The term "habitable star" refers to those stars which, by our best estimates, are capable of hosting habitable planets. Habitable planets are defined, in this context, as those which have life, no matter how primitive.

Because we have never observed any life other than that on Earth, all our current searches consider first the zone around a star in which a planet could have liquid water on its surface (termed the habitable zone), and then consider whether the star would last long enough for life to naturally evolve there, given a timeline of evolution similar to life on Earth. Without having examples of extraterrestrial life to look at, we must assume that life on Earth evolved neither anomalously quickly nor anomalously slowly relative to life elsewhere. This constrains the range of habitable stars to those rather like our Sun, for a number of reasons.

First, habitable planets are likely to exist only around main sequence or possibly subgiant stars (those expanding due to depletion of hydrogen in their cores - stars can remain stable in this form for a long time, though not as long as their main sequence lifetime). Giant stars are actually the last stage in a star's life, shortly before it explodes or peters out. Their increased size and luminosity will fry any planets that were in its habitable zone, and the star won't last long enough in its giant phase for life to evolve on any worlds in the newly-widened habitable zone.

The very hottest stars, those of spectral class O, evolve off the main sequence very rapidly, from a few million to as little as a few hundred thousand years. This is probably too rapid for planets to even form, much less acquire ecosystems. Additionally, class O stars emit so much radiation that the radiation pressure serves to blast the material that could coalesce into planets clear out of the system. The next cooler class, B, may have similar problems with radiation pressure. Even if they don't, class B stars are still very short-lived, burning through their hydrogen fuel in tens of millions of years.

This neatly excludes the most spectacular stars, the giants and O or B class main sequence stars that form the majority of the naked eye visible stars. The next cooler class of stars, class A, can definitely host planets. In particular, the star Fomalhaut has a large, Jupiter-like world orbiting it. We haven't spotted any Earth-like planets around class A stars, but then, we haven't spotted such planets around any stars at all, and the technology that would let us is only just now coming on line. The issue with class A stars, though, is longevity. They might last one to two billion years on the main sequence, which is probably not enough time for life to evolve. Even if it is, the first forms of life will just be beginning to get a foothold when the star expands into a giant and burns the hapless planet - and all its inhabitants - to irradiated ash. So, no aliens from Sirius, it seems.

That leaves spectral classes F, G, K and M. These aren't terribly spectacular-looking in our night sky, except for Alpha Centauri, and that only because it's so close. Class F is marginal - the hottest F stars, termed F0V through about F5V, have much the same longevity problem as A stars. Cooler Fs, though - F6V through F9V - last between seven and eleven billion years, more than enough time for life, or even civilization to become established.

Class G, the spectral class of our Sun, is pretty much universally suitable. Even hot G stars like the Sun or Beta Canum Venaticorum last more than long enough. Cooler G stars like Tau Ceti and 82 Eridani are some of the strongest candidates for extrasolar habitable world hosts.

K stars have no particular problems with longevity. All of them last a bloody long time, pretty much universally longer than the universe has existed (13.5 billion years). The problem is that the habitable zone around cooler K stars might be too small. This has two implications: one, there's less space and so the probability that a planet exists in the zone is smaller, and two, the orbit might be so small that the planet would be tide-locked. Tide locking can play hell with conditions on the planet, and might preclude habitability. Still, hotter K stars, down to about K3V or maybe even as low as K6V, are good candidates for hosting habitable planets.

That leaves red dwarfs, which are class M and bottom-range class K main sequence stars. These have the small habitable zone problem described above. Also, lower-mass stars tend to flare quite energetically. Indeed, the smaller the star, the more powerful the flare is relative to the star's normal output. A flare from a red dwarf like Gliese 581 is as energetic as a flare from the Sun, which is many times more powerful under normal circumstances. These flares can bombard any nearby planets with heat and radiation, with devastating effects to any life on those planets. Ultraviolet and X-ray radiation can directly damage living cells. Particle radiation is even worse, if not deflected by a powerful magnetic field, like Earth's. Even if it is deflected, it can ionize the upper layers of the atmosphere and carry them away into space. Over time, this can strip a planet of its atmosphere entirely, which is likely what happened to Mercury, and is happening to Mars right now. (Mars has a magnetic field, albeit a weak one, and is relatively far from the Sun - at the extreme edge of the habitable zone - which slows down air loss significantly.)

All that said, there are many red dwarfs in the galaxy - several times more than even K stars, the next most common type. Some of these don't flare, or at least, flare less violently. All this means that there might be habitable planets around red dwarfs, and even if they're proportionally much rarer than habitable worlds around F, G and K stars, there still might be more of them in absolute numbers.

