# Planet Year

Kelzan
How far from the sun would a planet of roughly Earth's mass have to be from its sun to have a year the length of two Earth years and what type of star would be needed for it to be in its system's habitable zone? Or is this widely variable?
Would the star of such a planet have a lifespan long enough for complex life to evolve?

## Answers and Replies

Simon Bridge
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Kepler is the man you want:
http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html

... from the distance, you'll get how much hotter than the Sun it would have to be (i.e. 2x radius = 4x hotter) which tells you the star type. There are also tables for goldilocks zones for different star types.

Life like us seems to take 4 or so billion years ... you can check lifetimes on the main sequence assuming that's typical.

SteamKing
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In the solar system, the orbital period of Mars is almost twice that of earth (687 days v. 365 days).

You can find out how far Mars is from the Sun here:

http://en.wikipedia.org/wiki/Mars

It's possible for a star to be hotter than our sun without being more massive and speeding up the orbits. This will happen later in our sun's lifetime - but might not last more than a few million(?) years. Fine for terraformers, but no good for life to evolve.

Kelzan
In the solar system, the orbital period of Mars is almost twice that of earth (687 days v. 365 days).

You can find out how far Mars is from the Sun here:

http://en.wikipedia.org/wiki/Mars

Thank you.
I know the planet distances and that Mars is outside the habitable zone. But does it's lower mass change the time it takes to orbit around the sun? If an planet with Earth's mass were there instead would it have the same year as Mars?

Kelzan
It's possible for a star to be hotter than our sun without being more massive and speeding up the orbits. This will happen later in our sun's lifetime - but might not last more than a few million(?) years. Fine for terraformers, but no good for life to evolve.

Unfortunately the planet in my story is not terraformed, there are no aliens but there is complex life.
The planet has milder seasons because its axial tilt is less than Earth's and it has a 2 year revolution around its sun.
It's not a huge part of the story so I can change the year if I have to but I wanted to be accurate so that's why I asked.

Borek
Mentor
But does it's lower mass change the time it takes to orbit around the sun? If an planet with Earth's mass were there instead would it have the same year as Mars?

Have you checked the page about Kepler's law, the one Simon linked to? It addresses (between other things) exactly this question.

SteamKing
Staff Emeritus
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Thank you.
I know the planet distances and that Mars is outside the habitable zone. But does it's lower mass change the time it takes to orbit around the sun? If an planet with Earth's mass were there instead would it have the same year as Mars?

Mars is currently not habitable for reasons other than its location relative to the sun. The lack of a substantial atmosphere means that almost nothing could survive on the Martian surface due to the cold temperatures and the lack of protection from solar radiation.

OTOH, Venus, although located closer to the sun than earth, is uninhabitable for the opposite reason: it has too much atmosphere, and this traps incoming solar radiation, preventing heat from re-radiating back into space. High atmospheric pressure combined with blistering temperatures at the surface mean that no known biological organisms could survive these environmental extremes. Venus is also whack in that its 'day' is actually longer than its 'year':

http://en.wikipedia.org/wiki/Venus

The duration of the year for a given planetary body is not related to its mass, but to the mass of the sun and the distance of the planet from the sun.

Mars is currently not habitable for reasons other than its location relative to the sun. The lack of a substantial atmosphere means that almost nothing could survive on the Martian surface due to the cold temperatures and the lack of protection from solar radiation.
The bigger problem is dryness - water would tend to freeze or evaporate.
OTOH, Venus, although located closer to the sun than earth, is uninhabitable for the opposite reason: it has too much atmosphere, and this traps incoming solar radiation, preventing heat from re-radiating back into space. High atmospheric pressure combined with blistering temperatures at the surface mean that no known biological organisms could survive these environmental extremes.

The problem is not the amount of atmosphere but its composition. Carbon dioxide traps incoming sunlight.
If Venus had dense atmosphere that did not trap sunlight, for example 90 bar but all nitrogen, then the surface of Venus would be cooler and better for life. After all, there is a lot of life underneath 900 m seawater - same high pressure as on Venus, but livable cool temperatures. Man cannot live in 90 bar nitrogen, he would get nitrogen narcosis, but man would also drown in 900 m water, and fish for example don´t.

SteamKing
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The bigger problem is dryness - water would tend to freeze or evaporate.
Even if there were large quantities of water present on Mars, the extremely low atmospheric pressure would prevent this water from existing in the liquid phase. You want to make Mars wetter, first raise the ambient pressure at the surface.

The problem is not the amount of atmosphere but its composition. Carbon dioxide traps incoming sunlight.

Venus has too much atmosphere, regardless of the temperature at the surface. The mass of the atmosphere of Venus is approximately 90 times the mass of the earth's atmosphere.

If Venus had dense atmosphere that did not trap sunlight, for example 90 bar but all nitrogen, then the surface of Venus would be cooler and better for life. After all, there is a lot of life underneath 900 m seawater - same high pressure as on Venus, but livable cool temperatures.
It's not clear if the life found in the depths of earth's oceans evolved there or evolved elsewhere and then migrated and adapted to its present environs.

In any event, the presence of large amounts of sulfur dioxide in the atmosphere of Venus produces a highly corrosive environment of sulfuric acid when mixed with any water vapor swirling around.

