What is the definition of a star's habitable zone?

• the range of distances from the star where planets with life have been detected

• the range of distances from the star where rocky planets can form

• the range of distances from the star where organic molecules can be stable on the surface of a suitable planet

• the range of distances from the star where liquid water can be stable on the surface of a suitable planet

A habitable zone

• exists around all stars

• only exists around stars which are identical to the Sun

• only exists around stars which are similar to our Sun

• only exists around one star, the Sun

Over time, as they age, the habitable zone of stars like our Sun

• move inward

• remain the same

• suddenly disappear

• move outward

Europa is located outside the Sun's habitable zone and yet may be habitable. How can this be?

• Europa's subsurface ocean contains lots of minerals that allow water to remain liquid at much lower temperatures

• Europa is continually being hit by comets and asteroids which keeps water beneath its surface liquid

• Europa is tidally heated, allowing liquid water to exist beneath its icy surface

• Europa is large enough to have appreciable heat trapped inside it to keep water beneath the surface liquid

The Moon is in the habitable zone of the Sun at the same distance as the Earth but is not habitable. How can this be?

• the Moon did have water on its surface in the past, but it was destroyed by high-energy particles from the Sun

• the Moon has never had water on its surface at any time

• the Moon is too small to retain an atmosphere necessary for liquid water to be stable

• the Moon did have water on its surface in the past, but it was blasted off the surface by impacts

Which of the following statements regarding the determination of surface habitability and subsurface habitability of a planet is correct?

• both surface and subsurface habitability can be determined only from robotic space probes

• surface habitability can be determined remotely by studying reflected starlight from a planet, while determining subsurface habitability would require a robotic space probe

• both surface and subsurface habitability can be determined by analyzing reflected starlight

• subsurface habitability can be determined remotely by studying reflected starlight from a planet, while determining surface habitability would require a robotic space probe

Apart from its distance from its parent star, what is the next most important factor that determines a planet's habitability?

• whether it has a large moon

• the size of the planet

• the chemical composition of its atmosphere

• the chemical composition of its surface

Theoretical calculations suggest that Earth-sized planets could be habitable on their surfaces outside of habitable zones if they

• have thick, hydrogen atmospheres

• have thick carbon dioxide atmospheres

• are tidally heated by several large moons

• have larger irons cores

Based solely on its distance from the Sun, we would expect the surface of Venus to be

• colder than the Earth such that water would freeze

• warmer than the Earth, but not so warm that liquid water could not exist

• roughly the same temperature as the Earth so that water would mostly be in liquid form

• very much warmer than the Earth such that liquid water could not exist on the surface

If Venus had the same atmosphere as the Earth, it would

• only be slightly warmer

• be at exactly the same temperature

• be very much warmer

• be slightly colder