Do Atmospheres Spin Worlds to Habitability?

The best place to look for nearby Earth-size planets are around the smallest, coolest stars. New research shows that any exoplanets tightly circling their stars might have a better chance of being habitable than previously thought.

This artist's conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf star only 20 light years away from Earth. The large planet in the foreground is the newly discovered GJ 581g, which has a 37-day orbit right in the middle of the star's habitable zone and is only three to four times the mass of Earth, with a diameter 1.2 to 1.4 times that of Earth. Lynette Cook

This artist's conception shows the planet GJ 581g, which has a 37-day orbit right in the middle of the star's habitable zone and is only three to four times the mass of Earth, circling its red dwarf star.
Lynette Cook

In the hunt for Earth 2.0, many astronomers are pointing their telescopes toward smaller, cooler stars. Not only are these so-called red dwarfs the most abundant type of star in the galaxy, but they’re also roughly one-quarter the Sun’s mass, bringing their habitable zones closer in and making it easier to spot any Goldilocks planets, either via their gravitational tugs on the star or when the planet passes in front of the star from our perspective.

There’s just one catch. A planet that orbits close enough to its dim star to be in the habitable zone could become tidally locked. Just as our planet sees one side of the Moon at all times, red dwarfs will only see one side of a close-in planet at all times. So one side of the planet will likely see continuous day and the other perpetual darkness, potentially destabilizing chemical exchanges between the atmosphere and surface or even (in extreme instances) causing the atmosphere to collapse. In short, tidally locked planets are likely uninhabitable.

New research, however, suggests not all is lost for tightly orbiting planets. Jérémy Leconte (University of Toronto and Pierre Simon Laplace Institute, France) and his colleagues think that an atmosphere’s effect might be strong enough to break any tidal locking, allowing the planet to rotate freely and exhibit a day-night cycle similar to Earth’s.

Leconte and his colleagues created a three-dimensional climate model (similar to those used in analyzing climate change on Earth) to predict the effect of a given planet’s atmosphere on the speed of its rotation.

It all goes back to the amount of starlight able to penetrate the planet’s atmosphere and reach the surface. Any temperature differences at the surface — between day and night and between the equator and the poles — drive winds. Those winds constantly push against the planet by running into mountains or creating waves on the ocean. Such friction then influences the rotation rate of the planet, helping to speed it up or slow it down.

“While gravitational tides and their associated torques tend to tidally lock the planet, thermal tides, produced by the star heating the atmosphere of the planet, tend to oppose the gravitational tides, and prevent the planets from becoming tidally locked,” says coauthor Norm Murray (University of Toronto).

Astronomers have long seen this effect on the planet Venus, where the atmosphere’s influence is so powerful that it forces the planet out of synchronous rotation into a slow retrograde rotation: to a Venusian, the Sun rises in the west and sets in the east. But Venus’s large atmosphere weighs in about 90 times heavier than our own, and planetary scientists didn’t think thinner atmospheres like Earth’s could throw their weight around as effectively.

Leconte’s simulations show that thinner atmospheres actually have a larger rotational effect on their planets. With less scattered sunlight, extra heat reaches the deepest atmospheric layer and creates stronger winds. If Venus were to have an atmosphere like Earth’s, it would spin 10 times faster. This is radically different from previous research, which suggested that it would spin 50 times slower.

An unlocked planet should have strong atmospheric mixing and relatively stable temperatures. “This greatly increases the chances for atmospheric stability — and, hence, for life — on any of these bodies, provided they are Earth-like in terms of mass, water content, and maybe their atmospheres,” says exoplanet expert René Heller (McMaster University, Canada).

In addition, it avoids many problems created on tidally locked planets, Take the cold trap, for example. “Liquid water on the sunny side tends to evaporate, and is thence transported by winds (driven by the temperature gradient) to the dark side, where it precipitates as snow and forms large-scale ice sheets,” says Murray. “Since the back side never sees the light of the host star, the ice sheets may well be permanent.” Eventually all the liquid water would move to the dark side, making life impossible.

Although the researchers show that a large number of known terrestrial exoplanets should have a day-night cycle, potentially rendering them habitable, the duration of their days could last between a few weeks and a few months. So Heller cautions that these planets would still be far from Earth-like, with only a few days per year.

Hopefully the theoretical results don’t remain in the observational dark for too long. Astronomers can determine the temperature of exoplanets when they pass behind their host stars. But it won’t be easy to do this for Earth-size worlds. Leconte thinks it might be within reach of the James Webb Space Telescope (slated to launch in 2018) if there is a particularly favorable planet to observe. If not, astronomers might have to wait for the European Extremely Large Telescope, whose first light is tentatively scheduled for 2024.

Reference:

Jérémy Leconte et al. “Asynchronous Rotation of Earth-mass Planets in the Habitable Zone of Lower-mass Stars.” Science. Published online January 15, 2015.


Learn more about the weird weather on alien worlds in a special report featured in our May 2014 issue.