Cloud behavior expands habitable zone of alien planets

Jul 01, 2013
A planet with clouds and surface water orbits a red dwarf star in this artist's conception of the Gliese 581 star system. New findings from the University of Chicago and Northwestern University show that planets orbiting red dwarf stars are more likely to be habitable than previously believed. Credit: Lynette Cook

A new study that calculates the influence of cloud behavior on climate doubles the number of potentially habitable planets orbiting red dwarfs, the most common type of stars in the universe. This finding means that in the Milky Way galaxy alone, 60 billion planets may be orbiting red dwarf stars in the habitable zone.

Researchers at the University of Chicago and Northwestern University based their study, which appears in Astrophysical Journal Letters, on rigorous of cloud behavior on alien planets. This cloud behavior dramatically expanded the habitable zone of , which are much smaller and fainter than stars like the sun.

Current data from NASA's Kepler Mission, a searching for Earth-like planets orbiting other stars, suggest there is approximately one Earth-size planet in the habitable zone of each red dwarf. The UChicago-Northwestern study now doubles that number.

"Most of the planets in the Milky Way orbit red dwarfs," said Nicolas Cowan, a postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics. "A thermostat that makes such planets more clement means we don't have to look as far to find a habitable planet."

Cowan is one of three co-authors of the study, as are UChicago's Dorian Abbot and Jun Yang. The trio also provide astronomers with a means of verifying their conclusions with the James Webb Space Telescope, scheduled for launch in 2018.

The formula for calculating the habitable zone of —where they can orbit their star while still maintaining liquid water at their surface—has remained much the same for decades. But the formula largely neglects clouds, which exert a major climatic influence.

This illustration shows simulated cloud coverage (white) on a tidally locked planet (blue) that would be orbiting a red dwarf star. Planetary scientists at UChicago and Northwestern are applying global climate simulations to problems in astronomy. Credit: Jun Yang

"Clouds cause warming, and they cause cooling on Earth," said Abbot, an assistant professor in geophysical sciences at UChicago. "They reflect sunlight to cool things off, and they absorb from the surface to make a greenhouse effect. That's part of what keeps the planet warm enough to sustain life."

A planet orbiting a star like the sun would have to complete an orbit approximately once a year to be far enough away to maintain water on its surface. "If you're orbiting around a low mass or dwarf star, you have to orbit about once a month, once every two months to receive the same amount of sunlight that we receive from the sun," Cowan said.

Tightly orbiting planets

Planets in such a tight orbit would eventually become tidally locked with their sun. They would always keep the same side facing the sun, like the moon does toward Earth. Calculations of the UChicago-Northwestern team indicate that the star-facing side of the planet would experience vigorous convection and highly reflective clouds at a point that astronomers call the sub-stellar region. At that location the sun always sits directly overhead, at high noon.

The team's three-dimensional global calculations determined for the first time the effect of water clouds on the inner edge of the habitable zone. The simulations are similar to the global climate simulations that scientists use to predict Earth climate. These required several months of processing, running mostly on a cluster of 216 networked computers at UChicago. Previous attempts to simulate the inner edge of exoplanet habitable zones were one-dimensional. They mostly neglected clouds, focusing instead on charting how temperature decreases with altitude.

Clouds exert a major influence on Earth¹s climate. If clouds only cooled the planet by reflecting solar energy back to space, Earth would completely ice over. But if clouds only warmed the planet by absorbing and reemitting infrared energy back to the surface, Earth would fry under a runaway greenhouse effect like the one on Venus. Credit: Norman Kuring, NASA GSFC

"There's no way you can do clouds properly in one-dimension," Cowan said. "But in a three-dimensional model, you're actually simulating the way air moves and the way moisture moves through the entire atmosphere of the planet."

These new simulations show that if there is any surface water on the planet, water clouds result. The simulations further show that cloud behavior has a significant cooling effect on the inner portion of the , enabling planets to sustain water on their surfaces much closer to their sun.

Astronomers observing with the James Webb Telescope will be able to test the validity of these findings by measuring the temperature of the planet at different points in its orbit. If a tidally locked exoplanet lacks significant cloud cover, astronomers will measure the highest temperatures when the dayside of the exoplanet is facing the telescope, which occurs when the planet is on the far side of its star. Once the planet comes back around to show its dark side to the telescope, temperatures would reach their lowest point.

But if highly reflective clouds dominate the dayside of the exoplanet, they will block a lot of infrared radiation from the surface, said Yang, a postdoctoral scientist in geophysical sciences at UChicago. In that situation "you would measure the coldest temperatures when the planet is on the opposite side, and you would measure the warmest temperatures when you are looking at the night side, because there you are actually looking at the surface rather than these high clouds," Yang said.

Earth-observing satellites have documented this effect. "If you look at Brazil or Indonesia with an infrared telescope from space, it can look cold, and that's because you're seeing the cloud deck," Cowan said. "The cloud deck is at high altitude, and it's extremely cold up there."

