Rotation of planets influences habitability

August 8, 2014 by Amanda Doyle,
In astronomy, a habitable zone is a region of space around a star where conditions are favorable for life as it may be found on Earth. Planets and moons in these regions are the likeliest candidates to be habitable. Our sun has a temperature of about 5800K. For stars cooler than our sun (M dwarfs, also known as red dwarfs, at 3000-4000K) the region is closer in. For hotter stars (A dwarfs at 10,000K) the region is much farther out. Credit: NASA

There are currently almost 2,000 extrasolar planets known to us, but most are inhospitable gas giants. Thanks to NASA's Kepler mission, a handful of smaller, rockier planets have been discovered within the habitable zones of their stars that could provide a niche for alien life.

The habitable zone of a star is typically defined as the range from a star where temperatures would allow liquid water to exist on the surface of a planet. At the inner edge of this zone, the star's blistering heat vaporizes the planet's water into the in a runaway greenhouse effect. At the outer edge of the habitable zone, temperatures are so cold that clouds of carbon dioxide form and the little solar energy that does arrive bounces off the clouds, turning the planet into a frozen wasteland.

However, this concept is rather simple. In reality, many other factors come into play that could affect a planet's habitability. New research has revealed that the rate at which a planet spins is instrumental in its ability to support life. Not only does rotation control the length of day and night, it can also tug on the winds that blow through the atmosphere and ultimately influence cloud formation.

The paper has been accepted to Astrophysical Journal Letters and a preprint is available online at Arxiv.

Air circulation and rotation rates

The radiation that the Earth receives from the Sun is strongest at the equator. The air in this region is heated until it rises up through the atmosphere and heads towards the poles of the planet where it subsequently cools. This cool air falls through the atmosphere and is ushered back towards the equator. This process of atmospheric circulation is known as a Hadley cell.

If a planet is rotating rapidly, the Hadley cells are confined to low latitudes and they are arranged into different bands that encircle the planet. Clouds become prominent at tropical regions, which are important for reflecting a proportion of the light back into space. However, for a planet in a tighter orbit around its star, the radiation received from the star is much more extreme.

This will decrease the difference between the equator and the poles and ultimately weaken the Hadley cells. The result is fewer clouds at the tropical regions available to protect the planet from the intense heat, and the planet becomes uninhabitable.

Rotation of planets influences habitability
The traditional habitable zone is outlined in blue, showing that Venus is currently well outside of the zone. However, for slowly rotating planets under the right atmospheric conditions, this zone will be extended so that it is much closer to the star. Image Credit: NASA

If, on the other hand, the planet is a slow rotator, then the Hadley cells can expand to encompass the entire world. This is because the atmospheric circulation is enhanced due to the difference in temperature between the day and night side of the planet. The days and nights are very long, so that the half of the planet that is bathed in light from the star has plenty of time to soak up the Sun. In contrast, the night side of the planet is much cooler, as it has been shaded from the star for some time.

This difference in temperature is large enough to cause the warm air from the day side to flow to the night side, in a similar manner as opening a door on a cold day results in warm air fleeing from a room. The increased circulation causes more clouds to build up over the substellar point, which is the point on the planet where the star would be seen directly overhead, and where radiation is most intense. The clouds over the substellar point then create a shield for the ground below as most of the harmful radiation is reflected away.

The high albedo clouds can allow a planet to remain habitable even at levels of radiation that were previously thought to be too high, so that the inner edge of the habitable zone is pushed much closer to the star.

"Rotation can have a huge effect, and lots of planets that we previously thought were definitely not habitable now can be considered as candidates," says Dorian Abbot of the University of Chicago, and a co-author on the paper.

Earth in Venus' orbit

The study used computer simulations to show that a slowly rotating planet with the same atmospheric composition, mass, and radius of the Earth could potentially be habitable even at Venus' distance from the Sun. Under the typical boundaries of a habitable zone, Venus is situated closer to the Sun than the inner edge of the zone. In the study, the simulated planet received almost twice as much radiation as the actual Earth did, and yet the surface temperature was cool enough for life to thrive due to the shielding clouds.

Despite the slow rotation, Venus itself is actually a scorching hot planet with a atmosphere so dense that it would crush a person on the surface in seconds. This goes to show that just because a planet is rotating slowly does not automatically mean that it is habitable, rather it has the potential to be habitable if the right conditions exist.

