A clean hut house and protected from rain may be a habitable home for some people. But, not necessarily habitation is habitable. To be categorized as habitable, the house must meet the healthy living standards of the Ministry of Health.
Another example, an airplane. Airplanes must meet safety requirements whose criteria are already established nationally and globally to be categorized as flying.
For the planet, we will use the word habitually to declare a planet already qualified to be inhabited by a living being.
Terms here is not a beautiful planet or clean, but a minimum requirement that the planet can support the growth and development of life. Keep in mind, the planet is habitually meaningless on the planet is definitely or has no life.
What is a habitable planet like?
How is the Planet Categorized As Lonely?
The search for a planet is habitable starting from an idea that has existed for centuries. The human dream to know if there is another life outside the Earth. The concept of the existence of another life beyond Earth has been raised since 450 BC by Epicurius, a Greek philosopher. He posits the concept of the existence of another world outside the Earth, which resembles or is not similar to the planet we live in. This view then developed into the modern era, and the search for another life or another Earth was done. Before reaching the era of exoplanet explosion as now, there are so many failures that astronomers encounter when looking for planets in other stars. This is one of them caused by the limitations of the instrument.
History records the first planet in another star discovered in 1992 in a pulsar or pulsating star. In those days, speculation developed whether a planet might survive and still exist in the neutron star or star who had ended his life? It turns out that the pulsar can indeed have a planet.
The exoplanet era begins when planet Pegasi 51b is found in a Sun-like Pegasi 51 star. As of November 2016, 3540 exoplanets and 4706 unexplored planetary candidates have been found. Among them, there is 44 candidates planet habitable.
So how can we know a planet is habitable or not?
When we talk about another planet that has life, of course, the idea of life will be similar to Earth. Or maybe someone has an idea like the science fiction world that the aliens have different shapes?
Other lives can be like on Earth or even very different from Earth. But, if life is so different from that on Earth how do we recognize it? It would be like looking for a needle in between a haystack.
Do not believe? The universe has 2 trillion galaxies. One galaxy of its contents can be billions of stars. And every star, if you have a planet must have at least 1 planet. Well, how can we find a different life from Earth? To find similar ones is not easy.
Thus, astronomers narrow the search space. The definition of life sought is the known life of life on Earth. Think of this as a basic rule when looking for a planet that has another life. In addition, the life sought is also not exactly like the Earth that has evolved and has intelligent life. A planet with life that means there are microorganisms that can emerge and develop.
The next problem, how to recognize a planet that has a life like on Earth from a great distance away?
The planet is much smaller than the star. From Earth, the star just looks like a spot of light in the sky. To be able to find planets in other stars alone astronomers use indirect detection methods that give an indication of the existence of the planet from the behavior of the star. So how to recognize a planet that has a life like on Earth?
Another requirement is required. This time, astronomers are looking for key life-supporting components just like on Earth. There are many, but what is the main thing?
Water! In the liquid form!
On Earth, there is no life that does not need water. Humans, animals, and plants do not live from ice or water vapor. What is needed is water in the form of liquid and in sufficient quantities to sustain life. If we look at Earth, the most striking is the ocean that fills 2/3 Earth.
Planets that have water in the liquid form on the surface so the main requirement to look for a planet that has the potential to have a life. Potentially life because water is not the only condition. That's just the initial requirement.
How to recognize it? What is a certain Earth-like rocky planet? But what could be the benchmark of a planet potentially having an ocean on its surface?
Let's see Earth first. Earth has oceans. That is, Earth can maintain water in the form of liquid. Earth is not a dry rocky planet and not an ice planet. This is related to the appropriate temperature to maintain water in the liquid form. Related also to the Earth's location or the Earth's distance from the Sun. The Earth is at a fitting distance so that the heat received from the Sun does not evaporate water from the surface and is not so cold that the water freezes to ice.
The area around the warm stars and can make the planet to defend the water in this liquid form we call zones laik. If there is a planet inside the zone is habitable then the planet has the potential to retain water in the liquid form. And the planet also has the potential to sustain life as we know it.