Stellar habitability by spectral type

  • O - Hell no
  • B - No
  • A - Unlikely in the extreme
  • F - Unlikely to probable, depending on temperature and mass
  • G - Likely
  • K - Likely to possible
  • M - Unlikely but not impossible
Thruokoot Fnaaffpfaargaan hated space travel -- space was too damned cold, and no matter how stuffed the ship was with thermal reactors, it could never quite warm the place up enough for his liking. He longed for the vast seas of his home, where the warmth never ended. But he knew he might never see those seas again, for he was on a mission, a vital search for a new home for his entire kind.

A signal sounded in his pod, and Thruokoot deftly used his whispy tentacles to drop the ship out of superlight speed and into orbit around a Class G star. The system checked out; everything about the size and stage of the star seemed just right to support life. But then, he wondered, would there already be life there? He pulled into a lazy curve through the system, when -- there it was, another ship, a strange little one, but definitely unnatural, something created by intelligent life. He scanned the conditions of the vessel and found it incapable of supporting life. Perhaps it was a probe, employing some bizarre technology? Perhaps it had once supported life, but had lost power and been frozen in place? But then -- impossible!! He received a signal from it!! Some living thing inside that tiny, frozen vessel was trying to communicate with him!! His ship's translation devices crackled and sparked before finally completing a translation of the incoming signal: "Alien spacecraft, this is Earth Defense. Identify yourself and your purpose here."

The tone was ominous, but they had not attacked, and so it might be possible to reason with them. Thruokoot resolutely crackled his tentacles, knowing that he had to communicate to these beings the plight of his people, to beg their indulgence for the sake of the continued existence of his race. "I come in peace," he began. "My people originate many light years from here. As we speak, our star is on the brink of going supernova; if we do not find a new home, we will be extinguished."

A moment's pause. And then a reply begin to filter through. "We must be sure that your intentions are peaceful. We are going to send a boarding party--" and then in the background, suddenly, a different voice; "Captain, wait -- this -- this is impossible!! We can't board this ship, the temperature is over thirty thousand degrees!!"

Thruokoot tilted a whisp to the left -- it was a bit chilly in his pod.

The other ship remained silent for a moment longer, and then, with a slightly confused tone, its captain communicated again. "What.... what are your intentions towards the third planet of this system?"

"Third planet? Why would I have any intentions towards a planet? Nothing could survive in one of those; you'd freeze into a liquid, or even a solid." A quick scan of the satellite in question revealed it to be unlivable; three quarters of its surface was coated with toxic liquid dihydrogen monoxide; even the core was a chilly slurrly of liquid iron, not even 10,000 degrees.

And then realization dawned. These people, they were actually from the planet -- they were solids, solid life forms composed of some strangely suspended mix of frozen and liquid particles. Thruokoot had heard the scientists amongst his kind speculate that such a thing might be possible, but he could never conceive it to be so. Now understanding, Thruokoot made clear, "we have no interest in any of your planets. We are plasma life forms. We wish to live in your star. From the planet, you will not even know we are there."

Some quiet chatter could be heard from the other vessel, until the verdict finally came through. "If you wish to live on -- in, is that right, you want to live inside our star?"

"That is correct."

"Well that is -- beyond our jurisdiction, I guess, so you may carry on. We, uh, we look forward to sharing our system with your, uh.... people."

Within half an hour, Thruokoot was laying a few meters below the surface of the Sun, utterly content as its flames energized his aching whisps, and its delightful gravitational and magnetic fields washed over him. His ship remained in orbit, a quarter way between the surface and the first planet. The signal had been sent; his people were coming to their new home; the habitable star had been found.

If you have not read the story by Pandeism Fish in this same node, please read it first, thanks!

"Alien spacecraft, this is Earth Defense. Identify yourself and your purpose here."


The day just went from marginally annoying to decidedly annoying.

When I got to my office this morning all I wanted to do was to have a nice hot cup of brewed stimulants and then find the maintenance department to see if they, finally, could see about the flaky heads on two of my rig's satellites. The girls down in the maintenance pool were, understandably, behind on their PM schedules, just as I was behind on MY scheduled assignments.

Budget cuts.

But like just every other morning it seemed, within recent memory, my inbox had several red flagged messages and, joy of joys, an immediate action work order, "see job packet E-6739X5DT5, blah, blah blah," I picked up my communicator and buzzed my supervisor.

"Supervisor, Rethichev speaking. Ah! How are you doing this morning, Hurth!"

"You tell me, Spug. You tell me how I am doing. What's up with this IAW?"

"Oh, typical Yuxvit shit. Some bigwig in the Ministry of Galactic Zoology is all freaking out because some planet that she likes to have her family reunions on apparently, possibly has some sort of organism running rampant on it."

"Yeah so? What is the big whoop, are we behind on that one?"

"Just like every other class G planet out there, Hurth. But you have to get out there today because somebody's favorite species of wild fungus just might be in jeopardy."

"Oh, come on, Spug...."

"No, no, wait, this is a good one, check this out! The Ministeress of Zoology is sitting in her dentist's waiting room, waiting for her tusks to get polished. She is reading an old copy of the Pan-Galactic Inquirer and, you know that Strange Encounters section in the back?"

