Question

What is the radius of our solar system?
(A) $39.5$ light years
(B) $39.5$parsec
(C) $39.5$AU
(D) None of these

Answer 1

Hint
The radius of any object is the distance from its centre to the outermost point. For our solar system, the radius will be the distance from the Sun to the outermost planet in the system.

Complete step by step answer
Our solar system consists of 8 major planets and one dwarf planet, centred on the Sun. These astronomical objects orbit the Sun because they are gravitationally bound to it. The orbits are not exactly circular, but elliptical in shape. The orbits are also tilted at different angles and not aligned with the plane of the Sun’s diameter, the way we would expect.
Since the astronomical distances are very large, scientists use unique units to measure these easily. For example, the radius of the Sun alone is about 696340 km. Calculations using such big numbers would be tough to produce and may pose a problem of errors. Hence, the following astronomical units are devised to simplify the measurements:
One light year is the distance light would travel in vacuum in one Earth year. Since light has the fastest speed known to man, we can imagine how much distance it would cover in 365 days. One light year is estimated to be equivalent to 9460 billion kilometres!
A parsec is an even bigger unit than the light year. Its value is approximately equal to 33 trillion kilometres or $3.3$ light years. And an astronomical unit (AU) is defined as the average distance between the Earth and the Sun. This unit is widely used when estimating relative distances.
We know that Pluto is the farthest planet in our Solar System and the distance to it from the Sun is given as $39.5$AU.
Hence, option (C) is the correct answer.

Note
To simplify the extent to which these units disperse, imagine a football field. If we assume the Sun to be a coin of INR 1 kept on the goal line, the distance of Neptune would be about 90 feet! Neptune is the farthest planet in our Solar system, if we exclude the dwarf planet Pluto.