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From CTV News:
CTVNews.ca Staff
Date: Wednesday Nov. 30, 2011 6:55 AM ET
A new study has rated the planets and moons that have the greatest likelihood of harbouring life, or being able to support it.
The report, "A Two-Tiered Approach to Assessing the Habitability of Exoplanets" was published in the journal Astrobiology.
The authors, led by Dirk Schulze-Makuch of the University of Washington, devised a two-pronged system for determining the livability of planets.
One approach rated planets according to their so-called Earth Similarity Index (ESI), which rated far-away worlds, quite simply, on how similar or dissimilar they are to Earth.
It looked at factors such as radius, temperature and mass.
The second approach, dubbed the Planetary Habitability Index (PHI), looked for the building blocks needed to support life such as whether the planet has a rocky or frozen surface and whether the appropriate chemistry is present.
The PHI also looked at the availability of energy, either from its parent star or through "tidal flexing" whereby heat is generated through kinetic energy caused by the effects of gravity.
The PHI also looks at whether the planet might possess liquid solvents necessary for chemical reactions.
"These two indices represent a first attempt to categorize the many exoplanets and exomoons that are expected to be discovered in the near future with regard to their potential for harbouring life," the report states.
The authors of the report used the planets and moons' scores on both scales to plot them on a habitability graph.
The planet Gliese 581g -- part of a cluster of up to six planets orbiting the red dwarf star Gliese 581 about 20 light years away from Earth -- topped the list as most likely to be habitable. Two of its sister planets followed in second and third place.
Using Earth's score of 1.0 as the benchmark -- the only known livable planet in the universe -- here are the results:
- Gliese 581g (.89)
- Gliese 581d (.74)
- Gliese 581c (.70)
- Mars (.70)
- Mercury (.60)
- HD 69830d (.60)
- 55 Cnc c (.56)
- Moon (.56)
- Gliese 581e (.74)
- Gliese 581f (.46)
- Venus (.44)
The report points out that while two planets or moons might have the same score, such as in the case with Gliese 581c and Mars, they may actually have little in common with each other since the scale only grades them on their similarity to Earth, not to each other.
The article also says that out that of all the planets listed, only Gliese 581g falls into the "Earth-like category." However, it acknowledges the fact that some scientists have expressed doubt about the planet.
"The existence of Gliese 581 g still needs to be independently confirmed after its apparent initial discovery by Vogt (in 2010), as later reanalysis of the discovery data suggests that Gliese 581 g does not exist," the article says.
The Gliese 581 system comprises four or five planets orbiting a red dwarf star.
HD 69830d, an exoplanet orbiting a star in the constellation Puppis is believed to inhabit the so-called Goldilocks Zone -- the distance from a parent star that could result in life-supporting temperatures.
While Venus made the list, the authors acknowledge it has a surprisingly high score on the PHI scale, given that most scientists agree the planet is too hot to support life on or beneath its surface.
However, that doesn't completely rule out the possibility of life existing in some form on Venus, the article says.
"Liquid droplets at benign temperatures in the upper cloud layers could harbour microscopic forms of life, evolved from ground-dwelling organisms earlier in the planet's history," the article says.
The authors of the report stated their work is intended to be updated and enhanced as new missions and new technology such as the Keck Interferometer in Hawaii and the Large Binocular Telescope Interferometer in Arizona, which is currently under construction.
Both will have the ability to see large gas clouds and planets around distant stars.
Link:
Study rates planets and moons on potential habitability | CTV News
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