"Yeah, always something good in there..."

Well there is a blurb about a "breakthrough scientific discovery of the millennia" in some scientific journal of some Plasma based-race, describing how they discovered, while searching for a new star to inhabit, get this, a 'previously implausible carbon based life' on an orbiting planet."

"Yeah Strange Encounters always has a bit about one race 'discovering' another 'wild, bizarre' life form, so what?"

Well, the so what is that these particular carbon based life forms thought that the Plasmas were intending to invade 'their' planet and sent a welcoming party into space to intercept them."

I groaned and massaged my optic lobes,

"AND, the shit hit your desk, of course, and you had a radiological scan done and it come up positive for excessive EMR, right Spug?"

Yup. Saddle up and check it out. And don't forget the paperwork. Oh, and fill out your expense reports when you get a chance, girl, OK?"

Several Million Light Years later I am staring at the dark side of my assignment and, yup, just as I had expected, the thing is all lit up with artificial light.

By, Prime, that is a helluva infestation, I thought to myself as I checked my logs, I didn't think I was that overdue on this one!

But hey, what do they expect when they layoff over a quarter of the Galactic Parks and Recreation Ministry? Oh, no the GPR is just some unnecessary fiscal black hole until someone's favorite fungus patch might get messed up and then it's as if an entire solar system went nova!

"Alien spacecraft, this is Earth Defense. This is your last warning. Identify yourself and your purpose here, or we will *blurp! unknown expression*."

I did not need the translator to know what will come next. Great. This is always a pain in the ass.

"Earth Defense, this is Planetary Degaussing Specialist, Hurth F'rienne of Galactic Parks and Recreation. I apologize but your planet is behind on its scheduled preventative maintenance. This won't take very long."

There was a long pause, from the spacecraft that had come to share my orbit.

"Uh, could you hang on for a bit...I do not feel I am, uh, authorized to, uh...hang on, we'll get back to you."

"Yeah sure," I replied and went back to heat up some lunch.

After a bit, while enjoying a nice bowl of vegetables in a protein broth, someone "authorized" got back to me.

"Specialist, F'rienne, is it? Are you there?"


"My name is Tyrone Washington, speaker of The United Nations of the Planet Earth. We welcome you to our solar system and to our orbit."


'We, are unaware of the...governing body of which you represent, sir?"

"Uh, not Sir, I am a female of my species. And, well, I am not really qualified as a diplomat, Sentient Speaker, I am just a worker for Parks and Recreation. Just going to do some routine degaussing, no trouble really," I lied.

"Miss F'rienne, what exactly is it that you are going to degauss?"

This is where the alarm and threatening and begging starts...

"Your Planet. Won't take long. Just need to deploy my satellites"

"Hold on there, are you saying that you are going to de-gauss the entire planet?"

And here we go!

"Yes, that is what I am going do in just..."

"You can't do that!"

"Well, Sentient Speaker, that is my job, you know."

Right on cue, a pair of jet torpedoes of some kind fired from the craft which I was communicating to. Casually, I flipped a switch which had "Pulse Generator" taped above it.

The torpedoes detonated and I waited for...

"What did you just do?"

"Well, you attacked me." I continued calmly, "You shall find that all of your orbiting craft and sensors are now off-line Sentient Speaker. Sorry about that, but I do have a job to do."

"Look, the thing that you are going to do will cause the whole planet and everything on it to loose their magnetic field, correct?"

Yup, that's what a Planetary Degaussing Specialist does,"

"But our whole civilization is dependent on magnets in turbines and *blurp! unknown expression* for electricity!

"Sorry, just doing my job Sentient Speaker."

"Your 'Job' will deprive the whole planet of light and heat and power for every facet of our civilization! Without it our whole world will collapse! The lives of over 16 Billion people are at stake!"

"Sixteen billion sentients on this sized planet, are you pulling my Threegers??"

"No! Please, you shall plunge us into darkness and famine and *blurp! unknown expression*, what you are going to do will be genocide!"

"Now calm down, I think that you are over exaggerating the situation."


"You sentients seem to be fairly resourceful. Your race shall survive, hopefully."


"Yeah! With a little luck and a whole lot of faith in a little while you will get it right and some one will come down here again to make proper introductions.

"It is our fault really, if it weren't for these damn budget cuts, we would have been able to get to you before all this happened and you would have not gotten sidetracked."

"For the love of *blurp! unknown expression*, please have mercy on us!"

And with that, my day was completely ruined. They were now going to go on pleading and begging and fnonging on and would make me feel terrible.

"Now, now, take heart everything will work itself out in the end. Stand by, please."

I switched off the translator and settled deep into my seat. After taking a moment for a deep sigh, I set about deploying the satellites until a fault indicator "#4 launch pod failure" started flashing.

"Oh, for Prime's sake! You piece of *blurp! unknown expression*!!!"

Log in or register to write something here or to contact authors.