# Help in determining the features of a fictional planet

I'm writing a sci-fi novel about a planet outside of the Solar System. The planet can support human life and is similar to Earth in many ways (1G gravity, Nitrogen-Oxygen-Carbon atmosphere, soil and liquid water, plants). The main differences are that its orbital period is about 120 years and winters are very cold.

I'd appreciate ideas, insights and comments on the following:

1. If the temperatures in winter (which lasts for 30 years) are about -500°C (-868 F) and in summer they rise to 20-25°C (68-77 F), is it possible that spring and autumn are also quite warm (around -20 to 10°C (-4 to 50 F)) and not just take the average of -240°C (-400 F)? Basically, can winter start abruptly or the change must be gradual?

2. Will there necessarily be winter in one hemisphere while it's summer in another?

Also any ideas about the differences of season change in prolonged periods will be much appreciated.

P.S. Sorry for my English, it's not my native language.

• Hi there - have you visited Worldbuilding Stack Exchange? They are likely to be much more helpful - your question is off topic for this site. Dec 7, 2021 at 18:20
• The temperature figures you quote are impossible, "-500°C" can not exist as it would be below absolute zero. The orbital period/winter length are not consistent with any known orbital pattern. Could you A) Narrow it down to a single focussed question as per our help center, and B) Clarify what you are trying to achieve, as at present you've asked the impossible. Dec 7, 2021 at 18:29
• Might need another planet to shield it from the sun if you want it any colder than... -220 to -240 degrees C (Neptune/Uranus/Pluto). Dec 7, 2021 at 18:43
• Absolute zero, or -273 degrees Celsius, is assumed to be the absolute coldest, a stage where all particles stop moving entirely. As far as I'm aware negative absolute temperatures aren't 100% impossible in theory, but seem to be so in the scale you want them to be unless you start tempering with how certain laws work (or rather don't work) as they should in your planet (entering deep into the fi part of Sci fi) . Dec 7, 2021 at 19:14
• @ProjectApex the TL;DR on negative temperatures is that they're incredibly hot - so hot you can't add any more heat to it. so no, you might be getting below 0K, but you're never getting colder than 0K. Dec 7, 2021 at 19:20

Elliptical orbit.

https://en.wikipedia.org/wiki/Elliptic_orbit

Winter is when your planet is far from its sun. Winter might be longer than the other seasons. Look at the pink planet in the gif and use the red one as a clock. The pink planet with the most elliptical orbit spends the most time in the regions more distant from the sun, because it is moving slower out there. If it has a 120 Earth year orbit, winter might be 90 years. And summer on Pink is going to be seriously warm.

On our world with an orbit that is nearly circular, axial tilt is what makes the seasons because one hemisphere is relatively more distant from the sun than the other. On a planet with an elliptical orbit distance from the sun is going to make more difference than axial tilt.

L.Dutch asked me to do some calculations. I am flattered. But instead I will do comparisons. Let us consider Halleys comet.

https://www.ucl.ac.uk/~zcape78/CometHalley.html

The orbit of the Halley’s Comet is elliptical with a high eccentricity, which is equal to 0.97, compared to 0.0167 for the Earth, or in other words, the major axis of the ellipse is about four times greater than the minor axis.

At perihelion Halleys comes to 0.59 AU to the sun (Earth being 1 AU from the sun) and aphelion it is 35 AU away from the sun.

It gets nice and warm on the comet at 1 AU.

https://www.space.com/29024-how-to-live-on-a-comet.html

When the Giotto spacecraft visited Halley's Comet in 1986, the comet was 0.9 AU from the sun and had a surface temperature of about 170 degrees Fahrenheit (77 degrees Celsius).

170F is a little warmer than Portland, but not boiling water weather. Of course the comet gets closer to the sun than 0.9. But moving on...

As far as how cold, Neptune is at 30AU and Neptune gets to -225C, absolute zero being -273. So 35AU is colder than -225C. But Pluto is at 40AU and only -232 so probably the comet only gets a little colder than Neptune.

Thus my calculation: an eccentricity of 0.97 to produce the warm summer and cold winter requested.

Dutch I am going to keep working on that paean because you will earn it someday I am sure.

