Habitable Hotspots & ‘Dead Zones’ of the Milky Way –Update on the Search for Life


New findings from diverse fields are are being brought to bear of the central question of the 21st century: How common is life in the universe? Where can it survive, Will it leave a fossil record, How complex us it. The list below moves several key features of the Universe's habitable zones and "off-the-chart" of likely places to search for life.

Peter Ward and Donald Brownlee, both of the University of Washington have outlined a short list of conditions needed:

Right distance from a star; habitat for complex life; liquid water near surafce; far enough to avoid tidal lock; right mass of star with long enough lifetime and not too much ultraviolet; stable planetary orbits; right planet mass to maintain atmosphere and ocean with a solid molten core and enough heat for plate tectonics; a Jupiter-like neighbor to clear out comets and asteroids; plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field; not too much, nor too little ocean; a large moon at the right distance to stabilize tilt; a small Mars-like neighbor as possible source to seed Earth-like planet; maintenance of adequate temperature, composition and pressure for plants and animals; a aglaxy with enough heavy elements, not too small, ellipitcal or irregular; right position the galaxy; few giant impacts like had 65 million years ago; enough carbon for life, but not enough for runaway greehouse effect; evolution of oxygen and photosythesis; and, of course, biological evolution.

In stark contrast, the listing of "dead zones" compiled for Rare Earth -Why Complex Life is Uncommon in the Universe by Ward (Professor of Geological Sciences and Curator of Paleontology) and 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 eveloved into giants that are too hot for life o inner planets.

Globular Clusters: Although they contain millions of stars, the stars are too metal poor yo have inner planets as large as`earth. Solar mass stars have evolved to giants 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: Where 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.

Three years ago, astronomers at the European Southern Observatory in Chile announced that they had found what might be the first habitable planet outside the solar system. Known as Gliese 581c, the planet is only five times as massive as the Earth and inhabits a rare sweet zone around a dim red star in the constellation Libra where it is neither too hot nor too cold for liquid water.

Gliese is but a cosmic hop, skip, and jump from Earth -only some 20 light years away (or 120 trillion miles!). Voyager 1, now leaving the solar system at a speed of about 39,000 miles per hour, would need more than 300,000 years to travel that far. Or, maybe someday we'll actually invent a Star Trek-type transporter that reassembles our atoms and transports us to the farthest reaches of the Cosmos.

For decades, scientists have been debating the conditions that are needed to replicate an Earth-like probablility of complex beyond the microbial level. There's not much doubt in the minds of most astrobiologist that based on extremophile life we've discovered recently on Earth (see prior posts below), that life on the microbial level will be discovered sometime in the next twent years on Mars or on one of Jupiter or Saturn's moons.

The three recent key findings for astrobiology are extremophiles, extrasolar planets, and a sense that water may be more ubiquitous even in our own solar neighborhood (in meteors like the Mars' Lafayette, Europa, and the ice frost on polar Mars). This picture has evolved quite suddenly with 1300-plus extrasolar planets found in just the last decade (and none known before around 1995). We now know that the number of planets in our own galaxy could easily tally in the hundreds of billions. The discovery of Gliese was a visible clue that a great number of these could be carpeted in the dirty chemistry we call life. Life on Earth may be unique, but it might not be miraculous.

Even in the oldest globular cluster star systems in our Milky Way galaxy — choked with stars that were born more than 10 billion years ago — there's enough "metals" to make earth-like worlds. According to models of planet formation developed by Dr. Sasselov and his colleagues of the Geneva discovery team, such a planet should be about half again as large as the Earth and composed of rock and water, what the astronomers now call a “super Earth.”

The most exciting part of the find, Dr. Sasselov said, is that it “basically tells you these kinds of planets are very common.” Because they could stay geologically active for billions of years, he said he suspected that such planets could be even more congenial for life than Earth. Although the new planet is much closer to its star than Earth is to the Sun, the red dwarf Gliese 581 is only about a hundredth as luminous as the Sun. "So seven million miles is a comfortable huddling distance." But for evolved animal life to be present we need to find that sweet "Goldilocks" planet with an exceedingly complex host of conditions present that have given rise the "Rare Earth" hypothesis.

