A new study finds a particular class of stars called K stars, which are dimmer than the Sun but brighter than the faintest stars, may be particularly promising targets for searching for signs of life.
Scientists have dubbed K stars "Goldilocks stars." According to a new study, K stars are more likely to host habitat exoplanets than other types of stars.
If life is to be found outside planet Earth, the new research suggests K stars are where astronomers should look. K stars are orangish stars that register a bit cooler than the sun.
Scientists looking for signs of life beyond our solar system face major challenges, one of which is that there are hundreds of billions of stars in our galaxy alone to consider. To narrow the search, they must figure out: What kinds of stars are most likely to host habitable planets?
As detailed in the Astrophysical Journal Letters, K stars have several advantages. They live a long time, providing surrounding planets plenty of time for the development of life. More importantly, K stars feature less electromagnetic turbulence.
M stars do offer some advantages for in the search for habitable planets. They are the most common star type in the galaxy, comprising about 75 percent of all the stars in the universe. They are also frugal with their fuel, and could shine on for over a trillion years. One example of an M star, TRAPPIST-1, is known to host seven Earth-size rocky planets.
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But the turbulent youth of M stars presents problems for potential life. The larger, more frequent solar flares produced by M stars can strip away the atmosphere of inner planets, quashing the chance for the development of life.
"I like to think that K stars are in a 'sweet spot' between Sun-analog stars and M stars," Giada Arney, a planetary scientist at NASA's Goddard Space Flight Center, said.
Scientists developed models to predict what biosignatures might look like in the atmosphere of an exoplanet orbiting a K star. Because oxygen and methane typically destroy each other, their simultaneous atmospheric presence suggests some type of process -- possibly a biological one -- is continually producing the two gases.
Oxygen and methane can be hard to spot from thousands of light-years away, but the models created by Arney and her colleagues showed oxygen-methane biosignatures would be more evidence the atmosphere of exoplanets in K star systems.
However, because planets around other stars (exoplanets) are so remote, there needs to be significant amounts of oxygen and methane in an exoplanet’s atmosphere for it to be seen by observatories at Earth. Arney’s analysis found that the oxygen-methane biosignature is likely to be stronger around a K star than a Sun-like star.
"When you put the planet around a K star, the oxygen does not destroy the methane as rapidly, so more of it can build up in the atmosphere," said Arney. "This is because the K star's ultraviolet light does not generate highly reactive oxygen gases that destroy methane as readily as a Sun-like star."
Because K stars are dimmer, the exoplanets orbiting them should be easier to spot. In their new paper detailing habitability in K star planetary systems -- published this week in the Astrophysical Journal -- authors identified several nearby K stars to be explored during future astronomical surveys.
"I find that certain nearby K stars like 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be particularly good targets for future biosignature searches," said Arney.