Man cannot live in 90 bar nitrogen, he would get nitrogen narcosis, but man would also drown in 900 m water, and fish for example don´t.

Man would asphyxiate in an atmosphere of 1 bar nitrogen. You need oxygen to sustain life.

Even if there were large quantities of water present on Mars, the extremely low atmospheric pressure would prevent this water from existing in the liquid phase.
The atmospheric pressure in Mars closely matches, and slightly exceeds, triple point of water.
You want to make Mars wetter, first raise the ambient pressure at the surface.
If large quantities of water were present on Mars, they would evaporate, causing the ambient pressure to rise since the atmosphere would start to consist mostly of steam.

But is it likely that there were large quantities of water present on Mars?

There are valleys present on Mars which plainly were eroded by large amounts of some liquid flowing. Specifically the dendritic valley networks must have been eroded over long time by runoff originating over extended areas.

What kind of liquid was it? Liquid carbon dioxide? Its triple point pressure is 5,1 bar. Or water?

Precisely what was the atmospheric pressure and composition on Mars when dendritic valley networks were flowing?
In any event, the presence of large amounts of sulfur dioxide in the atmosphere of Venus produces a highly corrosive environment of sulfuric acid when mixed with any water vapor swirling around.
Yes, and in even slightly cooler climate that sulphuric acid would rain to ground and be quickly neutralized by corroding the rocks.
Man would asphyxiate in an atmosphere of 1 bar nitrogen. You need oxygen to sustain life.
True. But a man will lose consciousness and die in 10 bar nitrogen mixed with the 0,2 bar oxygen.

SteamKing
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It's not clear why you are worried so much about nitrogen. The atmosphere on Venus is about 96.5% CO2 and only 3.5% N2.

Although Venus receives additional solar radiation from the sun because it is closer than the earth, the composition of its atmosphere keeps the surface temperature elevated because it traps the incoming solar radiation and prevents this heat from re-radiating back into space. The atmosphere of Venus has been cited as an example of a 'greenhouse effect' run amok. So much heat is trapped by the atmosphere of Venus, its surface temperature is higher than that of Mercury, even though the latter planet is much closer to the Sun and Venus receives on 25% of the solar radiation that Mercury does.

It appears that a large part of the water on Mars is now frozen in its polar ice caps. It's not clear if Mars ever had enough of an atmosphere to allow CO2 to exist in the liquid phase.

It's not clear why you are worried so much about nitrogen. The atmosphere on Venus is about 96.5% CO2 and only 3.5% N2.

Although Venus receives additional solar radiation from the sun because it is closer than the earth, the composition of its atmosphere keeps the surface temperature elevated because it traps the incoming solar radiation and prevents this heat from re-radiating back into space.
And my point is that if the atmosphere of Venus were 100 % N2, but the same total pressure of 92 bar, Venus would be much cooler.
It appears that a large part of the water on Mars is now frozen in its polar ice caps. It's not clear if Mars ever had enough of an atmosphere to allow CO2 to exist in the liquid phase.

So if Mars were warmer, would this cause large change in atmospheric composition and pressure as the polar ice caps evaporate into an atmosphere much denser than the existing one, and consisting mostly of water vapour?

SteamKing
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And my point is that if the atmosphere of Venus were 100 % N2, but the same total pressure of 92 bar, Venus would be much cooler.

Obviously, if you change the composition of Venus' atmosphere, and remove the gases responsible for its greenhouse effect, a lower equilibrium atmospheric temperature should result, after the trapped heat is permitted to radiate into space. The trick is, getting rid of all that CO2.

So if Mars were warmer, would this cause large change in atmospheric composition and pressure as the polar ice caps evaporate into an atmosphere much denser than the existing one, and consisting mostly of water vapour?

The Martian atmosphere, what little there is, appears to be almost entirely CO2.

Since Mars no longer has a magnetosphere, the solar wind interacts directly with the upper atmosphere, stripping it away. There's nothing to indicate that the already ultra-low atmospheric pressure on the surface of Mars won't get lower over time.

If the Martian atmosphere did become thicker for some reason, this same process would gradually recur to strip gases from the upper atmosphere unless a means were found to replace the lost atmospheric mass.

Perhaps this is already occurring on Mars to some extent, and the atmosphere has reached a point of long-term stability. There appears to be plenty of water distributed in the crust of the planet outside of the polar regions.

The low atmospheric pressure on Mars indicates a low total atmospheric mass. CO2 on Mars is the same substance responsible for Venus' extreme climate conditions, but there is too little CO2 in Mars' atmosphere to trap sufficient solar radiation to warm it up much. (Mars gets only 43% of the solar radiation that earth does due to its distance further from the sun.)

The low atmospheric pressure on Mars indicates a low total atmospheric mass. CO2 on Mars is the same substance responsible for Venus' extreme climate conditions, but there is too little CO2 in Mars' atmosphere to trap sufficient solar radiation to warm it up much.

And yet, these 7 mbar of carbon dioxide on Mars is 18 times the pressure of carbon dioxide in the atmosphere of Earth. And, since Mars has a weak gravity of about 0,37 g, that weight means the mass of carbon dioxide in any column between Martian surface and Sun is 50 times bigger than on Earth.