Earth sits at the center of the Milky Way’s habitable zone (weekend feature)

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ESO's Milky Way Plane

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, summarizes: “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, sufficient heavy to form telluric planets, enough time for biological life. evolution and an environment free of life-extinguishing supernovae. We have identified the “Galactic Habitable Zone” (GHZ) as an annular region between 7 and 9 kiloparsecs from the galactic center that expands over time and is composed of stars that formed between 8 and 4 billion years ago. ‘years.

GHZ green region expands

“8.5 billion years ago the green region was very narrow and positioned 8.5 kpc from the galactic center…move forward 2 billion years ago (2 Gyrs) to 6.5 Gyrs and the green region is now spans 6-11 kpc – i.e. the GHZ has expanded over time,” says co-author Brad K. Gibsondirector, EA Milne Center for Astrophysics at the University of Hull in an email to The Everyday Galaxy.

75% of GHZ stars are older than the Sun

“This GHZ gives an age distribution of 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-age extraterrestrial life exists anywhere in the Milky Way, chances are it will live in the Galactic Habitable Zone (GHZ). And if they want to explore the Milky Way in search of other life-bearing planets, similar to Earth, they would only have to explore the GHZ, not the whole Galaxy.

In 2018, in a field of view that represents 1/400th of the Milky Way but in the heart of the GHZ, researchers scanned 14 worlds of the Kepler mission in search of “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 in our galaxy alone. As of February 1, 2018, a grand total of 3,728 exoplanets have been confirmed in 2,794 star systems, 622 of which have more than one planet in the GHZ. As of November 1, 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 about once every hundred million years.

GHZ – “Just for life”

Although the exact GHZ boundaries are unclear, the inner regions of the Milky Way contain lots of metals but are dangerous to life, while the outer regions of the thin disk are safer, but metal-poor and less susceptible. to contain earth. like planets.

In between is the GHZ, which is perfect for life, with our solar system sitting near its center. We live in a particularly favorable place for life in the Milky Way. We also live at a special time in the Milky Way for life. Until about 5 billion years ago, even apart from the shortage of metals needed for their origin, star birth activity and supernovae would have made life dangerous.

Unfathomable abodes of life? –Aquatic worlds of the Milky Way

It took almost 5 billion years for intelligence to evolve on Earth, and if that’s typical, we could be one of the first intelligent civilizations in our Galaxy. There is indeed something that seems unique about our place in the Milky Way, both in time and space.

Centered on a ring about 26,000 light-years from the galactic center

The GHZ is a region that expands over time and is made up 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 currently extends from about 23,000 light-years to about 29,000 light-years. The GHZ gives an age distribution of 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 disk where planetary systems like our solar system and planets like Earth are likely to be found, centered on a ring about 26,000 years- light from the galactic center reflecting the close connection between a star’s metallicity and the likelihood of it having planets.

The area began to emerge about 8 billion years ago.

The area began to emerge about 8 billion years ago and currently extends from about 23,000 light-years to about 29,000 light-years. The critical conclusion, with respect to the perspective of other intelligent beings existing in our Galaxy, is that this Galactic Habitable Zone is slowly expanding over time. There are now fewer life-threatening supernovae in the former central regions of the Milky Way, pushing the inner edge of the GHZ further inland. Meanwhile, star formation continues in the Milky Way’s disk, increasing the net metallicity of stars and thus pushing the outer edge of the GHZ further outward. But in the outer edges 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. .

Signals from extinct extraterrestrial civilizations – “Echo throughout the Milky Way”

The position of the GHZ depends on the type and frequency of hazards a potential habitat for life encounters on its journey through the Milky Way – in Earth’s case, every 230 to 250 million years. The peak of supernova activity among stars in the Milky Way appears to be about two-thirds the distance from the Sun to the center of the Galaxy where our central supermassive black hole, quiet today, but showing signs of having been active lately. past.

Deadly gamma ray burst – More common in the infant milky way

Much more powerful, but far less common, explosions than supernovae are revealed by the intense but short-lived gamma-ray bursts they emit, and are known as gamma-ray bursts – the most powerful explosion that occurs in the Universe today, emitting more energy in seconds than the Sun will emit in its lifetime.

Only in the past billion years has life had the chance to evolve to the point of producing at least one technological civilization.

Statistics suggest that such an explosion occurs in a galaxy like the Milky Way about once every hundred million years. Even if it were to occur in a distant region of the Milky Way, the burst of sterilizing radiation could be devastating to life on Earth (or any other inhabited planet in the Galaxy) and destructive to the ozone layer.

It’s possible that one of the reasons it took so long for intelligent life to emerge on Earth and possibly elsewhere in the Galaxy is that gamma-ray bursts were more common in the infant Milky Way, and that’s not the case. is that over the past billion years life has had the chance to evolve to the point of producing at least one technological civilization. Seeing all the evidence, line weaver and his colleagues at the Australian National University concluded that the GHZ contains no more than 10% of all stars ever formed in the Milky Way.

The last word

“Overall, our Galactic Habitable Zone framework has stood the test of time,” concludes Brad Gibson. “Undoubtedly, a significant number of worthwhile adjustments have been made to the basic concept, ranging from the inclusion of the radial and azimuth motions of stars from their birthplace, to the incorporation of the effects of isotopes radioactive to better characterize the types of earth elements likely to form; but again, that said, the essence of the GHZ remains a valid construct and a powerful tool for engaging the public in these important discussions.

Maxwell Moeastrophysicist, NASA Einstein Fellow, University of Arizona via Science, Brad K. Gibsonand 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 has mapped the entire area of ​​the galactic plane visible from the southern hemisphere at sub-millimeter wavelengths – between infrared light and radio waves. ESO/APEX/ATLASGAL/NASA/GLIMPSE/ESA/Planck Consortium. GHZ Green Zone, courtesy of NASA.

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