In the 2019 paper, “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way”, astrobiologist and lead author, Charles H. Lineweaver of Australia National University, summarized: “We modeled the evolution of the Milky Way 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, sufficient time for biological evolution and an environment free of life-extinguishing supernovae. We identified the ‘Galactic habitable zone’ (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago.”
Green Region of the GHZ Widens
“8.5 billion ago, the green region was very narrow and positioned at 8.5 kpc from the Galactic Center … go forward 2 billion years (2 Gyrs) to 6.5 Gyrs ago and the green region now spans 6 to 11 kpc –i.e., the GHZ has widened with time”, explains co-author Brad K. Gibson, Director, E.A. Milne Center for Astrophysics at the University of Hull in an email to The Daily Galaxy.
75% of the stars in the GHZ are older than the Sun
“This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun,” the paper notes.
If advanced, space-faring alien life exists 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.
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. As of 1 November 2021, there are 4,864 confirmed exoplanets in 3,595 planetary systems, with 803 systems having more than one planet.
Deadly Gamma Ray bursts occur roughly once every hundred million years.
GHZ –”Just Right for Life”
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 in the Milky Way, in both time and space.
Centers on a Ring about 26,000 Light Years from the Galactic Center
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, 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 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.
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. There are now fewer life-threatening supernovae in the older central regions of the Milky Way, pushing the inner edge of the GHZ further inward. Meanwhile, star formation continues within the disk of the Milky Way, increasing the net metallicity of stars and thereby pushing the outer edge of the GHZ further outward. But in the outer rims of the Milky Way 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.
Deadly Gamma Ray Burst –More Common in the Infant Milky Way
Explosions far more powerful, but far less common, than supernovae 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.
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.
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 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, Lineweaver and his colleagues at Australian National University have concluded that the GHZ contains no more than 10 percent of all the stars ever formed in the Milky Way.
The Last Word
“Broadly speaking, our Galactic Habitable Zone framework has stood the test of time,” concludes Brad Gibson. “Without a doubt, significant numbers of valid adjustments have been made to the basic concept, ranging from the inclusion radial and azimuthal motions of stars from their birth locations, through to incorporating the effects of radioactive isotopes to better characterize the types of terrestrial likely to form; but again, having said that, the essence of the GHZ remains a valid construct and a powerful tool for engaging the public in these important discussions.”
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Science, Brad K. Gibson, and John Gribbin, Alone in the Universe: Why Our Planet Is Unique. Pages 74 -75 Kindle Edition.
Image credits: The image of the Milky Way at the top of the page was released to mark the completion of the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The APEX telescope in Chile mapped the full area of the Galactic Plane visible from the southern hemisphere at submillimetre wavelengths — between infrared light and radio waves. ESO/APEX/ATLASGAL consortium/NASA/GLIMPSE consortium/ESA/Planck. GHZ Green Zone, courtesy of NASA.
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