Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

barbara boxer (born november 11, 1940) is an american politician.
barbara boxer

the gates of the forest is a book written by nobel peace prize-winning author elie wiesel.
the gates of the forest

the future of the earth will depend on many things, including increases in the brightness of the sun, loss of heat energy from the earth's core and change of the planet's orbit by the other things in the solar system.
future of the earth