While the Kepler Space Telescope has discovered more than 3,000 exoplanets with 709 confirmed that revolve around a star, new findings from diverse fields are being brought to bear of the central questions of the 21st century: How common is life in the universe? Where can it survive, Will it leave a fossil record? How complex is it? The list below moves several key features of the Universe off the chart of likely places to search for life.
Astronomers estimate that there are 100 billion galaxies in the Universe. If you want to extrapolate those numbers, that means there are around 50,000,000,000,000,000,000 (50 quintillion) potentially habitable planets in the universe.
In stark contrast to the optimism underlying the Kepler Mission, the zones and regions of the known Universe listed below are the ones that astrobiologists have concluded have little or zero chance of supporting life as we know it. The listing of "dead zones" was compliled for Rare Earth -- Why Complex Life is Uncommon in the Universe by University of Washington scientists Peter D. Ward (Professor of Geological Sciences and Curator of Paleontology) and Donald Brownlee (Professor of Astronomy and member of the National Academy of Sciences).
Early Universe: The most distant known galaxies are too young to have enough metals for formation of Earth-size inner planets. Hazards include energetic quasar-like activity and frequent super-nova explosions.
Elliptical Galaxies: Stars are too metal-poor. Solar mass stars have evolved into giants that are too hot for life on inner planets. The image at the top of the page shows NGC 4631 (the Whale Galaxy or Caldwell 32) is a spiral galaxy, 30 million ly away in Canes Venatici. It has a central starburst, and is interacting with dwarf elliptical galaxy NGC 4627.
Globular Clusters: Although they contain millions of stars, the stars are too metal poor to have inner planets as large as Earth. Solar mass stars have evolved to gaints that are too hot for life on inner planets.
Small Galaxies: Most of the stars are too metal deficient.
Centers of Galaxies: Energetic star building and black-hole processes prevent development of complex life.
Edges of Galaxies: Most stars are too metal poor.
Planetary Systems with "Hot Jupiters": Inward spiral of the giant planets drives the inner planets into the central star.
Planetary Systems with Giant Planets in Eccentric Orbits: Unstable environments. Some planets lost to space.
Future Stars: Uranium, potassium, and thorium too rare to provide sufficent heat to drive plate tectonics.
The "Rare Earth" theory says that there is no hope of finding alien life in space because conditions on all other planets are too hostile, according to Howard Smith, a senior astrophysicist at Harvard. Smith made the claim after an analysis of the 500 planets discovered outside our Solar System that showed that extreme conditions are likely to be the norm, and that the hospitable conditions on Earth could be unique.
“We have found that most other planets and solar systems are wildly different from our own. They are very hostile to life as we know it,” he said.
Smith pointed to stars such as HD10180, which sparked great excitement when it was found to be orbited by a planet of similar size and appearance to Earth, but turned out to be superficial similarities. The planet lies less than two million miles from its sun, meaning it is roasting hot, stripped of its atmosphere and blasted by radiation. Many of the other planets discovered to date have highly elliptical orbits which cause huge variations in temperature which prevent water remaining liquid, thus making it impossible for life to develop.
Claiming that we're the only life in existence is a combination of ignorance and self-importance that should have a livejournal, not a scientific journal. The important work is getting ourselves out there and seeing who and/or what we can find.
Recent figures place the total number of stars in the Milky way at an astounding three trillion. Which leads to this question, given such a ginormous figure, what does it mean to be rare? Even if the Earth is a one in a million occurrence, that means there are still 3 million Earthlike planets in the Galaxy (assuming one Earthlike planet per star).
On the other hand, if the Earth is a one in a billion occurrence, then there are still 3,000 Earths in the Milky Way.
We also have to keep in mind that the 3 trillion stars only accounts for what exists right now. There have been well over a billion trillion stars in our past Universe. As Charles Lineweaver of the Planetary Science Institute and the Research School of Astronomy and Astrophysics at the Australian National University has noted, planets began forming in our Galaxy as long as 9 billion years ago. We are relative newcomers to the Galaxy.
Recent breakthreoughs in the chemical analysis of the Universe suggests that we live in a Universe exceedingly friendly to life. What we see in the physical laws and condition of the Universe runs contrary to the expectations of the "Rare Earthers."
If we are one in a billion, then, and considering that there are only 0.004 stars per cubic light-year, what are the odds that another Earthlike planet is a mere 20 light-years away?
Indeed, given all this evidence, the Rare Earthers are starting to come under attack. Leading the charge these days is Alan Boss who recently published The Crowded Universe. Boss estimates that there may be billions of Earthlike planets in the Milky Way alone.
"I make the argument throughout the book that we already know that Earths are likely to be incredibly common -- every solar-type star probably has a few Earth-like planets, or something very close to it," says Boss. "To my mind, at least, if one has so many habitable worlds sitting around for five billion or 10 billion years, it's almost inevitable that something's going to start growing on the majority of them."
The Kepler space telescope has mapped more than 1,200 planets in one tiny corner of our Milky Way Galaxy. Based on that sample, scientists say that there are approximately 50 billion planets in the entire galaxy based on a conservative estimate of one planet per star in the galaxy, including 500 million that are theoretically capable of sustaining life.
In astronomer Milan Cirkovic's view, truly advanced technological civilizations (ATCs: those who survive the bottleneck presented by the threat of self-destruction through warfare or asteroid impact or other accidents) will tend to be located at the outskirts of the Milky Way. The very traits that make ATCs capable of migrating and utilizing resources with high efficiency will tend to make them systematically hard to detect from afar.
Benjamin Zuckerman, an astrophysicist and a professor in the Department of Physics & Astronomy at UCLA, proposed in 1985 that stellar evolution of stars far older than our Sun is an important motivation for civilizations to undertake interstellar migrations. It seems implausible that any but the most extreme conservative societies would opt to wait to be forced to migration by slow and easily predictable process such as their star leaving the Main Sequence.
The Daily Galaxy via planetquest.jpl.nasa.gov