Kepler-16b: Inside a Chilly Habitable Zone?

by Paul Gilster on January 10, 2012

The annual meeting of the American Astronomical Society is now in session in Austin, sure to provide us with interesting fodder for discussion in coming days. Just coming off embargo yesterday was news of further study of the interesting Kepler-16 system. This one made quite a splash last fall when the planet known as Kepler-16b was discovered to orbit two stars, with the inevitable echoes of Star Wars and the twin suns that warmed the planet Tatooine. This planet, though, was a gas giant more reminiscent of chilly Saturn than a cozily terrestrial world.

Image: An artist’s conception of the Kepler-16 system (white) from an overhead view, showing the planet Kepler-16b and the eccentric orbits of the two stars it circles (labeled A and B). For reference, the orbits of our own solar system’s planets Mercury and Earth are shown in blue. New work out of the University of Texas at Arlington explores the question of habitability in a system like this. Credit: NASA/Ames/JPL-Caltech.

You’ll recall, too, that Kepler-16b circles both a K-dwarf with about 70 percent of the Sun’s mass and a red dwarf of about a fifth of a solar mass. Although the planet’s orbit takes it within Venus-like distances of them, Kepler-16b’s central stars are small enough that temperatures would appear to be too cold for life. At the Austin meeting, however, researchers from the University of Texas at Arlington have made the case that an Earth-class planet could exist here as an exomoon orbiting the gas giant. They have no indication that such a planet actually exists, as Zdzislaw Musielak (UT-Arlington) is quick to point out, but the work is interesting nonetheless:

“This is an assessment of the possibilities,” said Musielak. We’re telling them where a planet has to be in the system to be habitable. We’re hoping they will look there.”

Making conditions on such a moon habitable would require an atmosphere with a strong warming effect that could be provided by high levels of greenhouse gases like carbon dioxide or methane. Such an atmosphere would widen what we would normally consider to be the habitable zone around the two stars. Al Jackson noted today in an email from Austin that all kinds of new ways to study Kepler candidates are coming to the fore, and remember that Kepler still has a long way to go before its primary mission is accomplished. As to exomoons, we’ve yet to identify one, but so much good work has been accomplished on how to achieve such a detection that it’s surely not going to be long before we have candidate exomoons to focus in on.

The paper on this work is not yet out, but I’ll announce it here when it’s available. Meanwhile, thoughts on how many habitable worlds are out there continue to be expansive. More on this tomorrow, as we return to news coming out of the Austin conference.

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{ 15 comments }

andy January 10, 2012 at 15:52

The immediate question that arises from this is whether such a habitable exomoon would have already been detectable via transit timing/duration variations in the existing data. What configurations can be ruled out so far?

Also have to wonder whether 1:1 resonant orbits (Trojans, horseshoe orbits or quasi-satellites) would be stable against perturbations from the central binary. Such orbits if stable may provide another way for an Earth-like world to remain in a habitable orbit in this system.

andy January 10, 2012 at 16:03

Oh, and regarding the habitability of the Kepler-16 system and the potential for seasonal variations induced by the varying distance between the planet and the stars, it may be worth taking a look at this arXiv paper.

Black Sabbath January 10, 2012 at 20:01

It seems to me that scientists are too quick to write off gas giants when it’s almost certain they each will be orbited by numerous moons.

Phil January 11, 2012 at 0:14

I think it would be neat to read a study about what the Saturn, or Jupiter system would be like if transported (some how) to Mars, Earth, and Venus like distances from our sun. How would this sudden warming effect those satellites already heated from within. Would the new external heating be enough to push these planets into a more earth like habitability?

ljk January 11, 2012 at 1:37

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A Saturn-Like Ring System Eclipsing A Sun-Like Star Discovered

Monday, 09 January 2012 22:33

MessageToEagle.com – A team of astrophysicists from the University of Rochester and Europe has discovered a ring system in the constellation Centaurus that invites comparisons to Saturn.

The scientists, led by Assistant Professor of Physics and Astronomy Eric Mamajek of Rochester and the Cerro Tololo Inter-American Observatory, used data from the international SuperWASP (Wide Angle Search for Planets) and All Sky Automated Survey (ASAS) project to study the light curves of young Sun-like stars in the Scorpius-Centaurus association—the nearest region of recent massive star formation to the Sun.