For instance, it is possible that Venus used to spin much faster, giving shorter days than it has now. Venus' atmosphere is enriched with deuterium, which indicates that an ocean might have once been present. Such a rapid rotation rate on a planet so close to the Sun would have led to a runaway greenhouse effect and the loss of the oceans. By the time the rotation of the planet slowed to its current rate the damage was irreversible.

A Hadley cell is created when warm air rises at the equator and moves to the poles. The air then cools, sinks, and heads back towards the equator. Credit: Lyndon State College Atmospheric Sciences

Finding the slow rotators

While it is difficult to measure planetary , future observations by the James Webb Space Telescope might be able to measure rotation if the right conditions were present. The James Webb Space Telescope is an infrared telescope due to launch in 2018, and it is capable of measuring the level of heat emitted by exoplanets.

The telescope would be able to measure the heat emitted from any high albedo clouds that are formed over the substellar point. An unusually low temperature at what is expected to be the hottest location on the planet could indicate that the planet is a potentially habitable slow rotator.

"From space, Earth looks like it is between -70 and -50 degrees Celsius over large regions of the western tropical Pacific because of high clouds there, even though the surface is more like 30 degrees Celsius," says Abbot.

A runaway greenhouse effect occurs when the stellar radiation becomes trapped in the atmosphere, the planet warms up, and the oceans evaporate. Credit: ESA

It is also known than many planets that orbit cool M dwarf are either tidally locked, meaning that the same side of the planet faces the star all the time, or they are slow rotators.

This research emphasises the importance of looking beyond the traditional habitable zone for planets that could host life, and it turns out that we once thought were too hot might actually be just right for life.

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1.5 / 5 (8) Aug 08, 2014
Well well, here we are on planet earth, all life has one common ancestor. How strange. And we expect it to be on a few rocky planets beyond. Why do we expect that when it arose in exactly one location here? Remember simple maths, there is the zero case, the one case, or the many. I bet both arms there is no life in the rest of the universe, because if there was, there would not be one common ancestor here, there would be many iterations on this oh so perfect little planet. In fact it would be arising spontaneously as I type in a remote deep thermal spring.
5 / 5 (6) Aug 08, 2014
Or maybe there were many ancestors here, but one managed to eat all of the rest.
Or, as some biologists have pointed out, there could be life here that our tests, looking for DNA, don't find.
Or maybe there is only one practical way to make life, but it arises in many places. In that case, it would look like a common ancestor, even if it wasn't.
Or maybe our life CAME from elsewhere. In that case, there could be many ancestors, but only one made it here.
5 / 5 (4) Aug 08, 2014
@erdave: Math has exactly _nothing_ to do with empirical processes, except it is invented to quantify for testing.

On to the science:

You should open an astrobiology 101 book. This is routinely discussed in the first chapters on life emergence. We know life (biological evolution) has common ancestors. We also know that it operates under differential reproduction (fitness), meaning there is competition. There could well be many original ancestors on a planet same as we see many today, but the most successful outcompeted the rest same as we see today. (E.g. extinct clades.)

You should also open a biology 101 book. Already Darwin knew why there is no emergence at the moment, because preexisting life tend to eat organics. We now also know that oxygen is lowering de novo organics production. Your arguments (math, spontaneous generation) is pre-science creationist. In case you are a creationist, why do you frequent science sites?

5 / 5 (5) Aug 08, 2014

In case you are not trolling anti-science, just oblivious:

I count at least four reasons why we expect to see life everywhere that it can arise.

First, it is a natural process since we know the universe started out sterilized (cold inflation, hot big bang - both killers) and now we observe life. If it happens once, it likely will happen (has happened) again.

Second, processes have to fiercely finetuned (unnatural) to result in singletons, unique objects. (Even inflation seems to give many universes.)

Third, chemical, geophysical and biological evolution is ubiquitous and seamless. We now know that geophysics spawns life by way of phylogenies.

5 / 5 (6) Aug 08, 2014

Fourth, life appeared fast on Earth. Earliest oceans are now known to have been present 4.4 billion years ago. First dated split, between bacteria and archaea, happened over 4 billion years ago. Since we can model life emergence as a stochastic process, even 1 observation is enough for control ability - it is informative. More precisely, the early date tells of a repeated and/or easy process,

I'll take your bet, if you put in two beers instead. We will know in 10-20 years, when astronomers have built the necessary telescopes to identify life. (Need to see O2, O3, H2O and CO2 spectroscopically to separate biotic processes from abiotic.)
5 / 5 (4) Aug 08, 2014
I am very pro science and appreciate your and nkalanaga responses. I read books trying to understand it years ago, and really got nowhere, took bio chem and physics to high school, a bit of tertiary. It all amazes me, Two beers it is, arms are quite useful.

