If advanced, space-faring alien life exist anywhere in the Milky Way, odds are they will live in the Galactic Habitable Zone (GHZ). And if they want to explore the Milky Way searching for other life-bearing planets, similar to Earth, they would only have to explore the GHZ, not the whole of the Galaxy.
Astronomers have modeled the evolution of the Milky Way Galaxy over the past two decades to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets similar to Earth, sufficient time for biological evolution, and an environment free of life-extinguishing supernovae or gamma ray bursts.
In 2018, within a field of view that represents 1/400th of the Milky Way but in the heart of the GHZ, researchers scanned 14 worlds From the Kepler Mission for “technosignatures”, evidence of advanced civilizations. Between Kepler‘s first and second mission (K2), a total of 5,118 candidates and 2,538 confirmed exoplanets have been discovered within our galaxy alone. As of February 1st, 2018, a grand total of 3,728 exoplanets have been confirmed in 2,794 star systems, with 622 having more than one planet all within the GHZ.
While the exact boundaries of the GHZ are not clear, the inner regions of the Milky Way have plenty of metals but are hazardous for life, while the outer regions of the thin disc are safer, but metal-poor and less likely to contain Earth-like planets.
In between, there is the GHZ, that is just right for life, with our Solar System sitting near its center. We live in a particularly favorable location in the Milky Way for life. We also live at a special moment in the Milky Way for life. Until about 5 billion years ago, even apart from the shortage of metals needed for like’s origin, the activity of star birth and the supernovas would have made life hazardous.
It has taken almost all of 5 billion years for intelligence to evolve on Earth, and if that is typical, we may be one of the first intelligent civilizations in our Galaxy. There is indeed something that appears to be unique about our place the Milky Way, in both time and space.
The GHZ is a region that widens with time and is composed of stars that formed between 8 and 4 billion years ago, centered on a ring about 26,000 light years from the galactic center, began to emerge about 8 billion years ago, and at present extends from about 23,000 light years to about 29,000 light years. The GHZ yields an age distribution for the complex life that may inhabit our Galaxy, where 75% of the stars are older than the Sun.
The concept of idea the ‘Galactic Habitable Zone’ (GHZ), the region around the thin disc where planetary systems like our Solar System and planets like the Earth are likely to be found, centered on a ring about 26,000 light years from the galactic center reflecting the close link between the metallicity of a star and the likelihood that it has planets.
The zone began to emerge about 8 billion years ago, and at present extends from about 23,000 light years to about 29,000 light years. The critical take away, as far as the prospect of other intelligent beings existing in our Galaxy is concerned, is that this Galactic Habitable Zone slowly gets larger as time passes. But farther from the center there is relatively less gas and less star formation, so even today the metallicity of stars in the outer regions is not increasing as rapidly as in the inner regions.
The position of the GHZ depends on the kind and frequency of hazards that a potential habitat for life encounters on its journey through the Milky Way –in the case of Earth, every 230-250 million years. The peak of supernova activity among the stars of the Milky Way seems to be at about two thirds of the Sun’s distance from the center of the Galaxy where our central supermassive black hole lurks, quiet today, but showing signs of having been active in the recent past.
Explosions far more powerful, but far less common, than super-novas are revealed by the intense but short-lived gamma ray bursts that they emit, and are known as gamma ray bursters–the most powerful explosion that occurs in the Universe today, emitting more energy in a few seconds than the Sun will emit in its lifetime.
Statistics suggest that such a burst occurs in a galaxy like the Milky Way roughly once every hundred million years. Even if one occurred in a distant region of the Milky Way, the blast of sterilizing radiation could be devastating for life on Earth (or any other inhabited planet in the Galaxy) and destructive to the ozone layer. It’s possible that a strong gamma ray burst could sterilize an entire galaxy.
It is possible that one reason why it has taken so long for intelligent life to emerge on Earth and perhaps elsewhere in the Galaxy is that gamma ray bursters were more common in the infant Milky Way, and it is only in the past few billion years that life has had a chance to evolve to the point of producing at least one technological civilization. Viewing all of the evidence, astrophysicist Charles Lineweaver and his colleagues at Australian National University have concluded that the GHZ contains no more than 10 per cent of all the stars ever formed in the Milky Way.
The first accurate 3D map of our galaxy shown at the top of the page reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)
The Daily Galaxy, Max Goldberg, via Science and Gribbin, John. Alone in the Universe: Why Our Planet Is Unique. Pages 74 -75 Kindle Edition.