The basic concept of the research is straightforward. Imagine yourself sitting in a park on a sunny afternoon and a softball passes between you and the sun. The intensity of light from the sun would appear to weaken for just a moment.

Then a bird then flies by, causing the intensity of the sunlight to again weaken—more or less than it did for the baseball, depending on the size of the bird and how long it took to pass. That’s the principle that allowed the researchers to discover a cosmic ring system.

A light curve is a graph of light intensity over time, and one star in particular showed dramatic changes during a 54 day period in early 2007. University of Rochester graduate student Mark Pecaut and Mamajek discovered the unusual eclipse in December 2010.

“When I first saw the light curve, I knew we had found a very weird and unique object. After we ruled out the eclipse being due to a spherical star or a circumstellar disk passing in front of the star, I realized that the only plausible explanation was some sort of dust ring system orbiting a smaller companion—basically a ‘Saturn on steroids,’” said Mamajek.

If a spherical object merely passed in front of the star, the intensity of the light would gradually dim and reach a low point before gradually increasing. That was not the case with the star identified as 1SWASP J140747.93-394542.6.

The Rochester team discovered a long, deep, and complex eclipse event with significant on-and-off dimming. At the deepest parts of the eclipse, at least 95% of the light from the star was being blocked by dust.

The shape of the light curve was very similar to that of a well-researched star (EE Cephei), suggesting similar traits in the companion objects. However EE Cephei differs in that it appears to be a thick protoplanetary disk transiting—or passing—in front a massive, hot star. “We suspect this new star is being eclipsed by a low-mass object with an orbiting disk that has multiple thin rings of dust debris,” said Mamajek. The star is similar in mass to the sun, but is much younger – about 16 million years old or 1/300th the age of the solar system – and it lies about 420 light years away.

Full article here:

http://www.messagetoeagle.com/index.php/space/34-astronomy/1002-a-saturn-like-ring-system-eclipsing-a-sun-like-star-discovered

FrankH January 11, 2012 at 16:14

@Phil,
All of the moons around Jupiter and Saturn are made of a mixture of ice (lots of it) and rock. If brought to Earth’s temperature, they would quickly loose whatever atmosphere they have (Titan, for example) since they are not massive enough to hold on to an atmosphere at HZ temperatures. Most of the ices would melt (some are not water ice) and the resulting water would sublimate and again, escape the moon. The end result would be a smaller, rocky, airless world.
The largest moons of ALL of the gas giants in our solar system seem to be hitting an upper limit in size. Theoretical work (http://www.nature.com/nature/journal/v441/n7095/full/nature04860.html) shows this to be around 0.01% – 0.02% of the mass of the planet, so it’s highly unlikely to find an exo moon massive enough to hold on to a sizable atmosphere at the the temperatures found in a HZ; they would have to be considerably more massive than Mars.
The only realistic way to get a habitable moon is if the gas giant captures an Earth sized planet as it migrates into the HZ.

andy January 11, 2012 at 16:57

A couple more circumbinary planets have been discovered using Kepler data: say hello to the Kepler-34 and Kepler-35 systems. Interestingly, Kepler-16, Kepler-34 and Kepler-35 all host roughly Saturn-mass planets close to the critical semimajor axis within which the orbits would be unstable. Such systems had previously been predicted by studies of planet migration in circumbinary systems, where Saturn-mass planets seem to be better able to survive against migration into the unstable zone than Jupiter-mass planets.

Zanstel January 11, 2012 at 17:03

@FrankH,

Or perhaps, the colision of two similar planets like Earth/Theia -> Earth/Moon.
Using the ratio of Earth/Moon mass and extrapolate to a Saturn/Uranus big planet, the moon should be similar to a rocky planet. Probably, by the formation, more dry, but enough big to mantain an atmosphere. If the collision occurs on the formation before the late bombarment, the moon could have enough water and CO2 (later O2) to became habitable.

An exomoon, with a big planet near, with to big suns ( because cold stars appears big in HZ independently of the real size). What a beautiful place!

andy January 11, 2012 at 18:59

All of the moons around Jupiter and Saturn are made of a mixture of ice (lots of it) and rock.

