The Perfect Stars to Search for Life On Their Planets

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.

<Click to Enlarge> From top to bottom: The plentiful M dwarfs or red dwarfs; the less plentiful K dwarfs; the relatively uncommon G-type star, like our Sun. 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.
Credits: NASA, ESA and Z. Levy (STScI)

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.

Tidally-locked exoplanets may be very common around red dwarf (M-dwarf) stars because of their close orbits. Image Credit: M. Weiss/CfA

M-dwarfs are extremely energetic and unsteady. They’re often flare stars, and their violent output of energy could easily strip away a planet’s atmosphere very early in its life, and destroy any organism that had gained a foothold on the planet. Some of these flares can double the star’s brightness in a matter of minutes.

M-dwarfs can also have extremely powerful magnetic fields that may overwhelm the protective magnetospheres of any planets orbiting them. A 2013 paper examined the effect that these powerful magnetic fields could have on any potentially habitable planets. That study said, “To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G <Gauss> to up to a few thousand G.” This is in comparison to Earth’s magnetosphere which is one Gauss.

M-dwarfs’ powerful magnetic fields combined with their flaring makes them almost certainly toxic to life. And even though this intense flaring and powerful magnetic field can settle down later in an M-dwarf’s life, by then planets in the habitable zone would have already lost their atmospheres.

“We’re not so optimistic anymore about the chances of finding advanced life around many M stars,” Guinan said.

K-dwarfs are different.

This is an artist’s impression of the well-known TRAPPIST-1 system, a red dwarf that hosts 7 exoplanets. Thought the planets are all terrestrial planets, and three of them are in the habitable zone, the observed flaring from the TRAPPIST-1 star means life is unlikely. Image Credit: NASA/JPL-Caltech

K-dwarfs don’t experience the same flaring and chaotic energy output that M-dwarfs do. They also lack the same intense magnetic fields, which are responsible for much of an M-dwarfs inhospitable nature. According to Guinan’s research, K-dwarfs emit only about 1/100th as much deadly x-rays as some M-dwarfs.

The Goldiloks Project measured the age, rotation rate, and the x-ray and far infrared outputs of a sample of cool G and K stars. They’re using the Chandra X-ray Observatory and the XMM-Newton satellite in the project, but they’re relying heavily on the Hubble Space Telescope. The Hubble is extremely sensitive to ultraviolet radiation coming from hydrogen, and they used that sensitivity to assess the radiation coming from 20 K-dwarfs.

“Hubble is the only telescope that can do this kind of observation,” Guinan said.

An artist’s illustration of Epsilon Eridani and two exoplanets. EE is a K-dwarf star that’s only 10.5 light years from the Sun. It may host exoplanets, but their existence is controversial. Epsilon Eridani is young, only a billion years old, and it’s undergoing a tumultuous youth at this age. But it’ll calm down eventually, and could be a stable, hospitable star for tens of billions of years. Image Credit: By NASA, ESA, G. Bacon –, Public Domain,

Guinan and Engle found that the levels of radiation around K-stars was much less harmful than around M-dwarfs. K stars also have longer lifetimes and therefore slower migration of the habitable zone. That makes K-dwarfs the ideal place to search for life, and these stars would allow time for highly evolved life to develop on suitable planets. Over the Sun’s entire lifetime — 10 billion years — K stars only increase their brightness by about 10-15%, giving biological evolution a much longer time-span to evolve advanced life forms than on Earth.

We already know of some K-dwarfs that host exoplanets, and others that might host them, but that we’re uncertain about. Guinan and Engle looked at three particularly interesting targets: Epsilon Eridani, Kepler-442, and Tau Ceti.

Kepler 442 is a K-dwarf star hosting a rocky exoplanet about twice as massive as Earth. The planet is in the star’s habitable zone and is an object of intense interest. Artist’s illustration. Image Credit: By Ph03nix1986 – Own work, CC BY-SA 4.0,

“Kepler-442 is noteworthy in that this star (spectral 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. So the Kepler-442 system is a Goldilocks planet hosted by a Goldilocks star!” said Guinan.

Guinan and Engle have spent 30 years observing different types of stars. They’ve determined relationships between a star’s type, its rotation, age, x-ray and UV emissions. That data is the foundation of their work on how a star’s high-energy radiation affects a planet’s atmosphere and prospects for life.


Evan Gough

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