The Hunt for Life
The Hunt For Life : To Infinity and Beyond
This post was written by Avni, Murtaza and Harshit for the third SpaceWalk by STAC IIT Mandi.
SpaceWalks is a series of talks where speakers will present on various topics in the fields of Space Technology and Astronomy.
We are part of a tree of life pondering over our roots. Astrobiology deals with these questions. Astrobiology is the study of the origins, evolution, distribution, and future of life in the universe.
After the Copernican revolution, we have realized that earth is not the center of the universe, humans are not the pinnacle of evolution and there is nothing special about our place in the cosmic universe. So thinking that other systems like that of our solar system can also exist isn’t that radical an idea.
What is a Planet ?
According to the IAU, a planet is a celestial body that satisfies 3 conditions
- It should be in orbit around the sun
- it should have sufficient mass for its gravity to overcome rigid body forces and assume a nearly round (hydrostatic equilibrium) shape.
- And the third one, the one due to which Pluto had been demoted from being a planet: that it should have cleared the neighborhood around its orbit.
The definition of an Exoplanet? It is simply a planet orbiting a star that is not the Sun
History Of Exoplanet Discovery
In 1992, an exoplanet was discovered but isn’t considered as one as it was orbiting a Pulsar, which is a dead star, and thus didn’t seem like a classic earth-like exoplanet
Then the first Exoplanet, 51 Pegasai B, was discovered in 1995 by astronomers working on binary stars in Switzerland. It was very unusual to have found it. It had an orbit period of 4.4 days, even Mercury, the planet with the shortest orbital period in our solar system, has an orbit period of 3 months. It was termed a Hot Jupiter due to being very massive and extremely close to its parent star.
After this, there was an increase in the number of Exoplanets being discovered The next landmark was the launch of the Kepler Spacecraft and the Kepler telescope in 2009, which advanced our knowledge of exoplanets immensely, helping us discover Earth-like planets.
And now we have more than 4000 discovered and confirmed exoplanets of various types
Types Of Exoplanets
The major types are Hot Jupiters, Mini Neptunes ad Super-Earths
Hot Jupiters: They are very massive planets, orbiting very close to their parent stars and have short orbital periods. These were the first kind of exoplanets found and have an upper advantage in their detection due to their large size
Mini Neptunes: As the name suggests, these are a bit smaller than our Neptune and resemble it in terms of their structure.
Super-Earths: The third kind, the most interesting and useful ones for us. These are terrestrial planets that may or may not have atmospheres, many of them lie in the habitable region in space, where life can exist. They are more massive than Earth but lighter than Neptune.
Methods to Discover Exoplanets
There are 2 major methods that have been used for the purpose One is Gravity or Doppler Shift method and the other is transit Method
The gravity method: It is an indirect method and we don’t have to see the exoplanet at all. The first exoplanet in 1995 was discovered using the gravity method and was used for finding 100s of other exoplanets in the decade that followed. Currently, we have discovered about 900 planets by this method. So the planet and the star system orbit about their common center of gravity, which is shifted towards the parent star due to its mass being more. The movement of the planet creates a wobble motion of the star around the center known as reflex motion. The reflex motion converts the wobble into a periodic Doppler shift in a sinusoidal form, which is observed using spectroscopic methods. When the star moves away from us, we go towards the red side of the spectrum, known as redshift and when it moves towards us we move towards the blue side of the spectrum, known as blueshift. We can measure the periodic shift in the wavelength of the light obtained. The magnitude of the Doppler shift is directly proportional to the radial velocity. By using Kepler’s laws and gravitational equations, we can deduce the mass of the exoplanet through this method.
We necessarily get the Mass times sine(i) where i is the inclination angle of the orbit with the line of sight.Now, That inclination is not measurable by the observation itself, and so that is an uncertainty in the mass. But through large statistical samples, it can be shown that on average the total mass gets underestimated by a factor of 2.
There are certain limitations to this method as well As the planet gets smaller or their orbital distance increases, chances of detection reduce as it creates barely detectable reflex motion. And this method Tells nothing about the size of the planet
Transit method: With the launch of the Kepler spacecraft, we got enormous data which led to the discovery of many new exoplanets by the transit method. This method of exoplanet detection is the most used method and has helped us discover more than 3000 exoplanets.
