The habitable zone (also known as the Goldilocks Zone) is the range of orbits around a star within which a planet’s surface can support liquid water. They are not so hot as to vaporize itself into thin air, nor so cold as to be freeze. While liquid water is a key component for life, it is not the only ingredient to help us determine a planet’s ability to hold life. According to new research, there are also Goldilocks Stars.
What are they?
Not all stars have the same temperature. Some, like OB Stars, are blazingly hot. Others, like the Red Dwarf, are low in temperature. Scientists argue that despite some of these temperatures being good, they wouldn’t sustain life since the Goldilocks Zone would be very close to those stars. Red Dwarfs tend to violently lash their surrounding spaces with their flares.
The sun, also known as a G-Type main-sequence star, is located between these two extremes. And although we know there is life in the our solar system, the sun is not considered a Goldilocks star. According to astronomers from Villanova University, the stars best suitable to hold life are closer to the Hertzsprung-Russell chart of stars, such as K-Type stars. These are cooler than the sun, but also warmer than a Red Dwarf.
Villanova astronomer and astrophysicist, Edward Guinan, explains that:
“The K 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 pumps up your chances of finding life.”
Over the last three decades, astronomers (Guinan, Engle, etc.) have been observing a number of stars (F-G type) in both ultraviolet and X-rays as part of the Sun in Time program, as well as M-type Red Dwarfs for a decade for the Living with a Red Dwarf program. These programs have been helping evaluate the natures and impact of the X-ray and ultraviolet radiation of said stars regarding the potential of habitability of their planets. In addition, they have recently expanded the investigation to include K-type stars. Once again, these stars have proven to be the most promising for life-sustaining conditions.
Despite the small size of the habitable zone, given by the K-type stars, they are considered to be more common than G-type stars. About 1,000 of them are located just 100 light years from the Solar System. They also have much longer sequence lifetimes, unlike the sun and Red Dwarfs.
Fun fact: the sun is nearly 4.6 billion years old, with a main-sequence lifetime of 10 billion years. With complex life only starting 500 million years ago, scientist have theorized that the planet will become uninhabitable in another billion years, as the sun begins to expand and push the Solar System’s habitable zone outwards. On the other hand, Red Dwarfs are still common, but, as explained above, they are volatile. They expose the space around them to radiations that can strip planets of their liquid water and atmospheres.
In comparison, K-type stars have lifetimes in a range of between 25 and 80 billion years, which supports the idea that they offer more conditions for life than G-type stars. They are also seen as much calmer than Red Dwarfs. The highlight of the research is that K-type stars have already been found around the vicinity of certain planets. Some of these stars include Kepler-442, Tau Ceti, and Epsilon Eridani.
Guinan said that “Keppler-442 is noteworthy in that this star (special classification, K5) hosts what is considered one of the best Goldilocks planets, Kepler-442B, a rocky planet that is a little more than twice Earth’s mass.” In other words, a semi-perfect habitable-like planet is supported by its very own semi-perfect star, giving us a combination of a Goldilocks Planet with a Goldilocks Star.
There are other factors to consider as well, of course. An elliptical orbit can produce extreme temperatures which would turn an otherwise Goldilocks Planet uninhabitable. There is also the probability that each galaxy has its own habitable zone. One thing, however, is certain: this research give us the incentive to wonder about life outside of this planet. Who knows? Sci-fi movies may have a point.