Which statement best describes the habitable zone around a star?

Which statement best describes the habitable zone around a star?

• A. The region around a star where life is guaranteed to exist on any planet.
• B. The region around a star where a planet will always have a stable climate.
• C. The region around a star where planets would lack any atmosphere.
• D. The region around a star where liquid water could exist on a planet’s surface, depending on atmospheric conditions and planetary mass.

Answer: (D)

The habitable zone is the range of distances from a star where a planet could maintain liquid water on its surface, given the balance between stellar heating and the planet’s atmospheric greenhouse effect. The key idea is that water can exist as a liquid only if the temperature and atmospheric pressure are just right, which depends on how much stellar energy the planet receives and how thick or effective its atmosphere is at trapping heat. Planetary mass matters because it influences whether the planet can hold onto a substantial atmosphere and generate the surface pressures needed for liquid water. So the statement that best describes the habitable zone is that liquid water could exist on a planet’s surface, depending on atmospheric conditions and planetary mass. It doesn’t guarantee life or a perfectly stable climate, and it doesn’t require having no atmosphere.

The habitable zone is the range of distances from a star where a planet could maintain liquid water on its surface, given the balance between stellar heating and the planet’s atmospheric greenhouse effect. The key idea is that water can exist as a liquid only if the temperature and atmospheric pressure are just right, which depends on how much stellar energy the planet receives and how thick or effective its atmosphere is at trapping heat. Planetary mass matters because it influences whether the planet can hold onto a substantial atmosphere and generate the surface pressures needed for liquid water.

So the statement that best describes the habitable zone is that liquid water could exist on a planet’s surface, depending on atmospheric conditions and planetary mass. It doesn’t guarantee life or a perfectly stable climate, and it doesn’t require having no atmosphere.