Alien Life - Where are our eyes?
Life outside Earth
We have been thinking of life outside earth since time immemorial. And when it comes to the scientific search of extraterrestrial life, what and where are we searching for? The primary question - what, is thought worthy. We assume that the Life outside Earth is like the Life on Earth. We assume the life to be water-based and to share many such similarities. Why so? Life outside could be of any form. It really need not be water based. It really need not have the needs of the terrestrial life, say, oxygen. But, it is easier to look for life with this assumption, for, as the old joke goes, the old man who lost his keys at night will search for it under the streetlights. So, although we know that life on Earth could not be representative of life elsewhere, we take up the assumption while we scout for extraterrestrial life.
A similar approach is also usually taken while we decide where to hunt for life. We look for Sun-like stars and Earth-like orbits and temperatures, although we also agree that orange stars - stars slightly cooler and less luminous than our Sun, are potentially better for advanced life, because G-type yellow stars like our Sun are shorter-lived and less common in our galaxy, while orange dwarfs can burn steadily for tens of billions of years, allowing unlimited time for biological evolution to form life with infinite evolutionary experiments.
Consequently, exploring life outside Earth, we first look for Earth-like planets around Sun-like stars, which brings us to finding rocky planets the size of Earth. And we have found a lot of them! We assume the size to be comparable to Earth’s because our solar system has taught us that large gas-giants cannot host life.
Zooming out further, let’s have the galaxies in view and figure out the habitable kind of galaxies. While scientists believe dwarf galaxies are the least possible place for Life to happen, giant elliptical galaxies are strong candidates with lots of stars to host planets and at the same time, a low rate of star formation to avoid erasure of evolved life by supernovae. Our spiral Milky Way is in the range between these two extreme types of galaxies.
We look for low rate of star formation primarily because stars are ticking bombs and explode when it is time. These violent supernovae explosions wipe off life from nearby worlds. Furthermore, gamma ray bursts are common when stars are born and deter life. Thus, we pick out giant elliptical galaxies to be the right kind with the mass twice of the Milky Way and less than a tenth of stars waiting to explode.
These point out that life is more probable in a giant elliptical galaxy than our Milky Way. And we know for sure that there’s life in the Milky Way!
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