Image Proxima Centauri, which is in the constellation of Centaurus 4.2 light-years from Earth. Credit... ESA/Hubble & NASA

And the planet’s proximity to Earth gives hope that robotic probes could someday be zooming past the planet for a close-up look. A privately funded team of scientists and technology titans, led by the Russian entrepreneur Yuri Milner and the theoretical physicist Stephen Hawking, have announced Breakthrough Starshot Initiative, a project to develop and launch a fleet of iPhone-size spacecraft within two to three decades. Their proposed destination is the Alpha Centauri star system, which includes two larger, sunlike stars in addition to Proxima Centauri.

“We will definitely aim at Proxima,” said Avi Loeb, a Harvard astronomer who is the chairman of an advisory committee for Breakthrough Starshot. “This is like finding prime real estate in our neighborhood.”

This newly discovered planet is much closer to its parent star, about five million miles apart, than Earth is to the sun, 93 million miles. Even Mercury, the innermost planet of our solar system, is 36 million miles from the sun.

While Proxima b might be similar to Earth, its parent star, Proxima Centauri, is very different from the sun. It is tiny, belonging to a class of stars known as red dwarfs, with only about 12 percent of the mass of the sun and about 1/600th the luminosity — so dim that it cannot be seen from Earth with the naked eye.

Thus Proxima b, despite its closeness to the star, receives less warmth than Earth, but enough that water could flow on the surface. Whether the planet has liquid water or an atmosphere is “pure speculation at this point,” Dr. Anglada-Escudé said in a news conference.

If the planet formed close to the star, it could be dry and airless, but it might also have formed farther out and migrated inward to its current orbit. It is also possible that the planet formed dry and was later bombarded by comets or ice-rich asteroids.