UFO Sighting Report – Mundaring (20th April) – 4 May 2013
‘Alien Earths’ May Not Be Only Exoplanets Capable Of Supporting Life, Astronomer Says
By: Mike Wall
Published: 05/02/2013 02:08 PM EDT on SPACE.com
Scientists searching for signs of life beyond our solar system should keep an open mind, for planets very different than Earth may well be habitable, a prominent researcher says.
While it may seem natural to zero in on “alien Earths,” such a narrow focus would exclude many potentially life-supporting exoplanets, whose diversity continues to astound astronomers, says Sara Seager of MIT.
And researchers can’t afford to be so picky, she adds, since they’ll be able to get in-depth looks at just a handful of alien worlds for the foreseeable future. [9 Exoplanets That Could Host Alien Life]
“The number of planets that we’re going to be able to see in our lifetime — and look at their atmospheres for signs of life— is so small that we’re forced to be open-minded,” Seager told SPACE.com.
Seager discusses exoplanet habitability in a review article published online today (May 2) in the journal Science.
A dazzling diversity of alien worlds
Scientists discovered the first alien planet around a sunlike star in 1995. Since then, the tally has grown to more than 700 (or more than 800, depending on whose list is consulted), with thousands more candidates waiting to be confirmed by follow-up observations.
Some of these alien worlds are broadly similar to planets in our own solar system. But many others are truly alien — enormous “hot Jupiters” that whip around their parent stars at extremely close range, for example, or “rogue planets” that cruise through the cold depths of space alone, with no parent star.
“If there is one important lesson from exoplanets, it is that anything is possible within the laws of physics and chemistry,” Seager writes in the Science article. “Planets of almost all masses, sizes and orbits have been detected, illustrating not only the stochastic nature of planet formation but also a subsequent migration through the planetary disk from the planet’s place of origin.” [The Strangest Alien Planets]
Intriguingly, a number of planets have been spotted orbiting within the so-called “habitable zone” — that just-right range of distances from a star where liquid water is possible on a world’s surface. (Water is required for life as we know it here on Earth and has thus spurred astrobiologists to “follow the water” on other planets, Seager writes.)
Just where this habitable zone lies for each planet depends on a number of factors, most crucially its host star’s brightness and the planet’s atmospheric makeup.
“It’s really all about the greenhouse gases,” Seager told SPACE.com. “The greenhouse gases are like a blanket that moderates the temperature at the surface.”
Extending the habitable zone
The conventional definition of the habitable zone assumes a roughly Earth-like atmosphere, dominated by nitrogen, carbon dioxide and water vapor. But the huge diversity of alien worlds argues for a new way of thinking, writes Seager, who literally wrote the book on exoplanet atmospheres (“Exoplanet Atmospheres: Physical Processes,” Princeton University Press, 2010).
For example, large and/or chilly alien worlds could conceivably hang onto their gaseous molecular hydrogen, which long ago escaped from small planets such as Earth, Venus and Mars.
Hydrogen is a powerful greenhouse gas that could make liquid water possible on a number of worlds far beyond the outer edge of the traditional habitable zone — and perhaps even on seemingly frigid rogue planets, Seager writes.
Similarly, the habitable zone may extend inward, toward the host star, on “dry” rocky planets whose atmospheres have much less water vapor than Earth’s does. So it’s best to consider alien planets’ potential to support life individually, on a case-by-case basis, Seager says.
Looking for life
Seager and others stress that a better understanding of exoplanet habitability is key to the next phase of the alien life hunt, which seeks to search promising candidates’ atmospheres for water vapor and gases that may have been produced by life.
Astronomers have already scanned the air of a few dozen planets using NASA’s Hubble Space Telescope and other instruments, Seager said. But those were hot Jupiters with big, puffy atmospheres — relatively easy targets that aren’t intriguing from an astrobiological perspective.
Scientists plan to do the same with smaller, potentially habitable worlds soon, Seager said. They’ll use the Transiting Exoplanet Survey Satellite, which NASA recently approved for a 2017 launch, to identify promising candidates relatively close to Earth. Then NASA’s James Webb Space Telescope (which is scheduled to blast off in 2018) will follow up, getting an in-depth look at these worlds’ air.
Though JWST is designed to be incredibly powerful, the $8.8 billion instrument will probably only be able to investigate the atmospheres of exoplanets that lie within a few tens of light-years from Earth, Seager added.
Seager said she hopes her review article in Science helps her fellow astronomers make the most of this small pool of observable candidates.
“I hope it gets people to realize that so many types of worlds could be habitable, and that our chance of finding one is higher when we accept that,” she told SPACE.com.
New York at high risk of alien invasions
Aliens don’t strike everywhere in equal measure. The first comprehensive assessment of the risk posed by ballast water has identified 20 hotspots that are particularly vulnerable to being invaded by species stowed away inside the bowels of cargo ships.
San Francisco Bay, New York, Los Angeles, Hong Kong and Durban in South Africa are on the list, which could help focus counter-measures.
Empty ships routinely take on seawater as ballast before they set sail, to keep them stable while at sea. They later dump it in the port where they take on cargo. At any one time, an estimated 7 billion tonnes of ballast water is crossing the oceans – almost all of it carrying seeds, spores, eggs, larvae, bacteria and plankton native to wherever the water was loaded. It’s probably the top cause of alien invasions worldwide.
These biological time bombs were meant to be defused by a UN treaty agreed in 2004 requiring big ships to filter and disinfect ballast water before dumping it. But not enough nations have ratified the treaty for it to come into force.
Now a team led by Bernd Blasius of the University of Oldenburg in Germany says that most of the threat is to a handful of ports. Of more than 1400 ports studied, they estimate that just 20 carry 39 per cent of the risk of invasions. The risk for most other ports was negligible.
The authors mapped nearly 3 million ship journeys and assessed the danger their ballast water posed. An alien invader is most likely to be released if the journey is not too long – in which case the critter may die en route – and not too short – in which case it may already be present locally. The riskiest journeys turn out to be between 8000 and 10,000 kilometres – roughly the distance from Shanghai to San Francisco.
Invasions are also more likely if the start point is in warm water, which usually contains more species, and if the water at both ports is of similar temperature and salinity. That flags up busy tropical ports handling long-distance cargo, with Singapore and Hong Kong top of the at-risk list alongside the Suez and Panama canals.
The predictions match data from ports that have known problems with alien invasions. That includes San Francisco Bay, where the majority of inhabitants are alien species such as Chinese mitten crabs, New Zealand sea slugs and Japanese gobies – all brought by ballast water. The team expects relatively few alien species to survive discharge at colder ports like those around the North Sea, but caution that this could change with global warming.
David Smith at the Plymouth Marine Laboratory in the UK says the study is solid but fears that by pointing out that most ports are not so vulnerable, “it could be used as a reason for not ratifying the treaty”. He warns that the team did not look at bacteria, which might pose different risks.
The findings could help target efforts to stem invasions, says study co-author Hanno Seebens of the University of Oldenburg. “Usually port authorities cannot check all ships,” he says. “Our model can help inspectors identify the ships with the highest probability of carrying potentially invasive species.”
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