Red Dwarf Stars and the Planets Around Them

It’s tempting to look for habitable planets around red dwarf stars, which put out far less luminosity and so are less blinding.  But is it wise?

That question has been near the top of the list for many exoplanet scientists, especially those involved in the search for habitable worlds.

Red dwarfs are plentiful (about three-quarters of all the stars out there) and the planets orbiting them are easier to observe because the stars are so small compared to our Sun and so an Earth-sized planet blocks a greater fraction of starlight.  Because planets orbiting red dwarfs are much closer in to their host stars, the observing geometry favors detecting more transits.

A potentially rich target, but with some drawbacks that have become better understood in recent years.  Not only are most planets orbiting these red dwarf stars tidally locked, with one side always facing the sun and the other in darkness, but the life history of red dwarfs is problematic.  They start out with powerful flares that many scientists say would sterilize the close-in planets forever.

Also, they are theorized to be prone to losing whatever water remains even if the stellar flares don’t do it. Originally, it was thought that this would happen because of a “runaway greenhouse,” where a warming planet under a brightening star would evaporate enough water from its oceans to create a thick blanket of H2O vapor at high altitudes and block the escape of radiation, leading to further warming and the eventual loss of all the planet’s water.

The parching CO₂ greenhouse of a planet like Venus may be the result of that.  Later it was realized that on many planets, another mechanism called the “moist greenhouse” might create a similar thick blanket of water vapor at high altitudes long before a planet ever got to the runaway greenhouse stage.

Finally now has come some better news about red dwarf exoplanets.  Using 3-D models that characterize atmospheres going back, forward and to the sides, researchers found atmospheric conditions quite different from those predicted by 1-D models that capture changes only going from the surface straight up.

One paper found that using some pretty simple observations and calculations, scientists could determine the bottom line likelihood of whether or not the planet would be undone by a moist greenhouse effect.  The other found that these red dwarf exoplanets could have atmospheres that are always heavily clouded, but could still have surface temperatures that are moderate.

The new studies also enlarge the size of the habitable zones in which exoplanets could be orbiting a red dwarf or other “cool” star, making more of them potentially habitable.

 

“This is good news for those of us hoping to find habitable planets,” said Anthony Del Genio, a senior research scientist at NASA’s Goddard Institute for Space Studies (GISS) in New York, and co-author of a new paper in The Astrophysical Journal.

“These studies show that a broader range of planets could have stable climates than we thought.  This is a broadening of the width of the habitable zone by showing that we can get closer to a star and still have a potentially habitable planet.”

In a NASA release, the paper’s lead author, Yuka Fujii, said this: “Using a model that more realistically simulates atmospheric conditions, we discovered a new process that controls the habitability of exoplanets and will guide us in identifying candidates for further study.” Fujii was formerly at NASA GISS and now is a project associate professor  at the Earth-Life Science Institute in Tokyo.

Since telescope time available for exoplanets will be quite limited on observatories such as the James Webb Space Telescope — which has many astronomical tasks to accomplish — the Earth-sized exoplanets around red dwarfs seem to be the more technologically feasible target to observe.

Scientists have to observe Earth-size planets for a long time and for many transits in front of the star to get a good enough signal to interpret. So given that, it will be impossible to observe all, or even many, of the candidate Earth-size planets discovered so far or will be discovered.  Tough choices have to be made.

What the group found using their 3-D models is that unlike the predictions from 1-D models, this moist greenhouse effect does not set in immediately for a particular luminosity of the star. Rather, it occurs more gradually as the star becomes brighter.

That fact, Del Genio said, makes the findings from the new 3-D modeling studies additionally important because they can help observers determine which small, rocky exoplanets might be most promising in terms of habitability.

They do this by identifying — and then eliminating — exoplanets that have undergone what is called a “moist greenhouse” transformation.

A moist greenhouse occurs when a watery exoplanet orbits too close to its host star. Light from the star will then heat the oceans until they begin to evaporate and are lost to space.

This happens when water vapor rises to a layer in the upper atmosphere called the stratosphere and gets broken into its elemental components (hydrogen and oxygen) by ultraviolet light from the star.
The extremely light hydrogen atoms can then escape to space. Planets in the process of losing their oceans this way are said to have entered a “moist greenhouse” state because of their humid stratospheres.

What the group found using their 3-D models is that unlike the runaway greenhouse effect this moist greenhouse effect does not set it immediately at a particular temperature threshold.  Rather, it occurs more gradually, even over eons.

They came to this conclusion because the upper atmosphere heating turned out to be a function of the infrared radiation coming from the stars rather than from turbulent convective activity (as in massive thunderstorms) from the surface, as earlier believed.

The infrared radiation (which is at wavelengths slightly longer than the visible wavelength area of the spectrum) will warm the planet and cause what water is present to eventually. evaporate.

 

This paper comes on the heels of a related one in the August edition of  The Astrophysical Journal.

Ravi Kopparapu, a research scientist at NASA Goddard and Eric Wolf of the University of Colorado, Boulder came to a similar conclusion about surfaces on exoplanets orbiting red dwarfs. As they wrote in their abstract, the modeling  “implies that some planets around low mass (red dwarf) stars can simultaneously undergo water-loss and remain habitable.”

They also reported general circulation model 3-D modeling that showed moist greenhouse scenarios around red dwarfs were slow moving and took place at relatively low temperatures. As a result, oceans could remain for a long time — even billions of years — as they slowly evaporated.

Both groups use general circulation models (GCM), though different ones.  GCMs are an advanced type of climate model that looks at the general circulation patterns of planetary atmospheres and oceans.  They were initially designed to model Earth’s climate patterns, but now are used for exoplanets as well.

The original theory of the moist greenhouse scenario was put forward in the 1980s by James Kasting of Pennsylvania State University, who also did much original work on the concept of a habitable zone and helped popularize the concept.  Both the runaway greenhouse and the moist greenhouse have become important factors in exoplanet study.