Exomoons Could Be Abundant Sources Of Habitability

Europa is one of the moons in our solar system that could host life. 

What about beyond the solar system? 

Credit: NASA/JPL/Ted Stryk

With about 4,000 planet candidates from the Kepler Space Telescope data to analyze so far, astronomers are busy trying to figure out questions about habitability.

What size planet could host life? How far from its star does it need to be? What would its atmosphere need to be made of?

Look at our own solar system, however, and there's a big gap in the information we need. Most of the planets have moons, so surely at least some of the Kepler finds would have them as well. Tracking down these tiny worlds, however, is a challenge.

A new paper in the journal Astrobiology, called "Formation, Habitability, and Detection of Extrasolar Moons," goes over this mostly unexplored field of extrasolar research.

The scientists do an extensive literature review of what is supposed about moons beyond the Solar System, and they add intriguing new results.

A wealth of moons exist in our own solar system that could host life. Icy Europa, which is circling Jupiter, was recently discovered to have plumes of water erupting from its surface.

Titan, in orbit around Saturn, is the only known moon with an atmosphere, and could have the precursor elements to life in its hydrocarbon seas that are warmed by Saturn's heat.

Other candidates for extraterrestrial hosts include Jupiter's moons Callisto and Ganymede, as well as Saturn's satellite Enceladus.

Lead author René Heller, an astrophysicist at the Origins Institute at McMaster University, in Ontario, Canada, said some exomoons could be even better candidates for life than many exoplanets.

"Moons have separate energy sources," he said. "While the habitability of terrestrial planets is mostly determined by stellar illumination, moons also receive reflected stellar light from the planet as well as thermal emission from the planet itself."

Moreover, a planet like Jupiter, which hosts most of the moons in the Solar System that could support life, provides even more potential energy sources, he added.

The planet is still shrinking and thereby converts gravitational energy into heat, so that it actually emits more light than it receives from the Sun, providing yet more illumination.

Besides that, moons orbiting close to a gas giant are flexed by the planet's gravity, providing potential tidal heating as an internal, geological heat source.

Finding the first exomoon

The first challenge in studying exomoons outside our Solar System is to actually find one. Earlier this year, NASA-funded researchers reported the possible discovery of such a moon, but this claim was ambiguous and can never be confirmed.

That's because it appeared as a one-time event, when one star passed in front of another, acting as a sort of gravitational lens that amplified the background star.

Two objects popped out in the gravitational lens in the foreground, either a planet and a star, or a planet and an extremely heavy exomoon.

For his part, Heller is convinced that exomoons are lurking in the Kepler data, but they have not been discovered yet.

Only one project right now is dedicated to searching for exomoons, and is led by David Kipping at the Canadian Space Agency.

His group has published several papers investigating 20 Kepler planets and candidates in total. The big restriction to their efforts is computational power, as their simulations require supercomputers.

Triton’s odd, melted appearance hint that the moon was captured and altered by Neptune. 

Credit: NASA

Another limiting factor is the number of observatories that can search for exomoons.

To detect them, at least a handful of transits of the planet-moon system across their common host star would be required to absolutely make sure that the companion is a moon, Heller said.

Also, the planet with the moon would have to be fairly far from its star, and decidedly not those close-in hot Jupiters that take only a few days to make an orbit. In that zone, the gravitational drag of the star would fatally perturb any moon's orbit.

Heller estimates that a telescope would need to stare constantly at the same patch of sky for several hundred days, minimum, to pick up an exomoon.

Kepler fulfilled that obligation in spades with its four years of data gazing at the same spot in the sky, but astronomers will have to wait again for that opportunity.

Because two of Kepler's gyroscopes (pointing devices) have failed, Kepler's new mission will use the pressure of the Sun to keep it steady, but it can only now point to the same region of the sky for about 80 days at at time because the telescope will periodically need to be moved so as not to risk placing its optics too close to the Sun.

NASA's forthcoming Transiting Exoplanet Survey Satellite is only expected to look at a given field for 70 days.

In the future, the European Space Agency's PLAnetary Transits and Oscillations of stars (PLATO) will launch in 2024 for what is a planned six-year mission looking at several spots in the sky.

