Saturday, January 28, 2017
Monday, January 23, 2017
"Goldilocks zone of metallicity" on a galactic scale
What does the night sky look like to a planet within the Galactic Bulge? Cool Cosmos describes it as,
The concept of Habitable Zones around stars has been studied for a couple of decades. Life similar to ours can only exist on planets that are a certain distance from their sun. This is due to the amount of energy from the sun that is received by the planet. Too much energy, the planet is too hot. Too little energy, the planet is too cold, hence the Goldilocks reference.
There's another type of Habitable Zone at the galactic scale which uses a somewhat different set of criteria. Solar systems which have planets that can support life must themselves be made from material that has a lot of elements that are heavier than Helium. In astronomy, elements heavier than Helium are often referred to as metals. Metal content of a star is called its metallicity. The danger is that is if a solar system is made from material that is too rich in metallicity, Earth-sized planets may not be able to exist due to the likelihood of much larger (heavier) worlds displacing those Earth-size planets. Hence, "Goldilock zone of metallicity" is the idea that certain regions of a galaxy may be too metal-rich and other regions may be metal-poor in order to allow for the presence of Earth-like worlds.[002]
Where can solar systems with habitable planets reside within the Milky Way? According to the study The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way, the inner bulge component, diffuse halo component, and a thick disk component of our Milky Way Galaxy would not likely allow for Earth-size planets to exist within the right timeframe.[002] So, the Habitable Zone of our Milky Way Galaxy isn't even really based on distance from the galactic core. It's a somewhat washer-shape region in between all the places that Earth-sized planets cannot exist within solar systems.
Knowing our Milky Way's Habitable Zone helps us in the search for life on other worlds. We can focus more efforts on this space. This isn't to say that this is the only space where life can and does reside. The Galactic Habitable Zone is just our safest bet for finding evidence of life.
Primary reference:
C. H. Lineweaver,Y. Fenner, B. K. Gibson, Science 303:59–62, DOI: 10.1126/science.1092322, The galactic habitable zone and the age distribution of complex life in the Milky Way
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Stars in the Galactic Center are so concentrated that they typically are only a few light weeks away from each other. In contrast, our local neighborhood of stars are separated from one another by light years. If we found ourselves on a planet near the Galactic Center, our nighttime sky would light up in a blazing display every night, filled with stars as bright as the planet Venus looks to us.[001]However, would there be a habitable planet from which to see this sight? Is it possible to have life-supporting planets near the Galaxy's center?
The concept of Habitable Zones around stars has been studied for a couple of decades. Life similar to ours can only exist on planets that are a certain distance from their sun. This is due to the amount of energy from the sun that is received by the planet. Too much energy, the planet is too hot. Too little energy, the planet is too cold, hence the Goldilocks reference.
There's another type of Habitable Zone at the galactic scale which uses a somewhat different set of criteria. Solar systems which have planets that can support life must themselves be made from material that has a lot of elements that are heavier than Helium. In astronomy, elements heavier than Helium are often referred to as metals. Metal content of a star is called its metallicity. The danger is that is if a solar system is made from material that is too rich in metallicity, Earth-sized planets may not be able to exist due to the likelihood of much larger (heavier) worlds displacing those Earth-size planets. Hence, "Goldilock zone of metallicity" is the idea that certain regions of a galaxy may be too metal-rich and other regions may be metal-poor in order to allow for the presence of Earth-like worlds.[002]
It's not just the metals
Metallicity is not the only factor, however.Early intense star formation toward the inner Galaxy provided the heavy elements necessary for life, but the supernova frequency remained dangerously high there for several billion years.[002]If a solar system is too close to the galactic core, the intense supernova frequency in a young galaxy might've keep many worlds from supporting life. This is because they would have experienced numerous blast waves, cosmic rays, gamma rays and x-rays that are fatal to lifeforms.[002] As the collective of solar systems age and die, they would have contributed to increasing metallicity. This means, the right conditions for life on Earth-like planets may have never happened near the galactic core. Stars that are too close to the galactic core never had and never will have the right conditions to support Earth-like worlds with Earth-like life.
Where can solar systems with habitable planets reside within the Milky Way? According to the study The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way, the inner bulge component, diffuse halo component, and a thick disk component of our Milky Way Galaxy would not likely allow for Earth-size planets to exist within the right timeframe.[002] So, the Habitable Zone of our Milky Way Galaxy isn't even really based on distance from the galactic core. It's a somewhat washer-shape region in between all the places that Earth-sized planets cannot exist within solar systems.
Current Habitable Zone of Milky Way
Given all of these factors, the authors of the study The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way state,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.[002]
Primary reference:
C. H. Lineweaver,Y. Fenner, B. K. Gibson, Science 303:59–62, DOI: 10.1126/science.1092322, The galactic habitable zone and the age distribution of complex life in the Milky Way
Response:
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Article Series:
- Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
- Small stars may have stable Habitable Zones, but habitable planets might not be common there
- Habitable Planets around White Dwarfs
- Habitable Worlds Around Binary Star Systems might not match Sci-fi
- How many Earth-like planets are orbiting Sun-like stars?