5 / 5 (3) Aug 08, 2014
erdave: If yours was a serious question, I apologize, although my answers are still valid. I thought, apparently wrongly, that you were another of the ever-present trolls.

Textbooks don't always make things as clear as they could. I think some of the authors assume the students have more background than they do, and skip over some of the explanations that are "obvious" to those already in the field.
not rated yet Aug 09, 2014
variables such as the planets rotational speed and history are shown to make the difference between habitable earth in venus orbit and unhabitable earth in venus orbit. All from cloud formation. This leads to very scary possibilities of humans and anthropogenic warming (manmade global climate change) and the increases my estimate of the likelyhood that humanity actually could induce a runaway greenhouse scenario. I always thought just because of the long history of life on earth and my assumption we were in a habitable zone that greenhouse gasses released by man could only injure life on earth. Now I wonder if there are very real possibilities we could end life on earth for ever or extinct the human race from burning fossil fuels. This shows life is dependent on variables such as cloud formation that we are accidentally changing planet wide. very scary.
5 / 5 (1) Aug 09, 2014
ODesign: No, we can't cause a runaway greenhouse effect. There isn't enough carbon in the fossil fuels to do it. We can probably drive ourselves to extinction if we try, but we can't heat the Earth enough to cause the "cold trap" in the stratosphere to fail, so the oceans are safe. Most of the Earth's carbon is looked up in limestone and other carbonate minerals, and to get anywhere near Venus' atmosphere we'd have to release that. Baking the rocks could do it, but as long as the stratosphere stays cold, clouds will limit the temperature increase to well below that point.

Just the fossil fuels won't even destroy life on Earth. Bacteria and Archaea, at least, will survive, and quite possibly more advanced life. Also, civilization will probably collapse before we become extinct, and the survivors won't be burning the fossil fuels. Even now, much of what we're using would have been impractical to retrieve in the 1800s.
not rated yet Aug 09, 2014
I have time to propose that chirality and is it 4 amino acids? make all my ideas difficult to refute. I hope this thread continues. (My deceased dad 36 years ago was a professor of modal logic, so I was kinda raised a sceptic)
not rated yet Aug 09, 2014
You're right that we COULD be the only life in the universe. But as many planets as there it seems unlikely that it only happened here. The amino acids are fairly common in meteorites, and probably elsewhere in the universe, and when dealing trillions (at least) of potentially inhabitable planets, they probably came together somewhere else at least once.

As for chirality, that has been studied. Life as we know it has to be chiral, or the reactions won't work. If one assumes that the handedness doesn't matter, then it would seem likely that life started both ways on Earth. I read a paper once where the authors looked at the results of two types of life on the same world and determined that, if there was a single ocean, with no isolated basins, one form of life would almost inevitably dominate. Within a geologically short period of time it would be the only form to be found. As far as we know Earth's oceans have never been divided into two isolated basins for any extended period.
Whydening Gyre
1 / 5 (1) Aug 09, 2014
I have time to propose that chirality and is it 4 amino acids? make all my ideas difficult to refute. I hope this thread continues. (My deceased dad 36 years ago was a professor of modal logic, so I was kinda raised a sceptic)

Don't know about the numbers of amino acids, but I think you're deade on with the chirality aspect...
Whydening Gyre
1 / 5 (1) Aug 09, 2014
As for chirality, that has been studied. Life as we know it has to be chiral, or the reactions won't work. If one assumes that the handedness doesn't matter, then it would seem likely that life started both ways on Earth. I read a paper once where the authors looked at the results of two types of life on the same world and determined that, if there was a single ocean, with no isolated basins, one form of life would almost inevitably dominate. Within a geologically short period of time it would be the only form to be found. As far as we know Earth's oceans have never been divided into two isolated basins for any extended period.

I know, initially, they weren't, but - aren't they (relatively) separate now?
not rated yet Aug 10, 2014
Not in the sense the authors meant. They look separate, but it's still easy for life to spread from one place to another. An individual species usually doesn't today, at least on human time scales, without help, but that's because every place is already full of life. "Separate", as they meant it, was "little or no exchange of water between the basins". The Dead Sea and Great Salt Lake are examples of "separate" in this sense. Marine life can't travel between the two.

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