"PLATO is the next step, with a comparable accuracy to Kepler but a much larger field of view and hopefully a longer field of view coverage," Heller said.

Read the full article here

Scanning the skies for Exoplanets and Exomoons in other solar systems

The best prospect for habitable exomoons may be around gas giants. 

Credit: NASA

The first exoplanet was discovered in 1994.

Twenty years later, NASA's exoplanet catalog lists more than 1700 planets confirmed around other stars.

Most of these extra-solar-systems have been measured by changes in light spectra, in stellar motion or dust disks around stars.

Some exoplanets-more than 40 as of today-have even been directly photographed.

Jupiter's moons

One way or the other, we know that exoplanets are out there in abundance, in places we thought they would be and in places we didn't dream a planet could possibly exist. So what comes next? Finding moons.

Exomoons are naturally formed satellites circling around planets in other solar systems. Like the exoplanets themselves, we assume that exomoons are out there in relatively high abundance.

This assumption is based partly upon what we see around us in our own Solar System and partly upon our hypotheses about planetary formation.

Saturn's moons

This is what we observe in our own Solar System: moons are extremely common.

From Earth's one Moon to Jupiter's (currently known) fifty, every planet in the Solar System one astronomical unit or more from the Sun has a natural satellite.

Even Pluto, no longer officially classified as a planet, has a smaller companion circling around it.

Of note, the solid bodies such as Earth and Pluto have very few companions, while gaseous bodies Jupiter, Saturn, Uranus and Neptune have many.

Pluto and Charon

Furthermore, the masses of the Moon and Charon have a very specific relationship to Earth and Pluto in terms of mass: each satellite is about 10-2 the mass of their parent planet.

By contrast, the ratio of satellite masses to parent planet masses for the gas giants is very different: 10-4.

The differences in mass-ratio, how massive the moon is compared to the parent planet, and the differences in composition between the moons of solid planets and those of the gas giants led to a search for different formation scenarios for Earth's moon and the moons of the outer planets.

This is the current hypothesis: that there are two different methods of satellite formation at work in our Solar System.

Amy Barr Mlinar

Both methods were recently reviewed by Dr.Amy Barr Mlinar of Brown University at the Space Telescope Science institute Spring Symposium.

"This has been worked out starting about in the 1960's up through now," said Barr, "You have this [moon/planet] mass ratio of about 10-2 for solid planets, and a [moon/planet] mass ratio of about 10-4 for planets with a gaseous envelope."

Essentially, difference in mass ratios reflects the two completely different origins of our Moon and the satellites of Jupiter.

At the high end of the moon/planet mass ratio, 10-2 are the satellites of solid bodies (Earth and Pluto). These moons were formed from collisions.

Sometime in the distant past an object some large percentage of Earth's size struck the Earth, knocking material away that later coalesced into the Moon. The same is likely true of Charon, Pluto's companion.

Read the full article here

Alien moons could bake dry from young gas giants' hot glow

An Earthlike moon orbiting a gas giant host planet. 

Credit: NASA

When we think of where else life might exist in the universe, we tend to focus on planets but on a grander cosmic scale, moons could prove the more common life-friendly abode.

A single gas giant planet in the not-too-warm, not-too-cold habitable zone around its star, where Earth and Mars correspondingly reside, could host several livable moons.

At this early point in our hunt for exoplanets, most of the worlds we have found in the habitable zone are giants, not Earths.

It's possible that the first inhabited place we discover outside our Solar System will be a moon.

René Heller

It is this sort of consideration that inspires René Heller, a postdoctoral fellow in astronomy at McMaster University, in Ontario, Canada.

He studies how "exomoons" could form, what they might be like and how we might detect them with current or future astronomical instruments.

A major part of his work deals with gauging the habitability of exomoons, which is a bit trickier than planetary scenarios because they orbit another body besides their star.

Rory Barnes

A new paper by Heller and his colleague Rory Barnes, University of Washington and the NASA Virtual Planetary Laboratory examines how heat emanating from a freshly formed exoplanet, coupled with irradiation from the solar system's star, can roast the planet's moons.

Before the planet cools off sufficiently, its close-orbiting moons could lose all their water, leaving them bone-dry and barren.