• Can you calculate the eccentricity needed to achieve the requested -500°C?
– L.Dutch
Dec 7, 2021 at 18:40
• @L.Dutch - no, for a couple of reasons like no such thing as -500C (which I take to mean "very cold") and no idea of how hot the sun is. But if you can calculate it I will write a poem in your honor. More of a paean really. It will be a good one too; I actually already have the beginning worked out. Dec 7, 2021 at 19:08
• OK I take it back. I will put something up. Dec 7, 2021 at 19:13
• Re: "On our world with an orbit that is nearly circular, axial tilt is what makes the seasons because one hemisphere is relatively more distant from the sun than the other": That's not quite right. The orbit is indeed nearly circular, but the difference in distances between perihelion (closest to the Sun, in January) and aphelion (furthest from the Sun, in July) is nonetheless much greater than the difference in distances between the two hemispheres at any given time. The reason axial tilt makes the seasons is the angle itself: less solar energy per area. [continued] Dec 8, 2021 at 8:22
• [continued] To get an intuitive sense of this, try pointing a flashlight directly downward onto a horizontal piece of paper (simulating when the Sun is overhead) and then lifting the paper at an angle so it's no longer horizontal. You'll see that the paper is much more brightly lit in the former case. Dec 8, 2021 at 8:24

A binary star system

Helliconia is a fictional example of a planet that shares the characteristics that you want.

Helliconia orbits Batalix in 480 days; this is called the "small year". Each day of the small year comprises 25 hours, each of 40 minutes, which in turn are each 100 seconds long. Helliconia and Batalix's orbit around Freyr, the "great year", is highly elliptical and takes approximately 1,825 small years, equating to some 2,592 Earth years.[8] At periastron Batalix is 236 astronomical units from Freyr, whilst at apastron is 710 AU distant.[9] A Helliconian week is eight days. There are six weeks in a tenner, and ten tenners in a Helliconian small year.[10] While seasonal changes in the small year are slighter than those of Earth, the long seasons of the great year are much more marked. When distant from Freyr, Batalix's illumination is sufficient only to maintain ice-age conditions. However, Freyr's output is many times greater than Batalix's, so as Helliconia approaches Freyr, the tropics of Helliconia become hotter even than the tropics of Earth.

Your planet seems to be a little bit on the impossible side.

Part One.

You want you planet to be habitable for humans, and yet you want it to experience really extreme temperature changes which might exterminate all natural life on the planet. Including the plants that take carbon dioxide out of the air and convert it into the oxygen in the air.

Of course humans with sufficiently advanced technology could live on your planet. But the techiques they would need to shield themselves from the temperature swings would insulate them from the planetary enviromental conditions so much that they might as well living in an artifical space habitat.

Part Two:

You want the length of the planet's year to be 120 Earth years.

The length of a planet's year, defined as its orbital period around the planet, will depend on various factors. The equation to calculate the length of a planet's orbital period includes the mass of the star, the mass of the planet (which will almost always be so small comapred ot that of fhe star it can be ignored), and the distance between them.

Stars have different luminosities. A star with a specific luminosity will have a specific distance where a planet will receive exactly as much light and heat from the star as Earth gets from the Sun, and so (other factors being equal) would have exactly the same average temperature as Earth. I call that orbital distance the Earth Equivalent Distance or EED.

And the circumstellar habitable zone around the star will extend inward toward the star and outward from the star around the EED. The circumstellar habitable zone is where a planet would receive enough radiation from the star for water to be liquid on the surface of the planet. If other factors are good, a planet in the habitable zone can be habitable for life.

So how wide is the circumstellar habitable zone around the Sun?

The various estimates seen here:

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates

Show there is considerabledisagreement on wide the Sun's circumstellar habitable zone is.

To be safe, some writers might want to put their habitable exactly at the EED of their star to be certain that it is within the star's circumstellar habitable zone.

The average orbital distance of the Earth from the Sun is one Astronomical Unit (AU).

If a star has X times the luminosity of the Sun, its EED orbit will be at 1 AU times the square root of X.

If the star is 4 times as luminous as the Sun, its EED will be at 2 AU from the star.

If the star is 9 times as luminous as the Sun, its EED will be at 3 AU from the star.

If the star is 16 times as luminous as the Sun, its EED will be at 4 AU from the star.

If the star is 25 times as luminous as the Sun, its EED will be at 5 AU from the star.

