Lobster-shaped extrasolar oceans

Mar 13, 2014 by Charles Q. Choi
What the day side of a tidally locked exoplanet orbiting a red dwarf might look like, given atmospheric carbon dioxide levels similar to modern-day Earth. On the top frame, white represents ice while blue represents open water; on the bottom frame, colors represent surface air temperatures. The top image in each frame represents a computer model that does not take ocean heat flow into account; the bottom image in each frame does take such heat flow into account. Credit: Yongyun Hu

Alien planets circling the most common stars in the universe may often have strange lobster-shaped oceans on their surfaces, researchers in China now say.

These findings suggest the where life as we know it might dwell around these is smaller than previously thought, scientists added.

The most common type of star in the universe is the . These stars, also known as M dwarfs, are small and faint, about one-fifth as massive as the sun and up to 50 times dimmer. They make up to 70 percent of the stars in the cosmos, a vast number that potentially makes them valuable places to look for extraterrestrial life. Indeed, recent findings from NASA's Kepler space observatory reveal that at least half of these stars host rocky planets that are one-half to four times the mass of Earth.

Research into whether a distant planet might host life as we know it usually focuses on whether or not it has , since there is life virtually everywhere there is liquid water on Earth, even miles underground. Scientists typically concentrate on habitable zones, also known as Goldilock zones—the area around a star where it is neither too hot nor too cold enough for a planet to possess liquid water on its surface.

The habitable zones around red dwarfs are close to such stars because of how dim they are, often closer than the distance Mercury orbits the sun. This makes it relatively easy for astronomers to detect worlds in a red dwarf's habitable zone; since the orbits of these exoplanets are small, they complete their orbits quickly and often, and scientists can in principle readily detect the way these worlds dim the light of these stars by passing in front of them.

When a planet orbits a star very closely, the star's gravitational pull can force the world to become "tidally locked" to it. When a planet is tidally locked to its star, it will always show the same side to its star just as the moon always shows the same side to Earth, so that the planet will have one permanent day side and one permanent night side.

The uneven heating that tidally-locked planets experience could make them profoundly different from Earth. For instance, prior research speculated the dark sides of tidally-locked planets would become so cold that their atmospheres would freeze, leaving even the sunlit sides with little air. However, more recent models of atmospheric circulation have shown that winds on these planets would cause heat to flow enough to avoid this atmospheric collapse for terrestrial planets in the habitable zones around red dwarfs.

Recently astrobiologists suggested that tidally-locked exoplanets around red dwarfs might resemble giant eyeballs. Their night sides would be covered with icy, frozen shells, while their day sides would host giant oceans of liquid water constantly basking in the warmth of their stars.

However, planetary scientists Yongyun Hu and Jun Yang at Peking University in Beijing noted that past research into how tidally-locked exoplanets around red dwarfs might look did not consider the way in which heat might circulate within the oceans of such worlds.

Lobster-Shaped Extrasolar Oceans
Artist's impression of the planetary system around the red dwarf Gliese 581. Credit: ESA

Now these researchers find that when computer models account for the role that ocean heat transport can play, tidally locked exoplanets around red dwarfs might not resemble giant eyeballs at all. Instead, they might be mostly covered by icy crusts, save for somewhat lobster-shaped oceans on their day sides.

The simulations involved a computer model that comprehensively accounted for both atmospheric circulation and oceanic circulation and how they might influence one another on a planet orbiting a red dwarf about 5,660 degrees F (3,125 degrees C). The model used the same planetary parameters as that of an exoplanet called Gliese 581g located about 20 light years away, which may be the first known potentially habitable alien world—this world is a "super-Earth," a rocky planet 1.5 times wider than Earth. The researchers assumed the planet would have a global ocean about 13,125 feet (4,000 meters) deep, the average depth of Earth's oceans.

Because of the way ocean heat flows, the amount of open water on the day sides of these planets might be substantially larger than before thought. It also efficiently warms their night sides, preventing atmospheric collapse. If the starlight is bright enough, or if there are sufficiently high levels of heat-trapping greenhouse gases such as carbon dioxide, ocean heat flow could actually lead to a complete lack of ice on the planet's surface, even on the night side.

"This is the first work to demonstrate how a dynamic ocean can change the climate state of super-Earth-like exoplanets," Hu said.

Assuming this tidally locked exoplanet has roughly as much carbon dioxide in its atmosphere as modern-day Earth, it would have an ocean on its day side surrounded by ice. However, the computer model suggested this ocean would not be perfectly round like an eyeball's iris, but would rather have a vaguely lobster-like shape, with two "claws" on either side of the equator and a long "tail" along the equator.

"The lobster shape is created by ocean currents," Hu said.

It might be possible to see if these planets really do possess lobster-shaped oceans in the future, although current telescopes cannot do it, he added.

