HD 189733b is a boiling gas giant similar to Jupiter and Saturn in our Solar System. If HD 189733b -- 63 light years away -- is anything to go by, planets that support life are exceedingly rare. Think about it -- just Earth supporting life in the vast and desert space of 63 light years! So what makes the earth special?
We live on a blue planet. Seventy percent of the surface is covered in water, and many life forms have similar high water proportions in their tissues. Water itself has some unusual qualities. It is one of the lightest in gaseous phase, very dense in liquid form, and the cohesiveness of water molecules means the compound has an unexpectedly high boiling point and unexpectedly high freezing point.
4°C appears to be a magical value for H2O. Whether water is heated or cooled, it expands at this temperature. As such, an entire body of fresh water needs to be the same temperature to achieve the density of ice (rather than just a surface layer). Freezing occurs from the top down, which means biological life lower down is insulated.
But in the broad strokes of the planet, it is the heat capacity of our oceans that stabilise what would otherwise be vigorous and destructive temperature flux manifesting in dust and gas storms as we see on many other planets, including one of our closest neighbours, Venus. Ocean temperatures fluctuate only a third as much as land temperatures.
There are other anomalies besides, including water's crystalline structure in solid phase, its low viscosity, and that water is easily supercooled but not easily glassified. Similarly the Mpemba effect predicts that -- under specialised circumstances - warm water freezes faster than cold water.
All of these features add up to an exception substance with unique flexibilities and intolerances beneficial to life -- and this substance covers the surface of our planet.
Mars differs from Earth in two respects. Mars is an ice-cold waterless desert with no insulating atmosphere. The Earth's atmosphere is a blanket that also stabilises terrestrial temperatures. Beyond this, the atmosphere played a crucial role in protecting the early Earth from large incinerating meteorites.
The atmosphere thus also created beneficial stability for life to evolve.
The Earth travels at 29.8 kilometers per second around the sun, at an angle 23.5° from vertical. Thanks to the wobble of the Earth's axis, different parts of Earth experience different angles of direct sunlight for various periods of time. This is a controlled degree of instability -- a pattern that many of Earth's creatures have adapted to. The day/night cycle brings about the most basic adaptation -- which is sleep, an adaptation shared in different ways by many life forms on our planet.
The size of the planet determines its gravitational pull, and the extent to which muscles are stressed by the planet's pull. It is unlikely that very large planets could support life as we know it, as creatures would be crushed or have difficulty moving thanks to the force of gravity. On the other hand, a too small object, such as a large asteroid or mood would struggle to hold an atmosphere, let alone life forms moving on its surface.
The Moon is strangely large -- almost a quarter the size of our planet. As such, some scientists refer to the Earth and its satellite as a 'double-planet' system. The moon is an interesting contrast to Earth. It has no molten core, it is not building new mountains or terrains -- it is a fossil planet with no contiguous magnetic field. There have been reports of moonquakes that are believed to be the result of Earth-tides, and small gaseous eruptions.
The moon also has had a crucial stabilising effect. It holds the oceans in place, it may have some impact on our continental drift (spurring evolution of different creatures) and in some important sense it has provided a shield against a certain amount of space projectiles.
The stability of the sun is also important. A supernova or black hole in the galactic neighbourhood can't be good for the prospects of life. We already know that the sun is warmer today than it was before the dinosaurs.
If your planet periodically passes through asteroid or radiation belts, this can't be good for life. Incidentally, radiation reaching the Earth is deflected thanks to our Ozone later, which absorbs harmful UV rays.
Earth's landmass is still evolving, leading to new land and over time, the possibilities of different creatures forming. Earth's volcanic crust has also played a role in developing our atmospheric cushion, including during the Cryogenian period.
When one considers some of the massive meteorite impacts (such as those causing the 300km wide Vredefort dome in South Africa, and the extinction of the dinosaurs) it is easy to imagine how life, even where it manages to get a foothold, sooner or later gets wiped out by some intra-solar or interstellar projectile.
Perhaps life on Earth is so special that it is unique (for our purposes, and our quadrant of the galaxy certainly) in a hostile universe, and in cosmic time, will disappear again entirely.
Our chances of survival may depend entirely on some other civilisation rescuing us and transferring us to their home planet, but this presupposes their ability to reach us across the gulfs of space.
We - and our world - are about to face a tremendous test to survival, and if we graduate, we may be better prepared for a test not of our own making.
2008/12/12 오전 2:46
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