Although most potentially habitable worlds orbit red dwarf stars, we know that larger, brighter stars can support life. For example, a yellow dwarf star is known to have a planet teeming with life, perhaps even intelligent life. But how big and bright can a star be and still have an inhabited world? That's the question addressed in a recent paper in the journal Astrophysical journal.
Stable main sequence stars, such as the Sun, are classified by color or spectral type, with each type assigned a letter. Historical reasons The categories are not alphabetical. Red dwarf stars, the coolest and lowest-mass stars, are M-type. Then, with each category getting brighter, bluer, and more massive, there are K, G, F, A, B, and finally O. The Sun falls into the G category as a yellow star. Each spectral type is further divided into smaller sections, numbered 0 through 9. The Sun is a G2 star because it is at the warmer end of the G-type stars.
As you go up the scale, the potentially habitable zone gets farther away from the star, but it also gets larger. That makes it more likely that there's a planet in the zone. But larger stars also have shorter lifetimes, which might not give life enough time to evolve into a world. Plus, there are other factors that can be detrimental to life. Larger stars emit an enormous amount of ionizing radiation, which could strip planets of their atmospheres or sterilize a planet's surface. Because of this, larger B- and O-type stars are unlikely to host life.
But what about F-type stars? They're slightly brighter than the Sun and whiter than yellow in color. They're also stable for about 4 billion years — long enough for life to evolve and thrive. And they also emit more ultraviolet radiation, which may have helped life arise on Earth. What are the chances of a habitable F-type planet existing?
To answer this question, the team combed through the database of known exoplanets. About 80 F-type main sequence stars are known to have at least one planet. Of these, 18 systems have exoplanets that spend at least part of their orbit in the star's habitable zone. And in one case, the exoplanet 38 Virginis b, the planet is always in the habitable zone. Statistically, about 5% to 20% of F-type stars have the potential for life.
The interesting thing about 38 Virginis b is that it is a gas giant with a mass four times that of Jupiter, so it is not likely to be habitable. But it could have Earth-sized moons, similar to Jupiter's Galilean moons. A world orbiting a Jovian planet could be a perfect home for life.
F-type stars make up only 3% of the Milky Way's main sequence stars, and it's possible that their excess ultraviolet light could rule out the existence of habitable worlds. But alien astronomers could make similar arguments about G-type stars, like the Sun. As this study shows, we shouldn't rule out the Sun's brighter cousins in the search for habitable worlds.
Reference: Patel, Shaan D., Manfred Cuntz, and Nevin N. Weinberg. “Statistics and habitability of F-type star and planetary systems.” Astrophysical Journal Supplement Series 274.1 (2024): 20.
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