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Extrasolar Planets


An extrasolar planet is one that is not in our Solar system.  It had long been theorized that other stars have planets, but it was only in the latter part of the twentieth century that any were discovered.  According The Extrasolar Planets Encyclopaedia, as of the 19th of June 2009, 353 extrasolar plants had been discovered.   The rate of discovery is increasing very rapidly.

 Free Floating Planets
 
All the planets so far confirmed have been orbiting round stars.  Planets not orbiting stars do exist, but detecting them is difficult although at least three possibilities have been found.


These are sometimes called free floating planets.  They are defined in the paper "Free Floating Planets" by Annemarie Hagenaars, Ilja Rosenbrand and Charlotte de Valk as an object that does not orbit a star and has a mass less than 13 times that of Jupiter.  (This upper mass limit is to distinguish them from brown dwarf stars, and is roughly the mass below which sustained nuclear fusion takes place.)  I would also place a lower limit of mass on what we would consider a "Planet".

Naturally, nearly all the planets so far discovered are much bigger than the Earth.  This is because bigger things are easier to find, and does not suggest that Earth sized planets are rare.  The range of sizes of extrasolar planets discovered suggests that there will be plenty of Earth sized planets out there.

In 1992, a possible planet (Planet : PSR 1257+12 b) was discovered which is only a bit more than a fifth of the mass of the Earth, so we can expect increasing numbers of small planets to be discovered.

Habitable Zone



For us, or similar creatures, to live on a planet it generally needs to be orbiting a star in the fairly narrow region where water can be liquid on at least part of the planet, at least part of the time.  A planet also needs to be big enough to hold a reasonable atmosphere.  For example, our own Moon is too small.

A planet also should not be too big, although there has been plenty of speculation about what sort of life could live on gas giants.

Although no Earth sized planets have been discovered in the habitable zones of other stars, about 30 larger planets have. We can expect there to also be Earth sized planets.

Earth Sized Planets

I have been using this term loosely.  Perhaps we should consider what sized planet we could live on.  One as small as Mars
, if it had enough water and was the right distance from it star would do, although this is near the lower limit.

A planet much bigger than the Earth might tend to have an uncomfortably high gravity.  However, if it was of lower density that our Earth, it would have a bigger diameter and its surface gravity would not be so high.  From the limited sample we have available, it appears that the Earth is a high density planet.  It is probable that Humans could live on some planets with a bigger mass than the Earth as well as a larger surface area for living on.

Habitable Moons


Based of the limited information we have, we can expect that most planets will have moons, and that these will be of a huge range of sizes.

The 30 so big planets in the habitable zones of their stars may have Earth sized moons.  There is nothing impossible about a very big planet, like many of the ones found so far, having more than one Earth sized moon.

There could be another class of habitable moon.  If a big planet was orbiting further out than the habitable zone of its star, tidal forces could warm its moons enough to melt water.  This warming effect is clear in the moons of Jupiter.

No definite signs of life have been found on any extrasolar planet, but at present our techniques for observing them are not good enough to tell.  These techniques are rapidly improving.

There are certainly plenty of places in our galaxy that could support life like the Earth's.  We still do not know if extraterrestrial life exists, but the more we discover, the more likely it appears.

Sources


The Extrasolar Planets Encyclopaedia, Free Floating Planets,

 
Steve Challis
 
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