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Most liveable alien worlds ranked

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A Two-Tiered Approach to Assessing the Habitability of Exoplanets

EARTH SIMILARITY INDEX
Earth – 1.00
Gliese 581g – 0.89
Gliese 581d – 0.74
Gliese 581c – 0.70
Mars – 0.70
Mercury – 0.60
HD 69830 d – 0.60
55 Cnc c – 0.56
Moon – 0.56
Gliese 581e – 0.53

PLANET HABITABILITY INDEX
Titan – 0.64
Mars – 0.59
Europa – 0.49
Gliese 581g – 0.45
Gliese 581d – 0.43
Gliese 581c – 0.41
Jupiter – 0.37
Saturn – 0.37
Venus – 0.37
Enceladus – 0.35

Saturn's moon Titan bears many similarities to the early Earth

Scientists have outlined which moons and planets are most likely to harbour extra-terrestrial life.

Among the most habitable alien worlds were Saturn’s moon Titan and the exoplanet Gliese 581g – thought to reside some 20.5 light-years away in the constellation Libra.

The international team devised two rating systems to assess the probability of hosting alien life.

They have published their results in the journal Astrobiology.

In their paper, the authors propose two different indices: an Earth Similarity Index (ESI) and a Planetary Habitability Index (PHI).

“The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbour life,” said co-author Dr Dirk Schulze-Makuch from Washington State University, US.

“The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not.”

As the name suggests, the ESI rates planets and moons on how Earth-like they are, taking into account such factors as size, density and distance from the parent star.

The PHI looks at a different set of factors, such as whether the world has a rocky or frozen surface, whether it has an atmosphere or a magnetic field.

It also considers the energy available to any organisms, either through light from a parent star or via a process called tidal flexing, in which gravitational interactions with another object can heat a planet or moon internally.

And finally, the PHI takes into account chemistry – such as whether organic compounds are present – and whether liquid solvents might be available for vital chemical reactions.

The maximum value for the Earth Similarity Index was 1.00 – for Earth, unsurprisingly. The highest scores beyond our solar system were for Gliese 581g (whose existence is doubted by some astronomers), with 0.89, and another exoplanet orbiting the same star – Gliese 581d, with an ESI value of 0.74.

The Gliese 581 system has been well studied by astronomers and comprises four – possibly five – planets orbiting a red dwarf star.

HD 69830 d, a Neptune-sized exoplanet orbiting a different star in the constellation Puppis, also scored highly (0.60). It is thought to lie in the so-called Goldilocks Zone – the region around its parent star where surface temperatures are neither too hot nor too cold for life.

The highly rated worlds from our own solar system were Mars, with a value of 0.70, and Mercury, with 0.60.

The Planet Habitability Index produced different results. The top finisher here was Saturn’s moon Titan, which scored 0.64, followed by Mars (0.59) and Jupiter’s moon Europa (0.47), which is thought to host a susbsurface water ocean heated by tidal flexing.

The highest scoring exoplanets were, again Gliese 581g (0.49) and Gliese 581d (0.43).

In recent years, the search for potentially habitable planets outside our solar system has stepped up several gears. Nasa’s Kepler space telescope, launched into orbit in 2009, has found more than 1,000 candidate planets so far.

Future telescopes may even be able to detect so-called biomarkers in the light emitted by distant planets, such as the presence of chlorophyll, a key pigment in plants.
www.bbc.co.uk

Written by physicsgg

November 24, 2011 at 7:24 am

One-Third of Sun-Like Stars Have Earth-Like Planets In Habitable Zone

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Astronomers have calculated the likelihood of finding Earth-like planets around other stars using the latest data from the Kepler mission.

The period and radius of Kepler planets in the sample, around bright stars, are plotted. The lower right corner is relatively empty, probably owing to low SNR there, not because small planets are absent from long periods. The upper left corner is relatively sparse, in spite of an expected high SNR there, implying a deficit of large planets on shortperiod orbits. The left side of the diagram is relatively empty owing to an apparent paucity of planets of all sizes at periods less than 3 days. The right side of the diagram is not completely sampled in the current database, so should be ignored here.

The Kepler orbiting observatory is specifically designed to find Earth-like planets around nearby stars.

Earlier this year, the Kepler team released the mission’s first 136 days of data and it has turned out to be a veritable jackpot. In that time Kepler looked at some 150,000 target stars and found evidence for 1,235 potential exoplanets. That’s quite a haul….. Read the rest of this entry »

Written by physicsgg

September 28, 2011 at 2:08 pm

Posted in ASTROPHYSICS

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