- First round of life in the Universe might have been possible extremely early
- Factors a planet needs for suitability of life; perhaps
- "Goldilocks zone of metallicity" on a galactic scale
- Maybe we are the first
Sunday, January 22, 2017
Monday, January 16, 2017
Hypotheses, Theories, Laws and all that jazz
When I was in high school, I learned about hypotheses, theories, laws and principles. The problem is that I was taught that these were hierarchical. It took a long time for me to learn on my own that is not the case. They aren't necessarily stages in the understanding of our Universe. A single hypothesis does not become a theory. A theory does not eventually become a law. A law does not eventually become a principle. Furthermore, this list is missing the category of models. Each of these are different things that serve the Scientific Method in different capacities. Berkeley University of California states,
"Hypotheses, theories, and laws are rather like apples, oranges, and kumquats: one cannot grow into another, no matter how much fertilizer and water are offered. Hypotheses, theories, and laws are all scientific explanations that differ in breadth — not in level of support."[001a]
Hypothesis
The value of the hypothesis is that it allows us to simplify initial observations into a testable statement so that we can determine if the basis for the hypothesis is true or false. You can test to find supporting evidence for the hypothesis. You can also test to find refuting evidence which disproves the hypothesis.
Hypotheses are typically formed by one or a few persons who then conduct tests as experimenters to prove and disprove it in the pursuit to solve a problem. A hypothesis is often not a single point in research. Experimenters may test and reject several hypotheses before solving a problem. Disproving one particular hypothesis is just as important to Science is proving another hypothesis.[001b]
There is a subcategory of hypotheses called working hypothesis, which have some evidence to support them. As such, they are tentatively accepted as a basis for further study.
Theory
_shown_in_her_laboratory_in_1947.jpg "Barbara McClintock in her lab conducting genetic research")
Theories are testable and make falsifiable predictions. They allow for predictions to be made about a phenomenon, and they also explain the causes for the phenomenon.
Science historian Stephen Jay Gould said,
“...facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world′s data. Theories are structures of ideas that explain and interpret facts.”[002]
Theories are typically formed by consensus by many different people over a substantial period of time. Theories aren't just thought up by one individual and then magically accepted by everyone else. They are often heavily debated while they are being formed. This debate drives further hypotheses and experimentation that eventually helps develop the theory.
Theories are not absolute points. Once you have a theory, that doesn't mean the matter is settled. It just means that the evidence up to that point allows you to create a structure that provides both reliable predictions and explanations for phenomenon. Theories are often modified or replaced when better structures allow for better predictions and explanations.
A classic example of the process of debate to come to an eventual consensus is the Big Bang vs. Steady State theories debate, in which experimentation on both sides eventually lead to a much better understanding of our Universe.[003]
Theories are not absolute points. Once you have a theory, that doesn't mean the matter is settled. It just means that the evidence up to that point allows you to create a structure that provides both reliable predictions and explanations for phenomenon. Theories are often modified or replaced when better structures allow for better predictions and explanations.
A classic example of the process of debate to come to an eventual consensus is the Big Bang vs. Steady State theories debate, in which experimentation on both sides eventually lead to a much better understanding of our Universe.[003]
Model
A model seeks to represent empirical objects, phenomena, and physical processes in a logical and objective way. All models are in simulacra, that is, simplified reflections of reality which allow for useful approximations.
A model is evaluated by its consistency to empirical data. Inconsistency or irreproducibility of observations must force modification or rejection of a model. A model must be able to explain past observations, predict future observations and have refutability, just like theories from which the models are typically built.
Law
"Laws are descriptions — often mathematical descriptions — of natural phenomenon; for example, Newton’s Law of Gravity or Mendel’s Law of Independent Assortment. These laws simply describe the observation. Not how or why they work."[004]Laws are compact generalizations about data. As with other scientific elements, laws are not immutable. As more information is learned, laws can be changed.
It is important to note that laws can exist without theories. Sometimes laws exist for many years before theories explain their causes.[005]
Principle
Some persons have suggested that laws can typically be reduced down to precise math formulae, such as the Laws of Thermodynamics and Ohm's Law. Conversely, the suggestion is that principles are more general descriptions of the nature world.[006] Examples of such principles are Principle of Original Horizontality and Pareto Principle.
However, even this comparison is not an absolutely held distinction. For example, Heisenberg's Uncertainty Principle is highly mathematical in nature. Conversely, the Law of Superposition has no mathematical reduction. So even math provides no real distinction between the use of the words principle and law in Science.
Confusion
I guess the confusion about the relationships between laws, principles, hypotheses, theories and models is that it is not hierarchically ordered. It seems counter-intuitive that Science, being the mechanism that has brought so much order to our understanding of the world, is itself not similarly ordered. But, there's good reason for this. Science doesn't work in absolutes. Nothing is absolutely knowable. As such, everything we know is subject to be revised based on what we later learn. Having some sort of truth gradient would slow down the progress of learning since managing such grading would be an unnecessary distraction from the search for knowledge.
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