Where is the edge of the solar system?
The Sunday Nation
Nairobi,
31 August 2014
Dear readers, some of the questions that I get are quite straightforward.
They leave me wondering why you are asking instead of simply doing the
calculation for yourself. Some one recently asked me how many telephone
numbers can be accommodated in one prefix code, say, 0733. It's easy: we
can only get 0733-000000, 0733-000001,
0733-000002....0733-841451....0733-999999. You don't have to count them
to know that there are exactly one million possible numbers. I am yet to
figure out why this reader asked that question.
Another reader asked this seemingly simple question: “how long would it
take for light to travel from the edge of the solar system to earth?”
Now the speed of light is known; it is 299,792,458 metres per second –
this is usually rounded off to 300,000km/s.
But since we are more accustomed to kilometres per hour, the speed of
light in these units comes to approximately one billion kilometres per
hour. Therefore, if we can get distance to the edge of the solar system
in billions of kilometres, it will be easy to work out how many hours a
beam of light would take to traverse it.
The only remaining problem is to define what we mean by “the edge of the
solar system”. That is not a simple problem. In fact, astronomers have
never defined it! However, common sense tells us that this should be the
place where the sun ceases to be the dominant object.
There are many ways of determining whether the sun is the dominant
object. We may say it is the point where the sun is not the brightest
star (that is, it looks just like the other stars); or where its gravity
is no longer the greatest force; or even where its magnetic field
becomes insignificant.
But I suspect that when most people talk about “the edge of the solar
system”, they mean the orbit of the farthest planet. Unfortunately, that
is also not straightforward. It depends on when you went to school!
Between 1930 and 1979, Pluto was the last planet, then on February 1979,
it change places with Neptune.
That situation continued for 20 years until February 1999 when Pluto
again became the farthest planet. But a more dramatic change came in
2006 when astronomers agreed that Pluto is not actually a planet! So as
of today, the farthest planet if Neptune. It is approximately 4.5
billion kilometres from the sun and therefore, travelling at one billion
kilometres per hour, sunlight takes about 4.5 hours to reach Neptune.
But I must insist that the orbit of Neptune is NOT the edge of the solar
system. There are thousands of heavenly bodies existing far beyond this
planet and they all have orbits around the sun. So they are part of the
solar system.
The farthest known so far is Sedna. Its orbit extends as far as 77
billion kilometres from the sun. Sunlight takes over three days to get
there. But this is still NOT the edge of the solar system!
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