If the oort cloud contains 1012 comets, and ten new comets are discovered coming close to the sun each year, what percentage of the comets have been "used up" since the beginning of the solar system?

The correct answer is 4.6 percent.

Explanation:

The Oort cloud refers to the clouds of icy objects, which is present in the outermost section of the solar system. These are the disperse layer of clouds found in the external layer of the solar system.  

In the given question, it is given that the number of comets in the Oort cloud is 10^12 comets.  

The number of comets getting close towards the Sun is 10 comets in a year.  

The age of the solar system is 4.6 * 10^9 years

The number of comets used since the start of the solar system is,  

(4.6 * 10^9 years) (10 comets/year)

= 46 * 10^9 comets

Thus, the number of comets disappeared since the beginning of the solar system is 46 * 10^9 comets.  

The initial number of comets at the start of the solar system is 10^12 comets.  

The percentage of comets that are used up are:  

(change in no of comets) / (initial no of comets) % = (46 * 10^9 comets) / (10^12 comets) * 100

= 4.6 percent.  

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a physical change is being made.