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TOPIC: Alien life


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Fermi paradox
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Title: Is there anybody out there?
Author: Luis A. Anchordoqui, Susanna Weber, Jorge F. Soriano

The Fermi paradox is the discrepancy between the strong likelihood of alien intelligent life emerging (under a wide variety of assumptions) and the absence of any visible evidence for such emergence. We use this intriguing unlikeness to derive an upper limit on the fraction of living intelligent species that develop communication technology \langle \xi_{biotec}
angle. \langle \cdots
angle indicates average over all the multiple manners civilizations can arise, grow, and develop such technology, starting at any time since the formation of our Galaxy in any location inside it. Following Drake, we factorize \langle \xi_{biotec}
angle as the product of the fractions in which: (i) life arises, (ii) intelligence develops, and (iii) communication technology is developed. In this approximation, the number of communicating intelligent civilizations that exist in the Galaxy at any given time is found to be N = \langle \zeta_{astro}
angle \langle \xi_{biotec}
angle L_\tau, where \langle \zeta_{astro}
angle is the average production rate of potentially habitable planets with a long-lasting (~ 4 Gyr) ecoshell and L_\tau is the length of time that a typical civilization communicates. We estimate the production rate of exoplanets in the habitable zone and using recent determinations of the rate of gamma-ray bursts (GRBs) and their luminosity function, we calculate the probability that a life-threatening (lethal) GRB could make a planet inhospitable to life, yielding \langle \zeta_{astro}
angle ~ 2 x 10^{-3}. Our current measurement of N =0 then implies \langle \zeta_{biotec}
angle < 5 x 10^{-3} at the 95% C.L., where we have taken L_\tau > 0.3 Myr such that c L_\tau >> propagation distances of Galactic scales (~ 10 kpc), ensuring that any advanced civilization living in the Milky Way would be able to communicate with us.

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RE: Alien life
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NASA Scientists to Discuss Search for Habitable Planets, Signs of Life off Earth

NASA scientists from across the agency will present their latest findings and perspectives on topics ranging from the origins and evolution of life on Earth to the search for habitable environments and life in our solar system and beyond during the 2017 Astrobiology Science Conference, April 24-28 in Mesa, Arizona
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Star clumps harbour 'sweet spot' in search for alien life

Ancient, tightly packed clumps of stars found at the fringe of the Milky Way are a good bet in the search for extra terrestrial intelligence (Seti), research suggests.
At an average age of 10 billion years (much wrinklier than the Sun, at four billion), globular clusters don't have many young stars, rich in the metallic elements needed to build planets.
But Dr Di Stefano, speaking in Florida at the 227th meeting of the American Astronomical Society, pointed out that recent discoveries had placed exoplanets - especially small, rocky ones like Earth - around stars much less metal-rich than our Sun.

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Title: On the visible size and geometry of aggressively expanding civilizations at cosmological distances
Author: S. Jay Olson

If a subset of advanced civilizations in the universe choose to rapidly expand into unoccupied space, these civilizations would have the opportunity to grow to a cosmological scale over the course of billions of years. If such life also makes observable changes to the galaxies they inhabit, then it is possible that vast domains of life-saturated galaxies could be visible from the Earth. Here, we describe the shape and angular size of these domains as viewed from the Earth, and calculate median visible sizes for a variety of scenarios. We also calculate the total fraction of the sky that should be covered by at least one domain. In each of the 27 scenarios we examine, the median angular size of the nearest domain is within an order of magnitude of a percent of the whole celestial sphere. Observing such a domain would likely require an analysis of galaxies on the order of a Gly from the Earth.

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Good night, sleep tight: advanced alien civilisations rare or absent in the local Universe.

Sensitive new telescopes now permit astronomers to detect the waste heat that is expected to be a signature of advanced alien civilisations that can harness enormous energies on the scale of the stellar output of their own galaxy. Professor Michael Garrett (ASTRON General & Scientific Director) has used radio observations of candidate galaxies to show that such advanced civilisations are very rare or entirely absent from the local Universe.
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Title: The Age Distribution of Potential Intelligent Life in the Milky Way
Author: Daniel Legassick

We investigated the habitability of the Milky Way, making use of recent observational analysis on the prevalence of Earth-sized planets, in order to estimate where and when potentially habitable star systems may have formed over the course of the Galaxy's history. We were then able to estimate the age distribution of potential intelligent life in our Galaxy using our own evolution and the age of the Sun as a proxy. To do this we created a galactic chemical evolution model and applied the following habitability constraints to the Sun-like (G-type) stars formed in our model: an environment free from life-extinguishing supernovae, a high enough metallicity for Earth-sized planet formation and sufficient time for the evolution of complex life. We determined a galactic habitable zone as the region containing all the potentially habitable star systems in our model. Our galactic habitable zone contains stars formed between 11 and 3.8 billion years ago at radial distances of between 7 and 14 kiloparsecs. We found that most potentially habitable star systems are much older than the Sun and located farther from the galactic centre. By comparing the ages of these systems we estimated that 77% of potentially habitable star systems are on average 3.13 billion years older than the Sun. This suggests that any intelligent life in the Galaxy is likely to be incredibly more advanced than we are assuming that they have evolved under similar timescales than we have. Implications and limitations of our study are discussed.

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The Search for Life in the Solar System and Beyond 

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Colourful life-form catalogue helps discern if we're alone

While looking for life on planets beyond our own solar system, a group of international scientists has created a colourful catalogue containing reflection signatures of Earth life forms that might be found on planet surfaces throughout the cosmic hinterlands. The new database and research, published in the March 16 Proceedings of the National Academy of Sciences (PNAS), gives humans a better chance to learn if we are not alone.
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Posts: 131433
Date:
Aliens
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Sarah Rugheimer | Harvard Horizons Symposium

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RE: Alien life
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Title: Life on a tidally-locked planet
Author: Ashok K. Singal

A tidally-locked planet in its orbit around a star keeps the same face towards the star. This happens when the rotation period of the planet around its own axis becomes equal to its revolution period around the star. Many questions then arise. What gives rise to a tidal locking? Are there any tidally-locked planets somewhere? What will be the atmospheric system of a tidally-locked planet? Could life be sustained on a tidally-locked planet? What will be a sort of DAY in the life of a denizen of a tidally-locked planet? We dwell on these questions.

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