Learn French: Ils espèrent protéger cette belle planète. - They hope to protect this beautiful planet.

This is a stem-changing verb. In the present tense, when the ending is silent (like the -ent for ils), the accent changes from é to è. This changes the pronunciation slightly so the stress falls on that syllable. The accent stays as é for nous and vous because we pronounce their endings (for example, nous espérons).

Protéger is also a stem-changing verb. Why doesn't its accent change to è here?

The stem change only happens when the verb is conjugated. Here, protéger follows the conjugated verb espèrent, so it must stay in its infinitive (dictionary) form. Because it is an infinitive, it keeps its original é. If you were saying "they protect," it would be conjugated and the accent would change: ils protègent.

Why doesn't protéger have a small word like à or de in front of it?

Certain French verbs, including espérer, can connect directly to a following infinitive verb without needing a preposition like à or de in between. Other common verbs that do this include vouloir (to want) and pouvoir (to be able to).