A transit occurs when a planet passes between a star and its observer.
In this method, we detect the small dips in the light curves of stars to detect exoplanets as they cross their parent stars in their orbit. Light curves, as can be seen in the video, are charts of the level of light being observed from the star. Based on the dip produced we can infer various properties like orbital radius, orbital period, and the size/volume of the exoplanet.
The size limits the observation, large planets which can block more light are more easily detectable as compared to Earth-like planets( though through advanced technology, Earth-like planets have also been discovered.) Some exoplanets with very fast transit periods are visible for very short durations of time and thus are hard to detect.
Some Other Ways of detecting Exoplanets
Direct Imaging It seems like the most obvious way of detecting exoplanets but is the most difficult one as exoplanets do not emit any light of their own and reflect light from their parent star and it becomes very hard to observe that with the light of the star. We can use a coronagraph for this method, it blocks the light of the parent star and the exoplanets become visible. In the animation (insert animation), you can see a multi-planetary system, with the light from the parent star being blocked and the exoplanets become visible. As an exoplanet-finding approach, this technology is still in its early stages, but there are great expectations that it will become a major instrument for discovering and characterizing exoplanets in the future.
Gravitational Lensing or microlensing The light from distant galaxies can get distorted due to the gravitational force of massive objects in their way, this is gravitational lensing To an astronomer, a lensing event looks like a distant star that gets gradually brighter over the space of a month or so, then fades away. If a planet happens to be lensed, it looks like a brief blip of light that happens during this brightening and dimming process. This process is the most sensitive process and can detect exoplanets of the size of our moon as well, but such events are not that common
Now that we have seen the various methods employed for finding distant planets and looking for Earth-like planets, you might be wondering if we have found a chance of life on any of these?
With such a vast universe comes the possibility of life on other planets, but how rare is life and how many planets exist with the possibility of life on them?
The Drake Equation and The Fermi Paradox
In 1961 Dr. Frank Drake was pondering upon the same questions in his lab. It was when he took the help of probability to determine something that would help us know the chance of finding life.
Considering different factors he gave an estimate on how many planets exist with intelligent life that can communicate with us. Have a look at the formula and you’ll observe that it is fairly simple and considers a wide variety of factors to do so. With the available data and pure estimates, Dr Drake began putting in values to come up with a number.
The most well-known factor was the formation of stars which is 10 per year, 2nd the fraction of these suns that have a planet which is 0.5, now considering our solar system the number of these planets that lie in the hospitable zone and can nurture life is 2 when the conditions are right the fraction of these planets where life would appear on is liberally considered as 1, he assumed that given enough time and resources life always appears. Now the chance that these lives can evolve into an intelligent life is half but again he proposed that if the life is intelligent it’ll surely develop communication, now the last factor i.e the mean amount of time they can remain detectable to us is 10000 years. So after plugging in all the values he came up with a magical number which is 50,000. That’s a huge number as it suggests there are 50000 possible planets with intelligent life of them…
But unfortunately, we haven’t even found any trace of life on another planet except ours.
With such a good number and so many exoplanets in the habitable zone with the possibility of life, the question that arrives is where Is everybody? And will we be ever able to find them? This is the Fermi paradox.
Considering the size of the universe you’ll agree that the speed of light is really not that fast, it still takes 10,000 years for even light to cross the milky way, with such a vast universe, everything beyond the milky way and our local group is basically inaccessible to us because of the ever expansion of the universe. So Even if life existed in different parts of the universe we would never notice or never know about it. Life has been on earth for around 3.5 billion years, out of which human civilization has been for only 200,000 yrs and from which we have acquired technological advances just a few 100 years back. imagine sitting at your home sending morse code signals around the world, nobody would ever notice or communicate back to you and you will think you are alone in the universe whereas life may be thriving in other parts of the world. so if you see on the universal timeline we have just been here for a few nanoseconds it is a
possibility that we do not have the technological advances we need to communicate with other civilizations.
