ตอนที่ 43 : Goldilocks Zone

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เขตอาศัยได้ (อังกฤษhabitable zone) ในทางดาราศาสตร์ หมายถึง ย่านหนึ่งในอวกาศที่ซึ่งดาวเคราะห์คล้ายโลกสามารถดำรงน้ำในสถานะของเหลวได้บนพื้นผิว และสามารถมีสิ่งมีชีวิตคล้ายกับสิ่งมีชีวิตบนโลก เขตอาศัยได้เป็นจุดตัดกันระหว่างสองเขตที่ต่างก็เอื้อต่อการให้กำเนิดชีวิต คือหนึ่ง ภายในระบบดาวเคราะห์ และสอง คือภายในดาราจักร ดาวเคราะห์และดวงจันทร์บริวารของมันที่อยู่ในเขตนี้มีโอกาสมากที่จะเป็นแหล่งอยู่อาศัยของมนุษย์ได้ จึงมีความเป็นไปได้ที่จะมีสิ่งมีชีวิตต่างดาวลักษณะคล้ายคลึงกับเราอยู่ที่นั่น

จะต้องไม่สับสนปนเประหว่าง เขตอาศัยได้ กับ ดาวเคราะห์ที่อยู่อาศัยได้ (planetary habitability) ซึ่งในข้อหลังนี้จะคำนึงถึงเงื่อนไขต่างๆ ของดาวเคราะห์ที่จำเป็นในการดำรงชีวิตของสิ่งมีชีวิตที่มีคาร์บอนเป็นพื้นฐาน (carbon-based life) ขณะที่ เขตอาศัยได้ คำนึงถึงเงื่อนไขเกี่ยวกับดาวฤกษ์ซึ่งส่งอิทธิพลต่อการดำรงชีวิตของสิ่งมีชีวิตที่มีคาร์บอนเป็นพื้นฐาน


Goldilocks Zone

The Goldilocks Zone or Habitable Zone or Life Zone is the relatively narrow range of distance from a star within which an orbiting planet (or in some cases a planet's moons) may be able to sustain life, at least the carbon-based forms we know and love, because the temperature is right for water to remain liquid.  Water is a common enough material in the universe and acts as a very flexible biochemical solvent. The phrase Goldilocks Zone is a reference to the story of Goldilocks and the Three Bears, since, like the porridge which Goldilocks chose to eat in the bears' house, conditions in the Goldilocks Zone are neither "too hot" nor "too cold", but are "just right".

The definition of a Goldilocks Zone has expanded to include moons such as Enceladus that exist within a zone where warming occurs through tidal friction by their planet (Saturn, in the case of Enceladus), as opposed to external heating by a star. 

Other forms of life unknown to scientists today may have evolved elsewhere in the universe to thrive in conditions that are not Goldilocks at all.

Earth

Earth is the only Goldilocks planet in our own solar system. We would be unable to survive on the nearest neighbouring planets, Venus (too hot) or Mars (too cold). Non-Goldilocks temperatures also correspond with differences in the physical geology and chemical composition of the environment.

Earth's almost circular orbit keeps us firmly within the Goldilocks Zone at all times. A planet in our location but orbiting the Sun in a much wider ellipse would dip in and out of the Goldilocks Zone during its orbit, and so possibly could not sustain life. Some scientists speculate that life may be able to survive in planets with elliptical orbits that take them out of the habitable zone from time to time. All that may be required is liquid water at some point in a planet's orbit. Life could hibernate during cold conditions or could shelter in watery places during hot conditions. 

Significance of Goldilocks planets

Identifying other habitable or inhabited planets is a major preoccupation in space exploration and colonisation, as well as a perennial theme in science fiction. The discovery of Goldilocks Zone planets attracts interest from the media and the public as well as the scientific community. Recent examples include planets in the Gliese 581 solar system, approximately 20 light years from Earth, including Gliese 581 c, d, and g. Gliese 581 g, discovered in 2010, is thought to be the planet most closely resembling Earth of any which have been discovered yet. 22% of stars like our sun may have Goldilocks planets similar to earth according to data from the Kepler telescope. 

The most compelling reason for this fascination with Goldilocks planets is the possibility that they could already support extraterrestrial life, which is arguably the Holy Grail of space exploration. If observing atmospheric conditions and life signs on such distant planets becomes possible, we could discover species unknown on Earth, and studying the development of life elsewhere could provide new insights into such aspects of biology as the origin of life and the degree of chance involved in evolution.

Many other factors are also required to make a planet suitable for life. Atmospheric composition is one, as the greenhouse effect can either raise the temperature enough to keep surface water liquid, or make it too hot for liquid water, and the mass of the planet is also important for maintaining this atmosphere. Planets that are too small do not have sufficient gravity to hold on to their atmospheres in the face of the onslaught of the solar wind.

The range of the Goldilocks Zone also changes throughout a star's lifespan, slowly edging out further away from the star as it ages. Therefore a planet needs to remain inside the zone as it drifts outward. This favours planets which form just on the edge of the zone early in a solar system's life, as the zone will then spread outwards slowly as the star ages and heats up.

Creationism and the Goldilocks Zone

The narrow range of the Goldilocks Zone, and shortage of planets comparable to Earth, is a common theme in modern creationism, in both its young Earth and old Earth variations. The most common argument, known as the argument from fine tuning, is that the Goldilocks Zone's conditions, and those of any habitable planet within it, are so specific that they could only have been created by intelligent design and not by random chance.

It is wise to remember that improbable things happen, and that with the vast number of stars in the universe, the possibility of some being orbited by planets in Goldilocks Zone conditions, and perhaps already supporting life, is not insignificant. 

Essentially, the fine tuning argument is an example of an argument from incredulity. It also emphasizes traditionally religious themes such as Earth's perceived special status as the centerpiece of God's creation. If other inhabited planets are discovered, this will tend to refute these notions, although of course creationists will insist that the space aliens are also part of God's master plan.

One last inconsistency, is that it contradicts a previous creationist idea that the entire universe was created by a wise god to be fine-tuned FOR the human race, but it seems the only location nearby Earth on which a human can survive is... none.

References

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