Consider a hypothetical planetary system, within the Milky Way Galaxy, orbiting a central star known as Los. The star and planets, known collectively as the Ralos System, are to be found about two-thirds of the way out from the Galactic centre.


(Q)
One of the other planets in the Ralos System is Htrae, a relatively small, compact body, with an atmosphere, oceans of liquid water and a hard rocky crust that provides a surface for its inhabitants to live on. The inhabitants of Htrae have determined that their planet has an absolute age of 4.5 × 109 years. Explain the principle of a method they might have used to determine their planet’s age. PLEASE BE AS SPECIFIC SA POSS THANKS.

Obviously they're going to do it the same way that we do here on earth: Isotopic ratios of radioactive compounds. You assume that when the earth solidified that the radioactive compounds were pure. This is a reasonable assumption since pure compounds tend to coalesce and crystallize on cooling.

As the radioactive starting materials decompose, they form the daughter products. By measuring the daughter products' concentration and the starting material concentration and knowing the half-life of the starting materials, you can figure out when they first solidified.

You should be able to easily research the materials that are commonly used to do this.