"An exomoon's habitability is of course constrained by its location in the stellar habitable zone, but it also has a second heat source, its host planet, that has to be accounted for," said Heller, whose paper has been accepted for publication in the International Journal of Astrobiology.

"With regard to this second source, our study shows that at close range, the illumination from young and hot giant planets can render their moons uninhabitable."

Researchers believe moons could serve as suitable abodes for life just as well as planets.

Even moons far beyond the habitable zone, such as Jupiter's Europa and Saturn's Titan, offer tantalizing hints of potential habitability thanks to the subsurface ocean in the former and the intriguing organic chemistry of the latter.

Still, a moon around an exoplanet in the habitable zone stands as a far better bet for life than these frigid candidates.

Heller's findings suggest that we ought to exercise caution, however, before declaring that an Earth-sized, habitable-zone exomoon is a real-life Pandora, the lush moon of science fiction fame in "Avatar."

Before assuming an exomoon is habitable based on its host planet's locale, the moon's current and conjectured past orbital distances will need to be assessed.

"Earth-size exomoons that could soon be detected by our telescopes might have been desiccated shortly after formation and still be dry today," said Heller.

"In evaluating a moon's habitability, it is crucial to consider its history together with that of its host planet."

More Information: International Journal of Astrobiology / FirstView Article pp 1-9 Copyright©Cambridge University Press 2014 DOI:dx.doi.org/10.1017/S1473550413000463

KOI-314c: Newfound planet is Earth-mass but gassy

KOI-314c, shown in this artist's conception, is the lightest planet to have both its mass and physical size measured. 

Surprisingly, although the planet weighs the same as Earth, it is 60 percent larger in diameter, meaning that it must have a very thick, gaseous atmosphere. 

It orbits a dim, red dwarf star (shown at left) about 200 light-years from Earth. 

KOI-314c interacts gravitationally with another planet, KOI-314b (shown in the background), causing transit timing variations that allow astronomers to measure the masses of both worlds. 

This serendipitous discovery resulted from analysis as part of the Hunt for Exomoons with Kepler (HEK) project. 

Credit: C. Pulliam & D. Aguilar (CfA)

An international team of astronomers has discovered the first Earth-mass planet that transits, or crosses in front of, its host star.

KOI-314c is the lightest planet to have both its mass and physical size measured. Surprisingly, although the planet weighs the same as Earth, it is 60 percent larger in diameter, meaning that it must have a very thick, gaseous atmosphere.

"This planet might have the same mass as Earth, but it is certainly not Earth-like," says David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA), lead author of the discovery.

"It proves that there is no clear dividing line between rocky worlds like Earth and fluffier planets like water worlds or gas giants."

Kipping presented this discovery today in a press conference at the 223rd meeting of the American Astronomical Society.

The team gleaned the planet's characteristics using data from NASA's Kepler spacecraft. KOI-314c orbits a dim, red dwarf star located approximately 200 light-years away.

It circles its star every 23 days. The team estimates its temperature to be 220 degrees Fahrenheit, too hot for life as we know it.

KOI-314c is only 30 percent denser than water. This suggests that the planet is enveloped by a significant atmosphere of hydrogen and helium hundreds of miles thick.

It might have begun life as a mini-Neptune and lost some of its atmospheric gases over time, boiled off by the intense radiation of its star.

Weighing such a small planet was a challenge. Conventionally, astronomers measure the mass of an exoplanet by measuring the tiny wobbles of the parent star induced by the planet's gravity.

This radial velocity method is extremely difficult for a planet with Earth's mass. The previous record holder for a planet with a measured mass (Kepler-78b) weighed 70 percent more than Earth.

To weigh KOI-314c, the team relied on a different technique known as transit timing variations (TTV). This method can only be used when more than one planet orbits a star.

The two planets tug on each other, slightly changing the times that they transit their star.

David Nesvorny

"Rather than looking for a wobbling star, we essentially look for a wobbling planet," explains second author David Nesvorny of the Southwest Research Institute (SwRI).

"Kepler saw two planets transiting in front of the same star over and over again. By measuring the times at which these transits occurred very carefully, we were able to discover that the two planets are locked in an intricate dance of tiny wobbles giving away their masses."

