Scientists recently announced the discovery of a new exoplanet, Proxima Centauri d, orbiting the star Proxima Centauri. This planet is about one-quarter the mass of Earth and is too close to its star to be in the habitable zone. However, its discovery is significant as it adds to our understanding of planetary formation around small stars.
What is the main subject of the text?
A) The discovery of a new exoplanet
B) The habitable zone around stars
C) The mass of Earth
D) Planetary formation