But life does not only rely on water, although water is the main factor. There are various other components that must also be taken into account such as the parameters of planets and stars where the habitable planet resides. This is all important not only for the beginning of life formation but also the evolution of life on the planet itself.
Matching Parent Star
Initially, astronomers are looking for such habitable planets on similar stars of the Sun or the yellow dwarf star. This is because life is known to exist on Earth and similar stars of the Sun are considered ideal to sustain life.
But why not all ideal stars as the parent planet is habitable?
Sun-like stars are considered ideal for the habitable planet, as it has a lifespan of several billion years. The star's lifespan for several billion years this year will be able to support the evolution of life on the planet.
For stars that are much hotter than the Sun, it lives only less than 1 billion years. This is certainly not enough for life to evolve.
Cold stars like red dwarfs are more likely to have planets habitable. Observations have proven that the planet is habitable enough in the red dwarf star. The longer lifespan also provides an opportunity for life to evolve. However, in a red dwarf, the habitable zone is very close to the star. As a result, the planet will experience a gravitational locking and only one side is always faced with the star. That means there is one side that will never experience starlight and this will be difficult for the evolution of life. One side will experience tremendous heat while the other side will experience endless cold. In addition, red dwarfs also occasionally release explosions that can be harmful to life on planets that are very close to the star.
Planet's atmosphere
Once exoplanets are found in the star-habitable zone, astronomers will investigate further whether the planets meet other requirements. Like the composition of the planet whether the planet is a rocky planet like the Earth or a metallic planet or gas planet like Jupiter and Saturn or ice planets like Uranus and Neptune. The composition of the planet is also related to the distance from the parent star. Planets of gases and ice can only be found in rarely distant from stars or outside zones of habitability.
Another condition is the planet's atmosphere. Astronomers must know whether the planet is habitable whether it has the atmosphere or not. A small mass planet will have a small gravity. As a result, the planet cannot maintain the existence of the atmosphere. The molecules that exist will easily escape from the planet when swept by a stellar wind or crash with other objects. Life can not grow and evolve on planets that do not have the atmosphere or the atmosphere too thin. For one thing, if life can grow, it will not be protected from harm. Thicker-mass planets such as Venus also cannot sustain life because the atmosphere is too thick causing a sustainable greenhouse effect and the planet's surface gets very hot.
Well, when tracing the planet's atmosphere is habitable, astronomers will look for an atmosphere that has a basic life content such as carbon dioxide, oxygen, nitrogen, water vapor, ammonia, and methane.
The habitable planet has been found
Until now, astronomers have yet to find another planet that has a life or another planet that is completely inhabited by humans.
What is found is a planet that has the potential to have the ocean in the area of habitable stars. Thus, it is expected that the planet can own or maintain a life like on Earth. Until now, 44 planets have been found habitable, including Proxima Centauri b that surrounds the neighboring Star of Proxima Centauri and Kepler-186f planets.
Reference :
https://www.sciencedaily.com/terms/planetary_habitability.htm
https://www.nasa.gov/mission_pages/kepler/news/kepler20130717.html
https://www.space.com/6240-habitable-planets-types-proposed.html
https://blogs.scientificamerican.com/life-unbounded/the-habitable-planets/
http://www.astronomy.com/news/2017/04/exoplanet-guide
http://www.slate.com/blogs/bad_astronomy/2015/10/06/_51_pegasi_b_the_first_exoplanet_discovered_orbiting_a_sun_like_star.html
https://www.scienceabc.com/nature/what-makes-a-planet-habitable.html
http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/habzone.htm
https://www.nasa.gov/feature/jpl/eso-discovers-earth-size-planet-in-habitable-zone-of-nearest-star
The article is so in depth. I didn’t know that a dwarf star can actually generate enough heat and light source for some planets to flourish. Such a magnificent article. Debates about habitable planets have becoming an important topic recently due to the over expanding astronomical knowledge. Thanks for writting this beautiful piece :)