There are millions of stars in a milky way with billions of planets but out of these, we have discovered only 4000 that can be potential candidates to harbor life. So you see we have only seen a grain of sand in the universe and a large number of them await to be discovered.
The Great Filters
life itself is super rare, you’ll understand that there are some things that we call the great filters which can be the possible solutions for the Fermi paradox.
- Biogenesis is the start of life,
- The evolution of life to complex form jump from unicellular to multicellular
- And the development of intelligence
Biogenesis: One of the biggest quests in finding life on other planets is life itself. There are many possible theories on how life originated, for this…. let’s consider our own planet, one of the possible theories for life formation is through the Miscelle, when the earth began to cool and oceans formed, water was rich with minerals and necessary ingredients for life. Some even call it the LiFe Soup, in the oceans, where phospholipids are the chain of fatty acids, with hydrophobic and hydrophilic ends. When placed in water these hydrophobic ends get pushed away from the water and automatically come together at the center while the hydrophilic ends make the outer layer and form a missile. Many miscelles come together to form a long chain of fatty acids which then turn around on themselves into a sphere. With the right conditions of pH, temp, and many other these spheres crushed in on themselves to form the early life. This was the stage that separated the nonliving world from the living.
Evolution From unicellular to Multicellular: The second step when life appears on any planet is to form complex structures and build itself into multicellular beings and life as we know it. but it’s not common for life to evolve because it requires many resources and a lot of energy to build up complex structures. If you look at any cell of today’s organism you’ll observe that it looks like 2 different bodies. One research suggests that at the beginning a bigger miscelle would have consumed the smaller one without harming the other. The smaller cell eventually starts providing energy and resources to build complex machinery of the cell, while the bigger is provided with raw materials and protection from the harmful environment outside. It still takes a few million years for life to evolve furthermore building cell by cell and form remarkable structures and multicellular beings. But it’s not as easy it may seem as it requires a lot of factors and necessary conditions for the process to occur. It takes just one meteor or one cosmic event to destroy everything and the miracle of life.
Development Of Intelligent Life: But if life survives all this it still needs to make a very big leap to intelligence. Let’s be honest, being intelligent is hard work, it requires lots of resources and energy to keep the brain functioning but doesn’t provide any physical advantage against a fight with a bear. So the chance of any life-developing intelligence is extremely rare. Look at the earth
for example out of thousands of species that have formed on earth 99% of them have been extinct, out of the rest only one had enough resources to evolve the complex brain and grow intellectually. But take a squirrel for example no matter how smart he is he’ll never understand the functioning of any machinery or modern communication. For the squirrel, the forest is his home and everything that exists. The same would happen for any other life. Let’s dive further and say on any planet only one advanced civilization can occur, now assuming they are as cruel as we are they’ll devour all other life on their planet. So now we have only one intelligent life on any planet with life on it. Till now we have known that it takes an immense amount of resources and conditions for life to even begin and then further grow, but if it existed how would it be?
On a broader scale, we can classify any civilization into 3 groups type 1, type 2, and type 3.
A type 1 civilization or planetary civilization would be one that came to use all the resources on its planet effectively and would build space crafts to further explore the solar system. We are on the verge of becoming a complete planetary civilization.
2nd is the type 2 or stellar civilization would beone that can consume all the resources of its solar system and would basically control everything there. They could build something as advanced as a Dyson sphere around their star to consume all its energy and build very advanced weapons, modes of communication, and travel.
Last but not least is type 3 civilization or galactic civilization , it is one that will be so advanced that they can basically colonize an entire galaxy and consume all the possible resources. Such a civilization would be like God to us. They can even control on which planet life would flourish, or which planet to allow flourishing and which to destroy.
But they can also collect so much energy and build advanced computers to upload all their consciousness into a virtual world, a world of never-ending bliss and ecstasy. And So we’ll never know about them as they have kinda left this universe.
Now we can also imagine that we’re also a civilization like this and are now living in a virtual universe.
At the end of the day it doesn’t matter how much we argue, debate and think about it, a possibility always remains. We can only think and search for ways of finding life in different parts of the universe. So the quest must never stop because life is a gift.