The claws are caused by ocean currents that rotate like cyclones, while the long tail is the result of something called a Kelvin wave, "which can be simply understood as a result of an equatorial ocean jet stream," Hu said.

Artistic representation of five known potential habitable worlds including Gliese 581g. Credit: The Habitable Exoplanets Catalog, PHL @ UPR Arecibo

Because the jet stream is eastward, it transports warm water from the day side to the night side, and transports cold water from the night side to the day side, explaining why the ocean is not symmetrical when one looks from east to west.

Although ocean heat flow suggests tidally-locked exoplanets orbiting red dwarfs might have more habitable open water on their surfaces than before thought, it could also mean that red dwarf habitable zones are actually narrower than previously suggested.

The scientists found that a dynamic ocean would be more likely to force planets to enter what is known as a runaway greenhouse effect. In this scenario, a planet absorbs enough heat from its star to cause too much water to vaporize. Steam is a greenhouse gas, and so that planet would trap even more heat from its star, causing still more water to vaporize. Eventually, all the water on that world boils away, rendering them uninhabitable—a phenomenon that could explain why Venus is so desolately dry today.

The researchers suggest this increased vulnerability to the runaway greenhouse effect means the inner edge of red dwarf habitable zones may be slightly farther away from these stars than previously thought, although Hu noted they were uncertain exactly how much smaller the habitable zones would be.

Astrobiologist Jim Kasting at Pennsylvania State University at University Park, Penn., who did not take part in this study, noted he would like to see the researchers calculate where the edges of habitable zones might be for tidally-locked exoplanets around red dwarfs. He would also like to see the scientists model the effects that a greater range of brightnesses from the stars would have on these planets.

Explore further: Researchers extend capabilities of computer simulation of tidally locked exoplanets

More information: Hu and Yang detailed their findings online Dec. 30 in the journal Proceedings of the National Academy of Sciences: www.pnas.org/content/111/2/629.full

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User comments : 12

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tadchem
not rated yet Mar 13, 2014
"Goldilock zones"? I thought it was "Goldilocks zones", given that the eponymous fairy tail character was named "Goldilocks."
GSwift7
5 / 5 (3) Mar 13, 2014
It looks like they are assuming banded flow in the ocean, but I wonder if a planet without much rotation would actually do that? A tidally locked planet will only rotate once per 'year'. IIRC that should be a matter of weeks for these planets. I wouldn't be too quick to apply the coriolis effect here (assuming that's what they did, which I realize may not be the case).

While I do appreciate this kind of work, I think these exoplanet simulations are very premature.
Caliban
5 / 5 (2) Mar 13, 2014
It looks like they are assuming banded flow in the ocean, but I wonder if a planet without much rotation would actually do that? A tidally locked planet will only rotate once per 'year'. IIRC that should be a matter of weeks for these planets. I wouldn't be too quick to apply the coriolis effect here (assuming that's what they did, which I realize may not be the case).

While I do appreciate this kind of work, I think these exoplanet simulations are very premature.


Agreed.

Plus, it appears that they are assuming a geodynamically dead world without any surface height variability, which would impose some constraints on the characteristics of any ocean found there, and therefore the global climate.
Captain Stumpy
3.7 / 5 (3) Mar 14, 2014
While I do appreciate this kind of work, I think these exoplanet simulations are very premature.

@GSwift
I can see your point (or do I?)
but, I also think that generating simulations like this can allow people to make inferences which are then testable, which would allow us to then search for certainties
IMHO - although it is a simulation, it gives the ability to explore new ideas that would give insights, as well as generate possible testable predictions or methodologies that can be used for searching and understanding

the article seems to support this opinion, showing how the prior research did not take something into consideration, which allowed for another prediction to be made (like the heat distribution in the oceans)

Or am I missing something?

(mind you that this is pure conjecture on my part)
Maggnus
3 / 5 (2) Mar 14, 2014
It looks like they are assuming banded flow in the ocean, but I wonder if a planet without much rotation would actually do that? A tidally locked planet will only rotate once per 'year'. IIRC that should be a matter of weeks for these planets. I wouldn't be too quick to apply the coriolis effect here (assuming that's what they did, which I realize may not be the case).

While I do appreciate this kind of work, I think these exoplanet simulations are very premature.
I don't know about that. Remember that although they would be tidally locked, they would also be whizzing around their stars in a matter of weeks. Furthermore, from a strictly thermal perspective, the ocean's water is going to want to flow from the hot sunlit equator to the cold night side, which should, in an of itself, set up a pretty good current flow. I think a bigger question would be the placement of continents.
GSwift7
3.7 / 5 (3) Mar 14, 2014
it gives the ability to explore new ideas that would give insights, as well as generate possible testable predictions or methodologies


Oh, I totally agree with that line of thought. But, we seem to be a long way off from actually testing this. In the meantime there are just so many theories of this type being generated, and many of them reach different conclusions. Just kinda seems like grasping at straws for now, and once we get our first direct observations, it will all change.

placement of continents


Yeah, certainly! Assuming a uniform global ocean is a big stretch to use for an initial condition.

