Leaving Exoplanets aside, Exomoons may harbour life too

Shooting for the exomoon. 

Credit: CBC11, CC BY-SA

In the Star Wars universe, everyone's favourite furry aliens, the Ewoks, famously lived on the "forest moon of Endor".

In scientific terms, the Ewok's home world would be referred to as an exomoon, which is simply a moon that orbits an exoplanet, any planet that orbits a star other than our sun.

Although more than 1,000 exoplanets have been discovered since the first one was found in 1995, only a handful of those are thought to be habitable, at least by life as we know it.

New research shows that exomoons, too, could provide habitable environments. Although we are yet to find exomoons, we have good reasons to believe that there should be many, even more than exoplanets.

Goldilocks zone
Perhaps the most habitable planet found to date is the recently announced Kepler-186f.

This is one of five exoplanets discovered by NASA's Kepler satellite, all orbiting a small, faint, red dwarf star, 500 light years away in the constellation of Cygnus.

Kepler-186f is an Earth-sized planet that orbits its star in only 130 days and is about as distant from its star as Mercury is from the Sun. But, because the red dwarf is much dimmer than the Sun, Kepler-186f receives only about one-third of the energy that the Earth does.

As a result, Kepler-186f lies at the outer edge of its star's "habitable zone".

This is the hypothetical region of space surrounding a star in which liquid water may conceivably exist on the surface of any exoplanets.

In our own solar system, Venus lies too close to the Sun and is too hot.

Mars lies too far from the Sun and is too cold.

But Earth, of course, lies within the critical "Goldilocks zone", where the temperature is just right.

Simply residing in the habitable zone, though, is no guarantee that an exoplanet has water oceans.

The climate of a planet is much more complicated than we can capture with a simple calculation based on the distance of a planet from a star.

We know that Mars probably had running water on its surface in the past, but now it is a frozen desert. Earth, meanwhile, was probably in a completely frozen "snowball" state about 650m years ago.

Duncan Forgan

Recent research by Duncan Forgan and Vergil Yotov at the University of Edinburgh highlights the various factors that may make an exomoon more or less habitable.

They investigate how the climate of an exomoon will be affected by tidal stresses which provide a source of internal heating for the exomoon as it is stretched and deformed by the gravitational pull of its planet.

They also investigated how light reflected from the exoplanet, and eclipses by the exoplanet, can also subtly alter the exomoon's climate.

The researchers lump theoretical exomoons into a number of classifications: "habitable", "hot", "snowball" or "transient".

Those in the first class have more than 10% of their surface at a temperature between the freezing and boiling points of water, with only a small fluctuation around the average temperature value.

Those in the second class have average temperatures above 100°C at all times, whereas those in the third class are permanently frozen, in both cases less than 10% of the surface is habitable.

Exomoons in the fourth, transient class are on average habitable, but the amount of habitable surface area varies widely with time.

Overall, this research shows that exomoon climates are rather more complex than previous research has supposed.

As yet, no exomoons have been discovered, but there are various techniques proposed for finding them.

One way is by studying the effects that an exomoon will have on the exoplanet it is orbiting – their gravitational connection means there will be a to-and-fro tugging between them.

This will cause variations in the times at which the planet transits in front of its star and in the durations of these transits, which we are able to measure.

These time variations will only be a few seconds at most, so very accurate measurements of the transits must be made to reveal the exomoon's presence.

If variations are detected then, in principle, both the mass and orbit of the exomoon may be calculated from the measurements.

It surely is only a matter of time before the first exomoon is discovered and the probability of finding one in the habitable zone of a star is reasonably high.

We may not find any Ewoks, but habitable exomoons may indeed offer the best prospect for hosting alien life.

Earth and Venus: Exoplanet Habitable Zone Around Sunlike Stars Bigger

The Earth and Venus, although not very different in size, have had a very different climate history. 

In the future, the Earth may look like Venus. 

Credit: Jeremy Leconte

Earth's place in the solar system is just right.

It's not too hot, like Venus, and it's not too cold, like Mars, and this "Goldilocks zone" of habitability around other stars like the sun just might be bigger than thought, scientists say.

A new study, unveiled today (Dec. 11), expands the habitable zone — the sweet spot in a solar system where liquid water and therefore life could potentially exist — surrounding stars like the sun.

Previous studies on the habitability zone around sunlike stars have placed the innermost edge of so-called Goldilocks zoneat about 0.99 AU (1 AU, or astronomical unit, is the average distance from Earth to the sun, about 93 million miles, or 150 million kilometers).

But a new computer model study pushes that border closer to its parent star, to a distance of about 0.95 AU (about 88 million miles, or 142 million kilometers).

The study in the journal Nature, led by Jeremy Leconte, now a postdoc at the Canadian Institute for Theoretical Astrophysics of the University of Toronto, used 3D computer modeling to find that the runaway greenhouse effect isn't an issue unless the planet is less than 0.95 AU from its star.

The new inner boundary for habitable zones might not make a big difference for scientists trying to determine if an alien planet is habitable, but it does make a big difference for future life on Earth, Leconte said.

Eventually, the sun will become brighter, heating the Earth and potentially creating a runaway greenhouse effect — a feedback loop that eventually causes a planet's oceans to boil away. But that eventuality is long way off, and it may now actually be farther off than previously expected, Leconte suggests.

"For example, if we believe that the limit is at 0.99, it means that Earth would start losing oceans around 150 million years from now," Leconte told reporters.

"Now, with our new estimate, it's not 150 million years, but it's actually 1 billion years, so almost an order of magnitude bigger."



More Information: 
Increased insolation threshold for runaway greenhouse processes on Earth-like planets doi:10.1038/nature12827

Exoplanet Cloud Behaviour Expands Habitable Zone

A new study that calculates the influence of cloud behaviour 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 computer simulations of cloud behaviour on alien planets.

This cloud behaviour dramatically expanded the habitable zone of red dwarfs, which are much smaller and fainter than stars like the Sun.

Current data from NASA's Kepler mission, a space observatory 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 alien planets -- 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.

"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 infrared radiation 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.