Our Sun is a G2V type star, or to use less formidable parlance, a yellow dwarf. It was inevitable that as we began considering planets around other stars (well before the first of these were discovered), we would imagine solar-class stars as the best place to look for life, but attention has swung to other possibilities in recent years, especially toward red dwarfs, which comprise a high percentage of all the stars in the galaxy. Now it seems that the problems of M-dwarfs are causing a reconsideration of the class in between, the K-class orange dwarfs.

Alpha Centauri B is such a star, although its proximity to Centauri A may raise problems in planet formation that we have yet to observe. Fortunately, our long-distance exploration of the Centauri stars is well underway, and we should have new information about what orbits the two primary stars here within a few short years. If we were to find a habitable zone rocky world around Centauri B, one thing that makes it interesting is the longevity of such stars.

Unlike our Sun, which is about halfway through its 10 billion year lifetime, orange dwarfs can live for tens of billions of years, offering abundant opportunity for life’s growth and evolution. While not as ubiquitous as M-dwarfs, K-dwarfs appear to be about three times more numerous than G-dwarfs like the Sun. These percentages are always being adjusted, of course, but I’ve seen estimates of G-dwarfs between 3 and 8 percent of the stellar population. A higher population of K-dwarfs, though, gives us plenty of search space for planets possibly bearing life.

How likely are the various kinds of stars to produce habitable conditions around them? M-dwarfs give us fantastically longer lifetimes (into the trillions of years), but at the recent meeting of the American Astronomical Society in Hawaii, Edward Guinan and Scott Engle (Villanova University) described the extreme levels of UV and X-ray radiation through flares and coronal mass ejections that planets in the habitable zones of these stars can receive, with the real possibility of atmospheres being stripped away. “We’re not so optimistic anymore about the chances of finding advanced life around many M stars,” Guinan said.

Guinan and Engle have been engaged in a project called GoldiloKs at Villanova, in which they work with undergraduate students to measure factors like age, rotation rate, and radiation exposure in a sampling of stars ranging across primarily G- and K-class stars. The Hubble instrument, Chandra X-ray Observatory, and ESA’s XMM-Newton satellite are involved in the observations, with Hubble particularly useful for assessing radiation from the K-dwarfs. Lacking the intense magnetic fields powering up X-ray and UV emissions, these stars produce a scant 1/100th as much radiation as would be received by a habitable zone world around an M-dwarf.

To Guinan, these orange dwarfs are indeed the 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.”

Image: This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs. The graphic compares the stars in terms of several important variables. The habitable zones, potentially capable of hosting life-bearing planets, are wider for hotter stars. The longevity for red dwarf M stars can exceed 100 billion years. K dwarf ages can range from 15 to 45 billion years. And, our Sun only lasts for 10 billion years. The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M dwarfs relative to our Sun, but only 5 to 25 times more intense for the orange K dwarfs. Red dwarfs make up the bulk of the Milky Way’s population, about 73%. Sunlike stars are merely 6% of the population, and K dwarfs are at 13%. When these four variables are balanced, the most suitable stars for potentially hosting advanced life forms are K dwarfs. Credit: NASA, ESA, and Z. Levy (STScI).

We have about 1,000 orange dwarfs within 100 light years of the Sun, making these interesting targets for future study. Whereas our own planet will face a habitable zone that gradually moves outward as the Sun begins to swell — we’re in deep trouble in a billion years or so — K-dwarfs see much slower migration of the habitable zone, with an increase in brightness by about 10-15 percent over the Sun’s entire lifetime. No wonder Guinan and Engle single out K-star hosts like Kepler-442 and Epsilon Eridani for extra attention. Indeed, Kepler-442 b is a rocky world circling a K5 star that Guinan calls ‘a Goldilocks planet hosted by a Goldilocks star.’

Addendum: I had inexplicably included Tau Ceti above as a K-class star (and yes, I had already had my morning coffee, so I have no excuses). Thanks to readers Alan and Michal Barcikowski for pointing out the error. Tau Ceti is a cool G8 dwarf, with mass about 70 percent of the Sun’s.

All this reminds us of how our views of our own circumstances have changed over time. It was natural enough to believe that in seeking out life elsewhere in the universe, we would look for places like the one we knew supported it. But we’re beginning to ask whether, habitable though it obviously is, the Earth is as ideally habitable as it might be. Let me point you to René Heller (McMaster University) and John Armstrong (Weber State University), who raised similar issues in a 2014 paper in Astrobiology. The duo use the term ‘superhabitability,’ and, although looking primarily at planetary types, also ask about the host stars:

Higher biodiversity made Earth more habitable in the long term. If this is a general feature of inhabited planets, that is to say, that planets tend to become more habitable once they are inhabited, a host star slightly less massive than the Sun should be favorable for superhabitability. These so-called K-dwarf stars have lifetimes that are longer than the age of the Universe. Consequently, if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and — once occurred — it would have had more time to ‘tune’ its ecosystem to make it even more habitable.

Back to Guinan and Engle, whose work over the past 30 years has included X-ray, UV and photometric studies of F- and G-class stars, a corresponding study of M-dwarfs that lasted a decade, and now the collection of similar data for K-dwarfs. My point here is that the K-dwarf work takes place within the context of a robust dataset painstakingly gathered across a wide range of spectral types, giving these two researchers’ conclusions substantial heft.

Is Earth, then, only ‘marginally habitable’ when compared to planets that could exist around stars more benign than our Sun? It’s a fascinating thought that demands we examine our own anthropocentrism while at the same time bolstering our target list for future observatories.

Heller and Armstrong’s paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract available. I’m sure a paper from Guinan and Engle is in the works. For now, however, have a look at Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (abstract).