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
Invitation to Link:
If any page of this website is relevant to your site, please feel free to link to our site.
|
|