To be pedantic here, among the major satellites of the gas giants there is a significant exception to this rule: Io doesn’t have much ice in it at all and is effectively a terrestrial planet…

Enzo January 11, 2012 at 19:00

@FrankH,

Io looks like an example of what you describe given that its surface would have been more than warm enough to have liquid water (even sporadically).

Large Jovian planets (~10 MJ) could offer more opportunities even though I don’t think they are particularly common in the HZ.
Can a 1/3 Me world hold onto its atmosphere and/or water ? I seem to remember that it could.
Would it have a strong enough magnetic field to protect itself from the radiation belts of a giant Jovian planet though ?

Nick January 12, 2012 at 2:59

FrankH: Great analysis. I’d summarize it as follows: if you moved a gas giant moon into the “habitable zone”, it would end up like earth’s moon. Ironically, your analysis indicates that gas giant moons are far more habitable (due to the occasional under-ice ocean) at Jovian distances than they would be in the “habitable zone”.

ljk: It’s fascinating that this data was recorded in 2007 and is only being explained now. I wonder how much other anomalous eclipse data is out there that doesn’t neatly fit into the eclipse=planet assumption. Large artificial structure(s), such as solar power or shading structure (s) with a surface area similar to that of a gas giant planet, would look at the first level of analysis like a ring: an eclipse that is much larger than can be explained by the mass of the eclipsing object if it were a planet. SETI folks should be chomping at the bit to find large artificial structures from this growing mountain of eclipse data. This is a far more likely approach than obsessing over an inefficient communications method that briefly predominated on earth during the last century. But I suppose that, just as fairies need or desire nothing to live except perfectly natural garden flowers, there are good reasons why of all those ETIs in our galaxy none of them ever build such a blatantly technological structure. They all just play gleefully among untouched rings and planets.

andy January 12, 2012 at 4:19

The paper has now appeared on arXiv: Habitability of Earth-type Planets and Moons in the Kepler-16 System. The case of Trojan planets is also considered in the paper, seems they could be stable.

FrankH January 12, 2012 at 15:55

@Andy,
Io is a rocky world because the tidal heating from Jupiter probably drove away all the remaining volatiles eons ago. Some of the volatiles were probably also lost after the formation off Jupiter – it was putting out a lot more heat than it is now.

In any case, Io at Earth’s temperatures would look much a slightly larger and more massive Moon. There are no gas giant moons that come even close to the mass of Mars (Ganymede has less than 1/4 the mass).

What bugs me the most about all these “habitable exo-moons” is that the search can potentially distract from the search of truly Earth-like worlds.
If I were on a telescope time allocation committee and had to choose between a project to look for extraordinarily unlikely Earth-like moons around known gas giants and a project to search for planets around stars known NOT to have a gas giant in their HZ… I know which one I’d pick.

Christopher L. Bennett January 12, 2012 at 16:51

FrankH wrote: “The largest moons of ALL of the gas giants in our solar system seem to be hitting an upper limit in size. Theoretical work… shows this to be around 0.01% – 0.02% of the mass of the planet, so it’s highly unlikely to find an exo moon massive enough to hold on to a sizable atmosphere at the the temperatures found in a HZ; they would have to be considerably more massive than Mars.”

Well, if the upper limit is 1/5000 the mass of the Jovian, then a Jovian with a mass of, say, 10 Jupiter masses (roughly the limit between Jovians and brown dwarfs) could have “native” (as opposed to captured) moons as large as 0.625 Earth masses, which could be enough to sustain habitability assuming that tidal stresses keep the planet partially molten and tectonically active. And you could potentially have Earth-sized planets around low-mass brown dwarfs.

Rob Henry January 12, 2012 at 20:10

I can’t help feeling that if the moons of Saturn and Jupiter were warmed, the results would be more complex that portrayed in these comments. Hundred kilometre deep oceans containing ammonia and hydrocarbons as well as water won’t instantly evaporate just after that ocean surface reaches melting point. My pick is that lots of interesting stuff will happen for hundreds of millions of years on the largest moons where atmospheric loss cannot be completely explained by rapid Jeans escape. If some of them had powerful magnetic fields, I think it likely that the period of interest at a “just thawed” temperature would stretch to several billion years.

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