Extraterrestrial life is defined as life that does not originate from

Earth. Possible forms of extraterrestrial life range from simple

bacteria-like organisms to more advanced than humans. It is

unknown whether any such forms of life exist or ever existed.

Panspermia is a theory that suggests that the seeds of life are

prevalent throughout the Universe and life on Earth began by such

seeds landing on Earth and propagating.

Scientists are directly searching for unicellular life within the solar

system, carrying out studies on the surface of Mars and examining

meteors that have fallen to Earth. There is some limited evidence that

microbial life might possibly exist in the universe.

Pre- Cambrianstromatolites in the Siyeh Formation, Glacier National

Park. It is in formations such as this that 3.5 billion year old fossilized

algae microbes, the earliest known life on earth, were discovered.

The Precambrian fossil record indicates that life appeared soon after the

Earth was formed. This would imply that life appeared within several

hundred million years when conditions became favourable. Generally

accepted scientific estimates of the age of the Earth place its formation

at about 4.55 billion years old.

The oldest known sedimentary rocks are somewhat altered Hadean

formations from the southern tip of Akilia island, West Greenland.

These rocks have been dated as no younger than 3.85 billion years.

Astrobiologists studying extremophiles as many organisms of this type

are capable of surviving in environments similar to those known to exist

on other planets. Some organisms have been shown to be more resistant

to extreme conditions than previously recognized, and may be able to

survive for very long periods of time. Some

Bacteria and animals have been found to thrive in oceanic hydrothermal

vents above 100 C; a study revealed that a fraction of bacteria survive

heating pulses up to 250 C in vacuum

Recent experiments suggest that if bacteria were somehow sheltered

from the radiation of space, perhaps inside a thick meteoroid or an icy

comet, they could survive dormant for millions of years. Deinococcus

radiodurans is a radioresistant bacterium that can survive high radiation

levelsR

Spores are another potential vector for transporting life through

inhospitable and inimical environments, such as the depths of interstellar

space. Spores are produced as part of the normal life cycle of many

plants, algae, fungi and some protozoan’s, and some bacteria produce

endospores or cysts during times of stress.

These structures may be highly resilient to ultraviolet and gamma

radiation, desiccation, lysozyme , temperature, starvation and

chemical disinfectants, while metabolically inactive. Spores germinate

when favourable conditions are restored after exposure to conditions

fatal to the parent organism.

Some research suggest that there are many more potential habitats for

life than Earth-like planets. The presence of past liquid water on Mars,

suggested by river-like formations on the red planet, was confirmed by

the Mars Exploration Rover missions.

Water oceans might exist on Europa, Enceladus, iverse. Triton and

perhaps other moons in the Solar system. Even moons that are now

frozen ice balls might earlier have been melted internally by heat from

radioactive rocky cores. Bodies like this may be extremely common

throughout the Universe.

A 2008 analysis of 12C/13C isotopic ratios of organic compounds found

in the Murchison meteorite indicates a non-terrestrial origin for these

molecules rather than terrestrial contamination. Biologically relevant

molecules so identified included uracil, an RNA nucleobase, and

xanthine.

These results demonstrate that many organic compounds which are

components of life on Earth were already present in the early solar

system and may have played a key role in life's origin. In August

2009, NASA scientists identified one of the fundamental chemical

building-blocks of life (the amino acid glycine) in a comet for the

first time.

Astronomers also search for extrasolar planets that they believe would

be conducive to life, such as Gliese 581 c, Gliese 581 g, Gliese 581 d

and OGLE-2005-BLG-390Lb, which have been found to have Earth-

like qualities.

1996, MARTIAN "FOSSILS" ARE

DISCOVERED IN METEORITE ALH84001

FROM ANTARCTICA

NASA scientists announced in 1996 that they had found what appeared

to be fossilized microbes in a potato-shaped lump of Martian rock. The

meteorite was probably blasted off the surface of Mars in a collision, it

was discovered in 1984 in Antarctica.

Careful analysis revealed that the rock contained organic molecules and

tiny specs of the mineral magnetite, sometimes found in Earth bacteria.

Under the electron microscope, NASA researchers also claimed to have

spotted signs of "nonbacteria".

