Researchers extend capabilities of computer simulation of tidally locked exoplanets

Dec 31, 2013 by Bob Yirka report
Spatial distributions of sea-ice fraction and surface air temperature. (Left) Sea-ice fraction (unit, %); (Right) surface air temperature (unit, °C); (Upper) 355 ppmv CO2; and (Lower) 200,000 ppmv CO2. In A and B, arrows indicate wind velocity at the lowest level of the atmospheric model (990 hPa), with a length scale of 15 m s−1 . In C and D, arrows indicate ocean surface current velocity, with a length scale of 3 m s−1 . Note that the color scale for surface air temperature is not linear. The substellar point is at the equator and 180° in longtitude. Credit: PNAS, Yongyun Hu, doi: 10.1073/pnas.1315215111

(Phys.org) —A pair of researchers at Peking University in Beijing China, has extended the capabilities of an existing computer simulation that is used to study tidally locked exoplanets. In their paper published in Proceedings of the National Academy of Sciences, Yongyun Hu and Jun Yang describe the improvements they've made and also how those improvements give a new perspective on the range of possible tidally locked exoplanets that may be habitable.

Prior to this new effort, most computer models that sought to recreate the conditions that exist on exoplanets that are tidally locked (they don't spin, thus only one side ever faces their star) relied mostly on the impact of atmospheric conditions. The new enhancements include possible impacts of ocean currents.

The main goal of the upgraded model is, like many others, to allow for predicting the likelihood of life existing somewhere other than here on Earth. Tidally locked exoplanets present a challenging prospect—on one hand, the side that points towards the star is likely warm enough to support life—on the other, the cold side may be so cold that gases freeze and are lost to space preventing the evolution of an atmosphere.

To try to get a better handle on what may go on with such exoplanets, the researchers extracted parts of models that try to predict ocean behavior here on Earth. Those parts were then modified to more accurately reflect what has actually been observed, namely, smaller, colder and less feature rich worlds.

Tidally locked generally exist close to a —they get locked because they move so close to their star. This means that the amount of heat hitting the star is much less, relatively speaking, than it would be for a planet that wasn't locked, because its star is colder. Space scientists tend to refer to such planets that might hold the potential for life as an "Eyeball Earth," because the dark side resembles a pupil.

The enhanced model, the team reports, allows for changing parameters (such as CO2 levels) and then for allowing many simulated years to pass to see what evolves as a result. Doing so, the team says, shows that given the right set of circumstances, heat from oceans can be transported around the globe allowing for a warmer planet than has been predicted before, though most outcomes suggest a narrower habitable zone.

The researchers note that much more work needs to be done on their model and others, noting that many are still too simplistic to render true approximations. One area of concern is that most models don't take into account land formations or uneven ocean bottoms, both of which can impact and hence heat transfer.

Explore further: How common are earths around small stars?

More information: Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars, PNAS, Yongyun Hu, DOI: 10.1073/pnas.1315215111

Abstract
The distinctive feature of tidally locked exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the nightside for terrestrial exoplanets in the habitable zone around M dwarfs. In the present paper, we carry out simulations with a fully coupled atmosphere–ocean general circulation model to investigate the role of ocean heat transport in climate states of tidally locked habitable exoplanets around M dwarfs. Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an "eyeball." For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs' habitable zone. This study provides a demonstration of the importance of exooceanography in determining climate states and habitability of exoplanets.

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philw1776
1 / 5 (1) Dec 31, 2013

How did this article get a one star vote?

"Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an "eyeball." For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs' habitable zone."

Interesting results. Without reading the paper I don't understand how the ocean currents heat transport effect reduces the width of an M star's HZ. Anyone here have some thoughts?
GSwift7
5 / 5 (1) Dec 31, 2013
The researchers note that much more work needs to be done on their model and others, noting that many are still too simplistic to render true approximations


Too bad we don't have at least one example that we can look at to base the models on. Mabye JWST will be able to see one of them clearly enough to get started. Until then, we don't really have any way to check the models.

One interesting factor is the coriolis effect (or lack of it). A tidally locked planet is only doing one rotation per orbit. So we are probably talking about a greatly reduced coriolis effect compared to Earth (not sure what the orbital period would be, but IIRC it's more like weeks rather than hours.) On the bright side, that seems like it should simplify the modeling job a bit.
GSwift7
5 / 5 (2) Dec 31, 2013
Interesting results. Without reading the paper I don't understand how the ocean currents heat transport effect reduces the width of an M star's HZ. Anyone here have some thoughts?


The extra heat transport takes what little heat is available and spreads it around the planet, making the outer portion of the previous HZ too cold. While it will extend the HZ farther in towards the star, the amount of eneregy received from the star increases exponentially as you get closer. So you don't get much closer before it is too hot, which means that you only gain a little on the inside but you lose a lot on the outside.

I could be wrong here, but I was under the impression that some estimates already take this into account (at least speculatively).
Torbjorn_Larsson_OM
5 / 5 (1) Dec 31, 2013
Note that there are lots of problems with this one, including their choice of a modified (more massive) "Earth" reference, no land masses et cetera. It is a first attempt, but I don't see that they have attempted to verify old results too much.

Else, GCM's is what in two consecutive papers recently predicted that Earth could be less than 100 % ice covered under the faint young Sun of the Archean. [The latest: "Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM", Charnay et al, Jour. Geophys. Res. 2013]

@phil: "a one star vote?" Being Yirka, anything is possible. Cf how he writes about "smaller, colder and less feature rich worlds" when the paper is about a superEarth example AFAIK.

@G7: ~ 30 days orbital period for a HZ planet around an M star, IIRC. Oh, and thanks for the HZ change excerpt! I haven't read the paper yet.
philw1776
not rated yet Jan 02, 2014
HZ orbital/rotational periods range from ~ 2 months for the "brighter" M0 stars (~0.07 sol) to ~ 2 weeks for M5 (~0.003 sol). Your milage will vary depending on which models you use for HZ zones around these dim stars.

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