If the star is 100 times as luminous as the Sun, its EED will be at 10AU from the star.

If the star is 1,000 times as luminous as the Sun, its EED will be at 31.6227 AU from the star.

If the star is 10,000 times as luminous as the Sun, its EED will be at 100 AU from the star.

If the star is 100,000 times as luminous as the Sun, its EED will be at 316.227 AU from the star.

If the star is 1,000,000 times as luminous as the Sun, its EED will be at 1,000 AU from the star.

The luminoisity of stars - at lest the main squence stars - depends on their mass. A difference in the mass of two stars of a specific percentage will cause a difference in their brightness which is significantly greater. The most massive stars are thousands of times as massive as the least massive stars, but are millions of times as luminous.

There are a few known planets which orbit their stars at distances of hundreds of AU, and which have years hundreds of thousands, or even about a million, Earth years long.

And there are stars with the right luminousity and mass that a planet with a year 120 Earth years long could orbit within their habitable zone.

So everything seems fine for the planet in your story.

Except.

Part Three.

It took billions of years for life on Earth to develop enough for planets to produce an oxygen rich atmosphere that multicelled animals, such as humans, could breath.

The Sun shone with a fairly steady brightness for those billions of years - otherwise it would have made Earth too hot or too cold and all life would have died.

For a planet to become habitable, it has to have existed for billions of years, and its star has to have shown with a fairly steady luminosity for all those billions of years.

Unless the writer wants to make the planet much younger than a naturlly habitable planet, which an advanced civilziatin has terraformed to become habitable with advanced technology.

And as it turns out, only some stars, main sequence stars similar to the Sun with a rather small range of mass and luminosity can shine with se teady luminosity for the billions of year necessary for a planet to naturally become habitable for humans.

Stephen H. Dole, in Habitable Planets for Man, 1964, discusses the types of stars suitable for having planets habitable for humans on pages 67 to 72.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

And to paraphrase a gambler, "read it and weep". many science ficiiton writers and readers have found those "Dole full" limitations on the types of stars which could have naturally habitable planets very frustrating.

Take a spectral class F2V star, which is about at the uppermass limit according to Dole, and which some astrbiologists would consider to be too massive to have a habitable planet. According to the table at:

https://en.wikipedia.org/wiki/F-type_main-sequence_star

an F2V star would have a mass 1.46 the mass of the Sun and a luminosity 5.13 the luminosity of the Sun. With 5.13 the luminosity of the Sun, it would have an EED at a distance of about 2.265 AU. I found an online orbital period calculator.

https://www.calctool.org/CALC/phys/astronomy/planet_orbit

Entering the mass of the stars as 5.13 suns, the mass of the planet as 1 Earth, and the semimajor axis as 2.265 AU, I get an orbital period of about 2.82066 Earth years.

It is possible that an Earth mass planet in the orbit of Mars might be habitable. The orbit of Mars has a semimajor axis of about 1.523 AU. So the Mars Equivalent Distance, or MED, of an F2V star should be 1.523 times 2.265 AU, or 3.449 AU. And that would give the planet an orbital period of 5.30016 Earth years.

Vega is a class A0Va star, with 2.135 the Sun's mass, and about 40 times the Sun's luminosity. So its EED and MED should be 6.324 times as far as the Sun's, at 6.324 AU and 9.6322 AU. The orbital period of a planet at Vega's EED would 10.8821 Earth years and at Vega's MED would be 20.4557 Earth years.

And Vega's calculated lifetime on the main sequence is only about a 10th that of the Sun's, and so roughly a billion years.

Component C of Beta Scorpii would be a class B star to try. It is spectral class B2V, 8 times the mass of the Sun, and 3,200 times the Sun's luminosity. It's EED would be 56.568 times that of the Sun, at 56.568 AU and its MED would be at 86.153 AU, with orbital periods of 150.396 and 282.674 Earth years.

The main star of Algol, Beta Persei Aai, has spectral class B8V, a mass of 3.17 Suns and a luminosity of 182 Suns. Its EED and Med would be 13.49 times a far as the Suns, at 13.49 and 20.546 AU, with orbital periods of 27.8236 and 52.2982 Earth years.

