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Habitable zone
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Habitable zone
In astronomy and astrobiology, the habitable zone (HZ), also known as the circumstellar habitable zone (CHZ), or as the Goldilocks zone, is the range of orbits around a star within which a planetary surface could potentially support liquid water. Liquid water is considered by many scientists as necessary for a planet to be habitable. The range depends upon the brightness of the star interacting with a planet's atmosphere. At the inner edge the star's light, trapped by greenhouse gases in a planet's atmosphere, boils off the planet's water. At the outer edge light from the star is insufficient even with help from atmospheric gas and the planet's water freezes. Many other factors are added in various habitable zone models. The habitable zone has become a key tool in the search for habitable planets because discoveries of exoplanets yield approximate orbital radii.
The alternative name, Goldilocks zone, is a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".
Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites because planetary mass moons in the HZ might outnumber planets.
The classical habitable zone concept was defined only for planetary surfaces where habitat depends on stellar energy. Deep biospheres are known on Earth, but they would not be detectable on exoplanets. Other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.
The concept of habitable zone arose in geography in the late 19th century. Alexander Winchell discussed planetary habitability in 1883, using a definition closer to what is now called a habitable zone for complex life. Possibly the earliest use of the term habitable zone was in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". Hubertus Strughold's 1953 treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars used the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang, an American astrophysicist developed the modern concept of circumstellar habitable zones in a series of papers in the 1950s and 60s.
The concept of habitability was further developed in 1964 by Stephen H. Dole in a RAND Corporation study, Habitable Planets for Man, in which he discussed the determinants of planetary habitability for humans, eventually estimating the number of habitable planets in the Milky Way to be about 600 million and about 50 such planets within 100 ly. Dole subsequently co-authored a less technical version of the study in a book with science-fiction author Isaac Asimov which appeared in print at the height of the space race. The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.
In 1993 James Kasting and colleagues developed a model for habitable zones across stars with different brightness using a simple climate model. Refinements of the climate model and extension to smaller stars led to new zone range estimates and the introduction of "conservative" and "optimistic" habitable zone concepts.
Practical application of the habitable zone concept arrived with improvements in telescope technology. In 2009, the Kepler space telescope was launched specifically to detect exoplanets in habitable zones. By the end of the mission the data suggested that at least 20% and perhaps as many as 50% of the stars visible at night have Earth-sized planets in their habitable zone. In 2018, the Transiting Exoplanet Survey Satellite continued the search.
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Habitable zone
In astronomy and astrobiology, the habitable zone (HZ), also known as the circumstellar habitable zone (CHZ), or as the Goldilocks zone, is the range of orbits around a star within which a planetary surface could potentially support liquid water. Liquid water is considered by many scientists as necessary for a planet to be habitable. The range depends upon the brightness of the star interacting with a planet's atmosphere. At the inner edge the star's light, trapped by greenhouse gases in a planet's atmosphere, boils off the planet's water. At the outer edge light from the star is insufficient even with help from atmospheric gas and the planet's water freezes. Many other factors are added in various habitable zone models. The habitable zone has become a key tool in the search for habitable planets because discoveries of exoplanets yield approximate orbital radii.
The alternative name, Goldilocks zone, is a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".
Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets. Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. About 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The HZ is also of particular interest to the emerging field of habitability of natural satellites because planetary mass moons in the HZ might outnumber planets.
The classical habitable zone concept was defined only for planetary surfaces where habitat depends on stellar energy. Deep biospheres are known on Earth, but they would not be detectable on exoplanets. Other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.
The concept of habitable zone arose in geography in the late 19th century. Alexander Winchell discussed planetary habitability in 1883, using a definition closer to what is now called a habitable zone for complex life. Possibly the earliest use of the term habitable zone was in 1913, by Edward Maunder in his book "Are The Planets Inhabited?". Hubertus Strughold's 1953 treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars used the term "ecosphere" and referred to various "zones" in which life could emerge. In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same concept in further scientific detail. Both works stressed the importance of liquid water to life. Su-Shu Huang, an American astrophysicist developed the modern concept of circumstellar habitable zones in a series of papers in the 1950s and 60s.
The concept of habitability was further developed in 1964 by Stephen H. Dole in a RAND Corporation study, Habitable Planets for Man, in which he discussed the determinants of planetary habitability for humans, eventually estimating the number of habitable planets in the Milky Way to be about 600 million and about 50 such planets within 100 ly. Dole subsequently co-authored a less technical version of the study in a book with science-fiction author Isaac Asimov which appeared in print at the height of the space race. The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.
In 1993 James Kasting and colleagues developed a model for habitable zones across stars with different brightness using a simple climate model. Refinements of the climate model and extension to smaller stars led to new zone range estimates and the introduction of "conservative" and "optimistic" habitable zone concepts.
Practical application of the habitable zone concept arrived with improvements in telescope technology. In 2009, the Kepler space telescope was launched specifically to detect exoplanets in habitable zones. By the end of the mission the data suggested that at least 20% and perhaps as many as 50% of the stars visible at night have Earth-sized planets in their habitable zone. In 2018, the Transiting Exoplanet Survey Satellite continued the search.
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