What is the extent of the solar system?

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

27 November 2011

Instead of asking “how big is the solar system?” like we did last week, perhaps we should ask “how far does the solar system extend to?”. After the huddles and pitfalls we encountered with the former, this latter question might prove easier. Let’s see…

As noted last week, the outermost planet is Neptune and it lies about 4.5 billion kilometres from the sun. Compared to the location of the Earth, that distance is so great that we wouldn’t notice much change whether the two planets are on the same side of their orbits or they are diagonally opposite each other.

You see; the Earth is only 150 million km from the sun, that is, 0.15 billion km. Thus when Earth and Neptune are on the same side with respect to the sun, they are 4.35 billion km apart and when they are on opposite sides, the distance between them increases to 4.65b km. The variation is about 300 million km but it represents only 3 per cent of the total separation between the two planets – you need a pretty good telescope (and keen observation) to record it.

From Neptune, the next major heavenly body is Pluto – the one that lost its “Planet” classification five years ago. Its orbit is so eccentric that at certain times it is nearer the sun than Neptune. This was the case from February 1979 to February 1999. Its distance from the sun varies from about 4.44b km to 7.4b km.

Many other objects have been discovered farther away and they come with exotic names such as Quaoar, Eris, Makemake and Sedna. Of these, Eris and Sedna interchange the recognition of “most distant known heavenly body in the solar system”.

Eris fluctuates from 5.7 billion km to 14.6billion km from the sun while Sedna moves between 11billion km and 140billion km. At the moment, Eris closer than Sedna, but that will change…

Clearly then, the extent of the solar system cannot be conclusively determined by the discovery of its most distant object – some one will always discover another body more distant than the last one!

For that reason, astronomers have come up with different methods of defining the extent of the solar system. One definition is based on the reasonable assumption that if one travels far enough, they will eventually come to a region where the sun’s gravitational pull is no longer the dominant force.

Empirical calculations show that the sun’s gravity is dominant up to about 18 trillion km away… now pause for a moment and contemplate that: 18,000,000,000,000km. It takes sunlight about two years to get there!

After that distance, the influence of other “nearby” stars begin to take hold. Thus if an object was found outside the 18 trillion km range, chances are that its motion is affected by another star (or stars) as well as the sun. Consequently, such a body cannot be described as “belonging” to the solar system. Clearly, this is a more conclusive definition than attempting to find the most distant object.