The Perfect Stars to Search for Life On Their Planets

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We tend to think of our Earthly circumstances as normal. A watery, temperate world orbiting a stable yellow star. A place where life has persisted for nearly 4 billion years. It’s almost inevitable that when we think of other places where life could thrive, we use our own experience as a benchmark.

But should we?

Our Sun is a G-type main sequence star with a lifespan of about 10 billion years. It’s about five billion years old and has powered life on Earth for almost 4 billion years already. G-type main sequence stars are not the most plentiful, nor are they the most long-lived. They make up only about 6% of the Milky Way’s stellar population, and they only live for about 10 billion years.

Most of the stars in the Milky Way (about 73%) are red dwarfs, or M dwarfs. M dwarfs are cooler than our Sun, and their habitable zones are smaller. But they’re much longer lived, by an order of magnitude. Their long lives might make them ideal stars for life to flourish around, given the right planets. But red dwarfs can be prone to deadly flaring, and their dangerous energy output may not be that hospitable for life as we know it.

There’s another type of host star that astronomers are starting to call Goldilocks stars. They’re more plentiful than the Sun, they’re longer lived than the Sun, and they don’t emit as much dangerous radiation as M dwarfs.

They’re called K dwarfs, also known as orange dwarfs.

“K-dwarf stars are in the ‘sweet spot,’ with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars).”

Edward Guinan, Villanova University

K dwarfs live between 15 to 45 billion years, they make up about 13% of the Milky Way’s population, and they emit only one-sixteenth as much deadly radiation as M dwarfs.

In new work presented at the 235th meeting of the American Astronomical Society, a pair of researchers used multiple telescopes to survey some G and K-dwarfs in our galactic neighborhood. They’re Edward Guinan and Scott Engle form Villanova University in Pennsylvanie. Their undertaking is called the Goldiloks Project.

In a press release, Guinan said that K-dwarf stars are true Goldilocks stars. “K-dwarf stars are in the ‘sweet spot,’ with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life.”

In a 100 hundred light year radius from our Solar System, there are about a thousand K-dwarfs. These stars are ripe for observation. And even though they’re far less plentiful than the M-dwarfs, some astronomers think we should shift our focus to K-dwarfs when it comes to searching for potentially habitable planets.

M-dwarfs are problematic when it comes to suitability for life. They’re plentiful and they host lots of exoplanets, but they’re dangerous. Since they’re so small, their habitable zone is very close.

That means that any planets in the habitable zone are probably tidally locked, which could diminish the chances for life to exist. One side would be in perpetual darkness, and the other side in perpetual light. That creates extreme, problematic temperature differences, where the frozen side could freeze the main gases out of the atmosphere, making the daylight side bone dry and barren.

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