If the James Webb Telescope detects this signal from an exoplanet, Abbot noted, "it's almost definitely from clouds, and it's a confirmation that you do have surface liquid water."

Explore further: Astronomer confirms a new "Super-Earth" planet

More information: "Stabilizing Cloud Feedback Doubles Frequency of Red Dwarf Habitable Planets," by Jun Yang, Nicolas B. Cowan and Dorian S. Abbot, Astrophysical Journal Letters, Vol. 771, No. 2, July 10, 2013.

Related Stories

How common are earths around small stars?

Jun 03, 2013

(Phys.org) —The Kepler mission has revolutionized the study of exoplanet statistics by increasing the number of known extrasolar planets and planet candidates by a factor of five, and by discovering systems ...

Three planets in habitable zone of nearby star (w/ video)

Jun 25, 2013

(Phys.org) —A team of astronomers has combined new observations of Gliese 667C with existing data from HARPS at ESO's 3.6-metre telescope in Chile, to reveal a system with at least six planets. A record-breaking ...

Can life emerge on planets around cooling stars?

Nov 20, 2012

(Phys.org)—Astronomers find planets in strange places and wonder if they might support life. One such place would be in orbit around a white or brown dwarf. While neither is a star like the sun, both glow and so could be ...

Recommended for you

Kepler proves it can still find planets

Dec 18, 2014

To paraphrase Mark Twain, the report of the Kepler spacecraft's death was greatly exaggerated. Despite a malfunction that ended its primary mission in May 2013, Kepler is still alive and working. The evidence ...

User comments : 7

Adjust slider to filter visible comments by rank

Display comments: newest first

Anda
5 / 5 (4) Jul 01, 2013
This explanation applies to the habitable zone of every star.
Why only talk about red dwarfs??
r2vettes
1 / 5 (1) Jul 01, 2013
Now we're all excited about so many "Goldilocks" worlds around red dwarf stars. Red dwarfs account for about 80% of all stars it is thought. Big problem and not to rain on the enthusiasm, but do a little research about red dwarfs. They typically spin very fast. Have a very strong, but unstable magnetic field and spew radiation via cme's and solar flares on a regular basis. The so called habitable zone planets would have to be very close to the star, right in line for massive doses of radiation. So it's best to look for a nice single, stable yellow star such as our own. Very bad things also would happen in a binary or triple star system via unstable tidal attraction.
Fleetfoot
5 / 5 (2) Jul 02, 2013
This explanation applies to the habitable zone of every star.
Why only talk about red dwarfs??


Because this effect would be most prominent for a tidally locked planet and those farther out around more massive stars are less likely to be locked.
GSwift7
5 / 5 (2) Jul 02, 2013
Planets in such a tight orbit would eventually become tidally locked with their sun


That's an interesting mistake which I keep seeing repeated in the comments here.

The habitable zones of red dwarves are around the orbit of Mercury, and Mercury is not tidally locked. A Mercury-sized planet orbiting a smaller star at the same distance as Mercury would be even less likely to be tidally locked than Mercury. A planet larger than Mercury at that same distance would be even less likely to be tidally locked.

but, like r2vettes said, flares are the big buzz-kill here. The above method, if it works, might measure liquid water and clouds on a totally sterile planet regularly engulfed by killing radiation.
Fleetfoot
5 / 5 (1) Jul 02, 2013
but, like r2vettes said, flares are the big buzz-kill here. The above method, if it works, might measure liquid water and clouds on a totally sterile planet regularly engulfed by killing radiation.


How deep into an ocean would that radiation penetrate? As long as life can originate at depth, the gradient to the surface would strongly favour radiation-hard species and the higher levels would speed up evolution.
GSwift7
not rated yet Jul 03, 2013
How deep into an ocean would that radiation penetrate? As long as life can originate at depth, the gradient to the surface would strongly favour radiation-hard species and the higher levels would speed up evolution


Anything that relies on photosynthesis would be in the danger zone near the surface. If you're talking about some kind of extremophile that doesn't need the sun to survive, then you no longer need to worry about the habitable zone anyway. At that point you can consider bodies like Titan, even if the ocean is covered by ice.
Fleetfoot
not rated yet Jul 03, 2013
How deep into an ocean would that radiation penetrate? As long as life can originate at depth, the gradient to the surface would strongly favour radiation-hard species and the higher levels would speed up evolution


Anything that relies on photosynthesis would be in the danger zone near the surface. If you're talking about some kind of extremophile that doesn't need the sun to survive, then you no longer need to worry about the habitable zone anyway.


No, I was wondering about the variation in absorption versus wavelength:

http://en.wikiped...ater.png

At wavelengths shorter than about 150nm, the radiation is strongly absorbed but that falls rapidly by about a factor of 10^7 by 200nm. There should be a range of depth where light from 200nm to 700nm penetrates but everything else is filtered out.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.