Dr. Sasselov noted that aliens could have been pointing their antennas at Earth for 4.6 billion years, without picking up a signal. "Maybe the inhabitants of Gliese 581c are at the level of the classical Romans . . . or maybe trilobites." We need to check out hundreds of thousands of Earthlike worlds.

A new map suggests that around 1.2 percent of all stars may have been capable of supporting complex life at some point in the history of the galaxy.

The analysis of the first 136 days of results from NASA's Kepler telescope – launched to 'search for habitable planets' – has ignited furious debate over the possibilities of intelligent life in space.  Kepler's scans have found 1,235 extrasolar planets in the Cygnus constellation of our galaxy.

The new study, a Model of Habitability Within the Milky Way Galaxy, led by the University of Hawaii's Michael Gowanlock, used computer models to 'find' habitable planets – and to weigh the possibly devastating effects of supernova explosions. The rapid formation of planets at the galaxy's core would 'outweigh' the negative effects of repeated supernova explosions – providing a haven for life, said the researchers.

Common wisdom has it that the galactic habitable zone is a torus about 30 lightyears in diameter around the center of the galaxy lessening the likelyhood that habitable planets will form close to the galactic centre or very far away from it. In contrast, however, Michael Gowanlock at the University of Hawaii in Honolulu and colleagues, developed a new map of the galactic habitable zone in which challenges this convention and suggests that the galactic habitable zone is much more complex than a simple torus.

The new map uses the latest findings about exoplanets to determine galactic habitability. The oldest stars in the early universe were formed entirely from hydrogen and helium but generated heavier elements when they ran out of fuel and exploded. The next generation of stars formed from the debris of these supernovas and so have higher levels of heavier elements or metallicity.
Astronomers have recently discovered that exoplanets are much more likely to form around  those stars that contain heavier elements.

These heavier stars are most likely to form in areas where there are lots of supernovas and in our galaxy that's near the centre (at distance of about 9 light years), and are more likely to have planets and therefore more likely to have a planet in the habitable zone

The Gowanlock team say that habitable planets are so common towards the center of the galaxy that even if many are wiped out by supernovas, there should still be plenty that survive for long enough for complex life to evolve.

"We find the majority of stars in our galaxy will be bathed in flux by a nearby supernova event during their lifetimes,' the team concluded – but even if a planet was irradiated, it could evolve life at a later stage, or even re-evolve life after a supernova blast.  Planets that might support avanced life could be more common than we realize."

Their model suggests that 2.7 per cent of stars in the inner galaxy should have habitable planets. And there should also be habitable planets further away too. Gowanlock and team say about 0.25 per cent of stars in the outer galaxy should have habitable planets.

"We predict that ∼1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy," says Gowanlock. But, their model also predicts that 75 per cent of these habitable planets will be tidally locked around their mother star, meaning that one side of this planet would burn under a scorching sun while the other would freeze.

Whether life could exist anywhere on a planet such as the superEarth around Gliese 581, which is close enough to the star to be in the habitable zone but is also probably tidally locked, is unknown.

Currently, arguments rage over what, precisely, constitutes a 'habitable' planet. Early statistical analysis of the Kepler results provoked furious debate when one researcher concluded that one-third of planets round sun-like stars (stars with the classifcation F, G or K) could have 'earth-like' planets, while other experts say that many of the "habitable" worlds would be shrouded in clouds of freezing gas

The image at top of page shows three massive star clusters: the Arches (upper right), Quintuplet (upper center), and the GC star cluster (bottom center), which is near the enormous black hole known as Sagittarius A. The massive stars in these clusters can themselves be very bright, point-like X-ray sources, when winds blowing off their surfaces collide with winds from an orbiting companion star. The stars in these clusters also release vast amounts of energy when they reach the ends of their lives and explode as supernovas, which, in turn, heat the material between the stars. The stars near the Galactic Center also can emit X-rays as stellar corpses — either in the form of neutron stars or black holes in binary systems — and are also seen as point-like sources in the Chandra image.

The Daily Galaxy via technologyreview.com, homas M. Brown (GSFC) et al., NASA and cfa.harvard.edu,   and Ref: arxiv.org/abs/1107.1286: A Model of Habitability Within the Milky Way Galaxy

Image credit: NASA/CXC/UMass Amherst/Q.D.Wang et al.


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