On a side note: I often wonder about how mechanisms both internal and on the surface of such a planet might cause the center of gravity of the planet to change over time. For example, accumulation of material towards the cool side? Maybe? Could the crust build up unevenly and eventually cause the planet to flip around?
GSwift7
3.3 / 5 (3) Mar 14, 2014
Sorry, I had one more thought:

In my previous post, I suggested material accumulating on the cold side, but I wonder if there could be even more profound differences?

For instance, could the core of the planet be lop-sided? Would the magma in the mantle (if it has one) differentiate in a non uniform way on the hot versus cold sides due to gravity/tidal force? We know from our own solar system that the inner moons of Jupiter and Saturn are deformed by the tidal force of their planet. I wonder what a similar effect would do to a body the size of a planet, and especially a body with a molten core, and possibly an atmosphere. The atmosphere should be noticably deformed by the gravity, not just the temperature difference.

I further wonder if the above modelers considered the differentiated gravity field in their circulation model. That should cause preferential flows that we don't have here on Earth.
Captain Stumpy
2.5 / 5 (2) Mar 14, 2014
But, we seem to be a long way off from actually testing this. In the meantime there are just so many theories of this type being generated, and many of them reach different conclusions. Just kinda seems like grasping at straws for now, and once we get our first direct observations, it will all change

@GSwift
ok. I see where you were going with that now.
but again... for every straw they grasp, they can then later narrow down the possibilities and perhaps generate a method for searching...
maybe I am thinking too optimistically ... I dont know.

I have to agree that once we get our first direct observations, the whole game will change.
TheGhostofOtto1923
2.7 / 5 (3) Mar 15, 2014
In my previous post, I suggested material accumulating on the cold side, but I wonder if there could be even more profound differences?

For instance, could the core of the planet be lop-sided? Would the magma in the mantle (if it has one) differentiate in a non uniform way on the hot versus cold sides due to gravity/tidal force? We know from our own solar system that the inner moons of Jupiter and Saturn are deformed by the tidal force of their planet. I wonder what a similar effect would do to a body the size of a planet, and especially a body with a molten core, and possibly an atmosphere. The atmosphere should be noticably deformed by the gravity, not just the temperature difference
Hmmm I wonder about these things too. But I wonder FIRST if they have ever occurred to scientists, because I actually want to know something about them.

Hey, here's a whole paper on the subject. Diagrams and all.
http://dspace.mit....1/74161
Captain Stumpy
5 / 5 (1) Mar 15, 2014
Hey, here's a whole paper on the subject. Diagrams and all.
http://dspace.mit....1/74161

thanks for sharing that otto, I found some of the reference material from that study more interesting than the study, like the "Cosmic Ray Impact on Extrasolar Earth-Like Planets in Close-in Habitable Zones" by Grießmeier et al.
We found that an Earth-like extrasolar planet, tidally locked in an orbit of 0.2 AU around an M star of 0.5 solar masses, has a rotation rate of 2% of that of the Earth. This results in a magnetic moment of less than 15% of the Earth's current magnetic moment. Therefore, close-in extrasolar planets seem not to be protected by extended Earth-like magnetospheres, and cosmic rays can reach almost the whole surface area of the upper atmosphere

I wondered if tidally locked planets had a magnetosphere, and how powerful they might be, but I had not had time to research it yet
GSwift7
5 / 5 (3) Mar 17, 2014
Hey, here's a whole paper on the subject. Diagrams and all


Yeah, that's great, and I was able to bring up all those point in less than 1000 words, without anyone having to follow links to another web site.

Besides, without direct observations to validate the theories, there's no way to judge the value of the paper you linked to. They could be totally wrong. The story above is a good example of a case where there are competing theories which reach different conclusions. That's kinda what Captain and I were talking about above.

Captain:

Maybe I'm just being too pesimistic. :)
GSwift7
5 / 5 (2) Mar 17, 2014
I have to agree that once we get our first direct observations, the whole game will change.


Yeah, and then when we get our second wave of observations it'll change again.

I was watching that documentary series "walking with dinosaurs" with my girlfriend's son yesterday. As they advanced through the different epochs, it was striking to me that the climate of the Earth changed so much over time. Using only the Earth as an example shows just how much variability we might find amongst any given class of exoplanet which might otherwise seem similar from a distance. Similarly, if you compare Earth, Mars and Venus, they would all look very similar if we were viewing them with our current telescopes from another star, yet again we see stark differences between otherwise similar planets.