And the vast majority of those stars — perhaps 75% or even 80% — are dim, red dwarf stars less than half the mass of the sun.
Sources: Interview with Abel Méndez; Smithsonian Insider
That's why scientists are so excited by Proxima b — an Earth-size planet that orbits the red dwarf star Proxima Centauri.
Source: Business Insider
The star is 4.24 light-years from Earth, making it the closest to our planet besides the sun.
Source: NASA
No one has directly photographed Proxima b, but we could soon with either ground- or space-based telescopes.
A photo of Proxima b would hardly be a pixel in size. But astrobiologist Abel Méndez says that bit of light could tell us a lot about dwarf-star systems.
Some purple would suggest a planet has oceans," says Méndez, since blue water looks that color in red light. "All brown might tell us it's bare rock.
If Proxima b passes in front of its star — called a transit — scientists may be able to analyze its atmosphere, if it has one.
Source: Interview with Sara Seager, a planetary scientist at MIT
That might reveal "biosignature" molecules like oxygen, methane, carbon dioxide, and other chemistry that's associated with life.
Source: Interview with Sara Seager, a planetary scientist at MIT
For example, worlds around a red dwarf may harbor jet-black plants to soak up that color of light.
Source: National Geographic News
Planets around red dwarfs are more likely to always have one side facing the star. Researchers call this "tidal locking."
Source: Astrobiology Magazine
Earth doesn't have this problem and spins once every 24 hours, soaking most of its surface evenly in sunshine.
Red dwarfs are much dimmer than the sun, so their "Goldilocks zone" — where water could be liquid — is much closer. One year on Proxima b is 11.2 Earth days, for example.
Source: Nature
Violent space weather that reaches a planet this close might trigger auroras more than 100,000 times stronger than those seen on Earth...
Source: Smithsonian Insider
...And bathe the surface in X-ray radiation that's hundreds of times more powerful than what the sun shoots at Earth.
And red dwarf stars burn for trillions of years — hundreds of times longer than the sun will last, increasing the number of opportunities for life to arise, recover from catastrophe, and thrive.
Sources: Interview with Avi Loeb, a physicist at Harvard University; Space.com