1. Pluto has been reclassified as a dwarf planet rather than a planet. How is a dwarf planet defined, and what led to this reclassification? (150 words)
2. The classification of living organisms has been revised with a "three domain" system replacing the previous "five kingdom" system. What led to this revised classification, and what are the main features of the new system? (150 words)
3. What is the current evidence for the presence of an ocean below the ice on Jupiter's moon Europa, and what tests could be carried out by future missions to confirm the existence of an ocean, and measure the thickness of the ice? (150 words)
4. What is meant by the "Early faint sun paradox" in the history of the Earth and how has the "paradox" been resolved? (150 words)