The second planet in the system, KOI-314b, is about the same size as KOI-314c but significantly denser, weighing about 4 times as much as Earth.

It orbits the star every 13 days, meaning it is in a 5-to-3 resonance with the outer planet.

TTV is a very young method of finding and studying exoplanets, first used successfully in 2010. This new measurement shows the potential power of TTV, particularly when it comes to low-mass planets difficult to study using traditional techniques.

"We are bringing transit timing variations to maturity," adds Kipping.

The planet was discovered by chance by the team as they scoured the Kepler data not for exoplanets, but for exomoons.

The Hunt for Exomoons with Kepler (HEK) project, led by Kipping, scans through Kepler's planet haul looking for TTV, which can also be a signature of an exomoon.

"When we noticed this planet showed transit timing variations, the signature was clearly due to the other planet in the system and not a moon. At first we were disappointed it wasn't a moon but then we soon realized it was an extraordinary measurement," says Kipping.

Magnetic shielding of exomoons

A new study on magnetic fields around extrasolar giant planets sheds first light on the magnetic environment of extrasolar moons. 

The work, authored by René Heller of the Department of Physics and Astronomy at McMaster University (Canada) and Jorge I. Zuluaga of the FACom group in the Institute of Physics of the University of Antioquia (Colombia), is the first to explore the complex magnetic environment of exomoons and its impact on the habitability of these peculiar bodies.

Regrettably the results are not completely encouraging. Even the most massive moons that can be expected from a formation point of view will be small compared to Earth.

Thus, the only possibility these moons can be magnetically protected from the stellar and cosmic high-energy radiation is that they are encased by their giant planet's magnetosphere.

Yet, in orbits close to the planet, these moons can be subject to enormous tidal heating, potentially making them uninhabitable.

These results represent just the beginning of an interesting research branch, which introduces a new key factor for the habitability of those "Pandora"-like environments.

Probably the first image that comes to our minds when thinking of an inhabited extrasolar moon shows the beautiful landscapes of Pandora, the hypothetical moon of James Cameron's movie Avatar.

But the environments of extrasolar moons seem to be less favored than the idealized version shown on the big screen.

Even if located around planets in the stellar "habitable zone," where the amount of incoming light allows for the existence liquid water and hence life, exomoons are subject to a number of other perturbing effects making things harsher for life than previously thought.

René Heller

In a paper accepted for publication in the Astrophysical Journal Letters, Prof. Jorge I. Zuluaga and Dr. René Heller have studied the relationship among the magnetic field of extrasolar giant planets in the stellar habitable zone and the so-called "habitable edge" around those planets.

This edge defines the minimum distance to the planet at which a moon would just avoid to undergo a runaway greenhouse effect—an uninhabitable state similar to that on Venus.

Heller and Zuluaga studied moons having a mass and size similar to that of Mars orbiting planets with masses and compositions ranging from that of Neptune to Jupiter.

Larger moons, with masses similar to that of Earth, are unlikely to have formed even around the largest planets, while moons lighter than Mars will not be easily detected in the near future.

Structure of the magnetosphere. Dashed lines represent the orbits of moons which are completely shielded (CS), partially shielded (PS) and unshielded (US)

The core of this new work is in the calculation of the size of the magnetospheres of giant planets that are located in the habitable zones of their host stars.

Planetary magnetospheres are "bubbles" made of fields and plasma created by the shock between the stellar wind and the intrinsic magnetic field of the planet.

These bubbles separate the immediate magnetic environment of the planet from the very different environment of the interplanetary space. Magnetospheres could be really huge.

In the solar system, for example, Jupiter is surrounded by a magnetosphere that ends at distances up to 50 times the size of the planet (3.5 million of kilometers or almost 10 times the distance from the Earth to the Moon) on the dayside and stretches out almost as far as the orbit of Saturn in the night or trailing side, creating an "invisible" plasma tail.

Although the magnetosphere and its elongated tail could cover billions of kilometers, the most important part in terms of permanent moon coverage is restricted to a region the size of the dayside radius, or "standoff radius."

Read the full story here