# How far away from a Sol-class sun could a planet be and still be habitable?

In my galaxy, I need a planet that is far enough away from its star to be relatively cold compared to Earth, but not so cold that liquid water cold not exist.

The planet has 2 continents, 1 northern and one southern. The southern continent's southern tip is around where the tropic of capricorn is on Earth, and its northern tip is basically the equator. However, the southern continent still only enjoys a Mediterranean climate in its warmest areas. The northern continent starts at around where the tropic of cancer would be on Earth and ends at around where the US state of North Carolina would be on Earth. The northern continent is highly mountainous and generally has a frigid subarctic climate similar to that of Scandinavia or northern Canada.

How far would this planet have to be from its star, assuming that both the star and the planet are about the size of our sun and Earth respectively and that the planet has an Earth-like atmosphere?

• This seems at once oddly specific and underspecified. For one thing, I imagine that the atmospheric composition could make a massive difference.
– user
Mar 22, 2019 at 18:25
• Calculation of Habitable Zones Mar 22, 2019 at 18:40
• @Alexander, add a paragraph of text explaining why that website is useful and you have an answer....
– JBH
Mar 22, 2019 at 21:35
• is the population human or can they be altered in a way to allow for various atmospheric conditions to accommodate your request? Mar 22, 2019 at 23:16

It sounds like you're asking about the habitable zone of a star, and for that we have a relatively good idea of where that is for a star like our Sun- a relatively up to date 0.95-1.67 AU from 2013. Wikipedia has some nice reading on the subject

But in case you were looking for a bit more variety in your stars, I direct you to the Hertzsprung-Russell Diagram as a handy 'star energy output' chart.

Any star that outputs more energy than our sun (either by being larger, or burning hotter, or both) is going to move that habitable zone outward, while anything that outputs less energy (either by being smaller, or burning cooler, or both) is going to pull that outer range in toward the star itself. SOURCE

With that said, there's some special phenomena that happens with other star types, such as:

• More destructive solar flares
• More interference from solar radiation
• Larger variation in sunspot cycles (I don't remember how that would affect us but still)

And then there's the stuff that happens as a result of shifting in the habitable zones:

• Intensity of sunlight
• likelihood of having a moon drops/raises the closer/further from a large body you are

That's about as much as I know, and you'd have to get into calculations and other stellar bodies (like gas giants and asteroid belts) to be significantly more specific than this

• 0.1 AU to 10 Au seems huge for the Sun' s habitable zone. That's from a quarter the distance of Mercury out to Saturn. Mar 22, 2019 at 19:00
• That's because I misquoted the value, it's actually 0.38 AU to 10 AU. Not much better, now that I think about it. I think that if Venus wasn't a toxic nightmare it would be right about perfect for human habitation though Mar 22, 2019 at 19:08
• Fair enough. The edge cases in linked the article made assumptions I'd never seen before in my casual reading. I'd probably recommend specifying the most common working definition , which wiki lists as 0.95- 1.37 au (Kasting et Al, 1993) Mar 22, 2019 at 19:16
• Good point, and with a bit more searching I found an update from 2013 with 0.95-1.67 AU Mar 22, 2019 at 23:16

Cool your planet by reducing the greenhouse effect.

Instead of moving your planet farther out to make it cooler, adjust its atmosphere. Just as we hear all the time about global warming from more greenhouse gas (CO2), if you decreased greenhouse gases you would get global cooling. Without a little greenhouse effect, the planet will radiate away more of the solar energy that hits it.

https://www.giss.nasa.gov/research/briefs/ma_01/

These gases are commonly referred to as "greenhouse gases" because they let in most of the incoming solar radiation that heats Earth's surface, yet prevent part of the outgoing thermal radiation from escaping to space, thus trapping some of the surface heat energy. Water vapor is also a major natural greenhouse gas, but its volatility, i.e., readily evaporating and condensing in response to temperature changes, complicates its role. Increases in the amount of atmospheric water vapor, under warmer conditions, reinforces the heat absorption by the other greenhouse gases. On the other hand, more clouds may form, as a consequence of increasing amount of atmospheric water vapor. Clouds can provide either a positive or a negative feedback by trapping outgoing thermal radiation or increasing the amount of solar radiation reflected back to space, respectively. At present, roughly 30% of the incoming solar radiation is reflected back to space by the clouds, aerosols, and the surface of Earth. Without naturally occurring greenhouse gases, Earth's average temperature would be near 0°F (or -18°C) instead of the much warmer 59°F (15°C).

Emphasis mine. Relevant gases would be CO2, methane, nitrous oxides and water.

Alternatively you could warm your planet by increasing the greenhouse effect. Mars is farther from Sol than Earth, but apparently had liquid water back when it had an atmosphere - presumably because the atmosphere provided some greenhouse effect to trap heat on the surface. I am not sure how hot you can get with the greenhouse effect but Venus is crazy hot due to its thick, thick atmosphere. If you moved Venus out to the orbit of Neptune, maybe it would be pretty nice. I think for a fiction you could safely assert this.

Nobody knows.

Go to the Wikipedia article "Circumstellar habitable zone" https://en.wikipedia.org/wiki/Circumstellar_habitable_zone1

And look at the table called Estimates of the circumstellar habitable zone boundaries of the Solar System. Note the vast, vast differences in the estimated inner and outer edges of the habitable zone of the Sun, and the vast differences in the estimates of the width of that zone.

So after that preliminary research you can do as much more research as you want to, perhaps starting by looking up the articles where various estimates of the habitable zone of the Sun were published and studying the methods used to estimate it.

And you can look up other discussions of the size of the habitable zone of the Sun on the internet.

And you can find other discussions of stellar habitable zones in this site.

Or if you want to be conservative you can always take the narrowest estimation of the habitable zone of the Sun, the one by Hart et al. in 1979, adjust it for the relative luminosity of your fictional star, and put your fictional planet in that narrow habitable zone.

Of course that zone seems much too narrow to contain two habitable planets if one goes by the relative sizes of known planetary orbits, though large enough to contain several habitable planets if one goes by the smallest known absolute distances between exoplanet orbits.

And on the other hand you might want to use a very optimistic estimate of the habitable zone so that your habitable planet can be as far away from its star as possible. I think that the most optimistic outer edge of the Habitable zone in the table is that by Pierrehumbert & Gaidos in 2011.

The average temperature on Earth is 14C/57F. Water freezes at 0C/32F. So you don't have much room to play with if you want to keep fresh water from freezing. You can not alter the atmosphere much, and still have humans able to breathe it. Earth could not be much further out from our sun as still support life as we know it.

• You could have elevated levels of CO2, up to ~5% (at about 6% its becomes toxic) or Methane, which has a greenhouse potential 104 times that of CO2. I am not sure on what atmospheric concentrations of methane would begin to become toxic, but if you had anything above 5000 PPM it would have a significant solar energy retention and the population could possible adapt to that concentration. Mar 22, 2019 at 23:14