According to the table at:

https://en.wikipedia.org/wiki/B-type_main-sequence_star

A B5V class star would have a mass of 4.7 Suns and a luminosity of 589 Suns. So its EED would be 24.269 AU and its MED would be 36.962 AU, with orbital periods of 55.1385 and 103.636 Earth years.

A B4V class star would have a mass of 5.1 Suns and a luminosity of 776 Suns. So its EED would be 27.856 AU and its MED would be 42.425 AU, with orbital periods of 65.0907 and 122.341 Earth years.

A B3V class star would have a mass of 5.4 Suns and a luminosity of 977 Suns. So its EED would be 31.257 AU and its MED would be 47.823 AU, with orbital periods of 75.1882 and 142.293 Earth years.

A B2V class star would have a mass of 7.3 Suns and a luminosity of 2,692 Suns. So its EED would be 51.884 AU and its MED would be 79.02 AU, with orbital periods of 138.298 and 259.938 Earth years.

So presumably a B2.5V class star would be about right to have a planet with a period of 120 Earth years orbiting between its EED and its MED.

Class B stars stay on the main sequence for only about a few tens of millions of years.

So a writer who wants a habitable planet orbiting one of them could have an advanced civilization terraform the planet to make it habitable. It might take that civilization thousands of years, but that might be considered worthwhile if the planet remains habitable for millions of years. And possibly after the terraforming civilization abandons the planet a native lifeform might develope intelligence and civilization and possibly develop spaceflight in time to escape, or humans might decide to settle there despite there being only a hundred thousand years before disastrous stellar changes.

Or you could take a look at the Mohs Scale of science Fiction Hardness and decide that you will be content with your story having a scale of only 1, and not worry abut the astronomical plausibiity of your system.

https://tvtropes.org/pmwiki/pmwiki.php/Main/MohsScaleOfScienceFictionHardness

Part Four:

Obviously if the planet has a year 120 Earth years long, the astronomical seasons will each be 30 years long. The meteorological seasons might be longer or shorter in various places on the planet. But such long seasons should cause each hemispehre to get colder and colder and colder during the winter, and hotter and hotter and hotter during the summer. It might be impossible for humans, or for any lifeforms at all, to survive such temperatures, even though they should vary much less than you stipulated in the question.

If the planet has a very eccentric orbit, that could cause seasons which were the same on both hemispheres of the planet at the same time. And the temperatures could get very cold during winter and very hot in the summer. But I doubt that anything could make them reach the extremes specified in the question.

And you might be interested in questions and answers about making seaons on a planet last much longer than its years.

How does a Game of Thrones-style hyperwinter occur?

## Why?

First, -500 Celsius is impossible, because the lowest possible temperature is absolute zero (-273.15 C). Let's assume the planet's winter temperature is 10 Kelvin. Your atmosphere would freeze and fall to the surface as snow. Liquid water would boil away as there is no pressure to keep it liquified, thus killing all life. Also, a planet will spend much more time at aphelion due to Kepler's third law, meaning that winter would last more than 80-90 years.

## What planet would you need?

You would need a planet in an eccentric orbit orbiting a binary star.

Why both, and not either???

Is it possible that spring and autumn are also quite warm?

Basically, can winter start abruptly or the change must be gradual

Spring and autumn are mainly transitional phases between summer and winter. For the transition to happen abruptly, you probably would need an eclipsing binary system with a period of half the period of the planet. The planet is warmest when it is at perihelion, where the more massive star eclipses the lower mass star. The planet grows colder slowly as the stars move apart, adding more sunlight, but the planet moves away and gets colder (autumn, spring but switched). When the planet goes to winter (at aphelion) the stars eclipse again, making it as cold as possible. Therefore, winter can start abruptly, but in a gradual manner (it can start quickly if the atmosphere freezes quickly)

The planet can support human life... plants.

No. Life is very unlikely because it would only have about 20ish years to develop and reproduce, then die out. The only way for a species to survive was if they lived in a 120-year cycle where they remain dormant underground as "seeds" during winter and live their short 20 year life to reproduce, then die-off as the atmosphere freezes over. Technically, humans could survive in spacesuits near aphelion. Major terraforming would have to occur for such a planet to be habitable (forced warming of the atmosphere to keep the planet warm during winter)

If you'd like specifications for such a planetary system, please let me know.