# Wikijunior:Solar System/Is there life out there?

The short answer is \"Nobody knows\" - but it\'s interesting to try to
make some guesses based on what we **do** know!

Scientists who have tried to make good guesses use an equation called
\'The Drake Equation\' which was named after Doctor Frank Drake who
first tried to work this out about forty years ago. The idea is that if
you know how likely it was for life to form on Earth - and if you know
how many planets there are that are more or less like the Earth - then
you can make a good estimate of how much life there is out there.

We can try to do that ourselves.

### How many stars are there in the Universe?

When you look up at the sky on a really dark night - far from the lights
of cities and roads, you can see about five thousand stars and four or
five planets. But nearly all of the stars and planets out there are far,
far away and much too dim to see.

We know that there are eight planets orbiting the Sun and that our sun
is a lot like the other stars out there in our galaxy. Our galaxy (which
we call \"The Milky Way\" because it looks like a splash of milk across
the night sky) has about three hundred **billion** stars in it. But the
Milky Way is a rather small galaxy and there are many, many other
galaxies.

With our very best telescopes we can see about a hundred billion
galaxies. With a hundred billion galaxies - each one with hundreds or
even thousands of billions of stars - astronomers calculate that there
are about fifty thousand, billion, billion stars in the part of the
universe that we can see! That\'s a big number. Written out in numbers
it\'s 50,000,000,000,000,000,000,000 stars!

### Do all of those stars have planets?

Well, it\'s rather hard to tell for sure - but astronomers have looked
carefully at some of the stars in our neighborhood and whilst planets
that are orbiting other stars are too small to see - even with our very
best telescopes, we can sometimes see a slight wobble in the light
coming from a star that is caused by the gravity of one or more planets
that are orbiting it.

Knowing that, we have found that many nearby stars have at least a few
planets that are big enough to wobble the lights from the star enough so
we know they are there. There are probably other planets too small to
produce a big enough wobble for us to measure - so we don\'t know
exactly how many planets to expect. But it seems that there are probably
more planets than stars in the universe, so there could easily be
50,000,000,000,000,000,000,000 planets for life to try to get started
on. That\'s a lot of places and we haven\'t counted moons, asteroids,
comets and dust clouds!

### How often does life actually get started on one of these planets?

This something we can really only guess at. We know that life formed
once on one planet because we are that life and we live on that planet -
but how do we know whether the Earth is just an amazingly special place
or whether there are Earth-like planets orbiting most of the stars in
the sky? Unfortunately, we don\'t know.

-   Many planets must be too hot - like Venus and Mercury in our own
    solar system. Life can\'t survive there because the chemicals that
    life needs don\'t work properly when things are that hot.
-   Other planets might be too cold - so there would not be enough
    sunlight for plants and animals to use as energy. The outer planets
    of our Solar system - Pluto and Neptune - must be far too cold for
    life to exist there.
-   It\'s possible that planets might not have the right mixture of
    life-giving chemicals. There is no life on our Moon because it has
    no atmosphere - in daytime it\'s too hot and at night too cold.
-   Other planets may form and then be destroyed before life has had
    time to form - and on others, life might have formed a long time ago
    and then died out for some reason,

But there are plenty of planets to choose from. Next time you are on a
beach or at a playground with a sand lot, take a handful of sand. There
are an awful lot of tiny sand grains in a fistful of sand - but there
are more stars and planets than there are grains of sand on all the
beaches of all the world! Surely they can\'t all be too hot, too cold,
too dry or too poisonous?

But we humans haven\'t yet looked closely enough in enough places where
life might be to really know for sure what life needs. We\'ve found
bacteria living in boiling hot water near underwater volcanoes. There
are tiny worms who live in the ice near to the South Pole. Bacteria have
been found living inside rocks in the deepest mine shafts. There is
hardly anywhere on earth where live can\'t survive somehow. Astronauts
from Apollo 12 who visited the moon in 1969 visited the Surveyor III
lander that had been sitting on the moon for more than two years. They
found that bacteria that had hitched a ride on Surveyor when it was
launched and were still alive after two years of living on the moon!

It seems then that life can survive and even prosper in some fairly
hostile places - this shows that a planet may not have to be a
comfortable place for humans in order to develop life. However, there
doesn\'t seem to be any native life on the Moon beyond what humans have
put there by accident and our robotic missions haven\'t found life on
Mars or Venus or any of the comets we\'ve looked at closely.