2001, MORE RIGOROUS CALCULATIONS

CONNECTED TO THE 1960S "DRAKE

EQUATION" SUGGESTS THAT OUR

GALAXY MAY CONTAIN HUNDREDS OF

THOUSANDS OF LIFE-BEARING

PLANETS

The Drake equation multiplies together seven factors including: the

formation rate of stars like our Sun, the fraction of Earth-like planets

and the fraction of those on which life develops. Many of these figures

are open to wide debate, but Drake himself estimates the final number of

communicating civilizations in the galaxy to be about 10,000.

2004, METHANE IN THE MARTIAN

ATMOSPHERE HINTS AT MICROBIAL

METABOLISM

In 2004 three groups - using telescopes on Earth and the European

Space Agency's Mars Express orbiting space probe - independently

turned up evidence of methane in the atmosphere. Nearly all methane in

our own atmosphere is produced by bacteria and other life.

h

19 May 1995: two scientists at Cal Poly showed that bacteria can survive

without any metabolism for at least 25 million years; probably they are

immortal.

24 November 1995: The New York Times described bacteria that can survive

radiation much stronger than any that Earth has ever experienced.

7 August 1996: NASA announced fossilized evidence of ancient life in

meteorite ALH 84001 from Mars.

27 October 1996: geneticists showed evidence that many genes are much

older than the fossil record would indicate. Subsequent studies have

strengthened this finding.

29 July 1997: a NASA scientist announced evidence of fossilized microscopic

life forms in a meteorite not from any known planet.

1998: a microfossil that was found in a meteorite and photographed in 1966,

was recognized by a Russian microbiologist as a magneto tactic bacterium.

26 April 2000: the German team operating the mass spectrometer on

NASA's Stardust mission announced the detection of very large

organic molecules in space.

19 October 2000, a team of biologists and a geologist announced the

revival of bacteria that are 250 million years old, strengthening that

case that bacterial spores can be immortal.

13 December 2000: a NASA team demonstrated that the magnetosomes

in Mars meteorite ALH 84001 are biological.

2 August 2004: Very convincing photos of fossilized cyanobacteria in a

meteorite were reported by a NASA scientist.

STILL UNDER INVESTIGATION

On May 11, 2001, two researchers from the University of Naples

claimed to have found live extraterrestrial bacteria inside a meteorite.

An Indian and British team of researchers led by Chandra

Wickramasinghe reported on 2001 that air samples over

Hyderabad, India, gathered from the stratosphere by the Indian Space

Research Organization contained clumps of living cells

In 2005 an improved experiment was conducted by ISRO. On April

10, 2005 air samples were collected from six places at different altitudes

from the earth ranging from 20 km to more than 40 km. three new

species showed that they were more resistant to UV radiation than

similar bacteria found on Earth. new species and named Janibacter

hoyeli.sp.nov (after Fred Hoyle), Bacillus isronensis.sp.nov (named

after ISRO) and Bacillus aryabhati (named after the ancient Indian

mathematician, Aryabhata).

LIMITATIONSLife as we know it requires the elements hydrogen, carbon, nitrogen,

oxygen, iron, phosphorus and sulfur (H, C, N, O, Fe, P and S

respectively) to exist at sufficient densities and temperatures for the

chemical reactions between them to occur. These conditions are not

widespread in the Universe, so this limits the distribution of life as an

ongoing process

Space is a damaging environment for life, as it would be exposed to

radiation, cosmic rays and stellar winds.

Environments may exist within meteors or comets that are somewhat

shielded from these hazards. However, the extreme resistance of

Deinococcus radiodurans to radiation, cold, dehydration and vacuum

shows that at least one known organism is capable of surviving the

hazards of space without need for special protection.

FUTURE MISSIONThe 'Living Interplanetary Flight Experiment'

The Living Interplanetary Flight Experiment, which is being developed

by the Planetary Society, will consist of sending selected microorganisms

on a three-year interplanetary round-trip in a small capsule aboard the

Russian Phobos-Grunt spacecraft in 2011. The goal is to test whether

organisms can survive a few years in deep space. The experiment will

test one aspect of transpermia, the hypothesis that life could survive

space travel, if protected inside rocks blasted by impact off one planet to

land on another.

CONCLUSIONThere are so many evidences and limitations also. So, more and more

research need for strong evidences to prove the life existence throught

the universe.

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