filter and we are the first. It means one of the steps we have passed to become what we are today, is almost impossible to take.
What could be that impossible step?
It may be that even if the necessary conditions for life are met then it is very difficult for life to emerge from dead things, there is a debate on whether life is unique or not.

It may be the formation of the complex cell. Before the beginning of complex structure formation, single cells used to feed on other cells, but once a cell captured another cell and provided it shelter in return the small cell provided nourishment and energy to host in return of protection. These cells developed more and more complex structures to evolve into living mammals, apes and us.
It may be that our galaxy is filled with bacteria covered planets but only on Earth, these bacteria evolved.

Case-2: the filter is in front of us.

This scenario is the most depressing one. It means we are already on the path of self-destruction. If this filter exists then it must be so powerful that every civilization that has ever achieved the level when they can use technologies to harness power from there planet, they are doomed.

It may be a planet-wide nuclear war, or a bio-war, or genetically mutated disease, or uncontrolled technology etc. powerful enough to wipe out their whole existence.
Or it may be that in the search of more and more resource every civilization has ruined their planet’s atmosphere making it completely inhabitable before even finding their second home.
And it must be true for every civilization, such that not one of them has yet survived. 
If the filter is ahead of us, then we are doomed to go out of existence once and for all. No one will ever give care that once a blue marble like planet ever existed, on which people existed and knew about their fate but still used to worry over petty things.


Even if life exists anywhere else in our galaxy, it may have not evolved into intelligent beings. Or they are so different from us that our means of communications are too primitive for them. They may have been sending us messages from a long time but we are not yet able to comprehend. Or maybe their senses are pretty different from us, they don’t know how to touch, feel or even see. Or even if they have seen us, we are not worth their visit. Or they are happy in their own kin. They don’t want to come and visit anyone. There could be a huge number of possibilities.

A scene from movie ‘Arrival’

In the galactic timescale we have just arrived and have opened our eyes, if there is anything for us to see then we have just started looking. And there is only one way to find out, just keep looking and exploring.

Credit for information- nasa, wikipedia.


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