With at fifty thousand billion billion planets in the universe, even if
each planet only had a one in a trillion chance of having life, there
could still be billions of planets teeming with life out there in the
universe. But if the chances are that bad then we might be alone in our
galaxy with the nearest other creatures living hundreds of millions of
light years away. Even if the odds for life are only a million to one
for each planet, we humans and our robots might have to travel for
thousands of years and search millions of planets and moons before we\'d
find them.

### So how many aliens are there?

That is where the drake equation comes into play:

`<big>`{=html}`<big>`{=html}R^\*^ × f~p~ × n~e~ × f~l~ × f~i~ × f~c~ × L
= The number of alien civilisations that we can talk to in our
galaxy`</big>`{=html}`</big>`{=html}

This equation might look complicated, but it\'s actually quite simple
when you understand it.

R^\*^ is how many stars appear a year. So, for example, if there was 10
stars being made every year, then you could scrible out R\* and put at
\'10\' there instead.

f~p~ is the chances that a star has planets. If, on average, there was a
1 in 2 chance that each new star had a planet, then the number that
replaced f~p~ would be 0.5 (1÷2).

n~e~ is the number of planets in each solar system that can have life.
In our solar system, n~e~ would be 1, because we only have one planet
that we know has life on it, Earth.

f~l~ is the chances that these planets have life. If all planets that
could have life have life, then 1 would go in the place of f~l~

f~i~ is the chances that the life on these planets is clever. A 1 in a
hundred chance of life would give us 0.01, 1÷100.

f~c~ is the chance that they will be able to talk to us. This number
might be 1 (100%), because if they are clever they will eventually find
out how to talk to other planets.

Finally, L is how long each alien civilisation will live (or want to
talk to us), in years. Though nobody knows the real value of this, Frank
Drake guessed 10,000 years.

Now, we can multiply them all together. 10×0.5×1×1×0.01×1×10000 = 500
alien civilisations in our galaxy. The values here are just guesses,
though, and other guesses have ranged from only one civilisation in our
galaxy to thousands.

### Little Green Aliens

If so many planets in the universe does this mean that we should expect
little green aliens to be popping out of their flying saucers soon?
Well, perhaps not. For beings living in another galaxy outside of the
Milky Way, if they noticed that we are here and set out to visit us -
traveling at the speed of light, they still wouldn\'t get here for
another 300 million years! They couldn\'t have set out millions of years
ago to visit us because humans haven\'t been on around for nearly that
long.

If we\'re going to be visited by aliens, they\'d have to live on a
nearby star and know how to fly very fast to get here. That means that
if we are to meet alien cultures they must come from within the very few
stars that are within a reasonable travel time. Life would have to be
very common indeed for there to be intelligent beings close enough to
meet us face-to-face.

### SETI - The Search for Extra-Terrestrial Intelligence

One group of scientists are searching for other intelligent life in our
galaxy by listening to the radio. With large radio telescopes they sweep
the stars looking for radio signals with interesting patterns in them
that would not come from natural radio sources. So far, they haven\'t
found anything - but that may mean that they have to keep on looking -
or it might be that the aliens are not transmitting anything in our
direction with enough power for SETI\'s sensitive radio telescopes to
pick up.

### Panspermia - Life from Space?

One other possibility is that life didn\'t just appear on Earth at all.
If life were really common throughout the universe, it\'s possible that
life arrived on Earth after it developed somewhere else. We know that
meteorites found on Earth have come here from the Moon and from Mars -
and probably from other planets too. We also know that debris in the
form of little bits of rock and ice rain down on the Earth from space
all the time. If life existed in that debris when the Earth was young,
there is a good chance that we Earthlings originally came from some
other planet entirely. It could be that all life on Earth originated on
Mars and we are the alien invaders!

### So what **IS** the answer?

We still don\'t know for sure that there is life anywhere beyond the
earth, and even if life were common in the universe, we don\'t know
whether we\'d ever be able to find it if there was.

The search for life beyond Earth is a very new thing. Humans have only
been searching for about 40 years - and we\'ve only been looking in the
most likely places for 10 years. If we humans are going to find signs of
life outside the Earth, it will probably happen in your lifetime. You
may one day be amongst the first humans to encounter alien life or to
hear a message from another world through SETI!

