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.

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MARS MARTE: Mars simulation chamber explores habitability of other planets

The simulation chamber, named MARTE, is designed to enable study of the behaviour of instrumentation and samples of different types and sizes in pressure ranges up to 10-6 mbar controlling the gas composition, with temperature control of samples in the range of 108K to 423K. 

Credit: J. Martín-Gago/ICMM

A research team in Spain has the enviable job of testing out new electromechanical gear for potential use in future missions to the "Red Planet."

They do it within their Mars environmental simulation chamber, which is specially designed to mimic conditions on the fourth planet from the sun, right down to its infamous Martian dust.

Mars is a key target for future space exploration, thanks to indications that the planet may have either been capable of supporting life in the past or is possibly even supporting it right now within its subsurface.

To answer the many questions about the habitability of Mars, it's critical to first develop new sensors and instruments capable of detecting the planet's atmospheric and surface characteristics.

In the journal Review of Scientific Instruments, researchers from Centro de Astrobiología, INTA-CSIC, and Instituto de Ciencias de Materials de Madrid describe their work mimicking conditions on Mars.

Jose Angel Martín-Gago

"Mars is a good place to learn about planets similar to ours and, as such, is the target of many NASA and European Space Agency (ESA) missions," explained Jose Angel Martín-Gago, a research professor at the Instituto de Ciencias de Materials de Madrid.

"Our group is primarily involved in the Mars Science Laboratory (MSL) mission to construct a meteorological station intended for future use on a rover to further explore Mars' surface."

By building here on Earth state-of-the-art vacuum chambers capable of reproducing the physical conditions of Mars, including temperature, pressure, gas composition, and radiation, the researchers can experimentally mimic these conditions to test instrumentation in "real" environmental operation conditions.

Vacuum chambers have already enabled the researchers to test some of the meteorological sensors currently used onboard the Curiosity rover, which is exploring the surface of Mars but they are now turning their attention to other challenges, such as Martian dust.

Jesus Sobrado

"We're simulating the effect of the Martian dust, one of the primary problems for planetary exploration, to gain a better understanding of how instruments behave when covered in dust," said Jesus Sobrado, the scientist in charge of the machine's technical development.

As part of its research effort, the team has designed and built vacuum chambers devoted to simulating spatial environments, such as the surface of other planets like Mars' surface or even Jupiter's icy moon Europa, the interstellar medium, and interplanetary regions.

More information: "Mimicking Mars: A vacuum simulation chamber for testing environmental instrumentation for Mars" by J.M. Sobrado, J. Martín-Soler, and J.A. Martín-Gago, Review of Scientific Instruments on March 25, 2014. DOI: 10.1063/1.4868592

The Grand Tack model: 'Rogue' asteroids may be normal

Credit: NASA/JPL-Caltech

To get an idea of how the early solar system may have formed, scientists often look to asteroids.

These relics of rock and dust represent what today's planets may have been before they differentiated into bodies of core, mantle, and crust.

In the 1980s, scientists' view of the solar system's asteroids was essentially static: Asteroids that formed near the sun remained near the sun; those that formed farther out stayed on the outskirts.

But in the last decade, astronomers have detected asteroids with compositions unexpected for their locations in space: Those that looked like they formed in warmer environments were found further out in the solar system, and vice versa. Scientists considered these objects to be anomalous "rogue" asteroids.

But now, a new map developed by researchers from MIT and the Paris Observatory charts the size, composition, and location of more than 100,000 asteroids throughout the solar system, and shows that rogue asteroids are actually more common than previously thought.

Particularly in the solar system's main asteroid belt—between Mars and Jupiter—the researchers found a compositionally diverse mix of asteroids.

The new asteroid map suggests that the early solar system may have undergone dramatic changes before the planets assumed their current alignment.

For instance, Jupiter may have drifted closer to the sun, dragging with it a host of asteroids that originally formed in the colder edges of the solar system, before moving back out to its current position.

Jupiter's migration may have simultaneously knocked around more close-in asteroids, scattering them outward.

Francesca DeMeo

"It's like Jupiter bowled a strike through the asteroid belt," says Francesca DeMeo, who did much of the mapping as a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences.

"Everything that was there moves, so you have this melting pot of material coming from all over the solar system."

DeMeo says the new map will help theorists flesh out such theories of how the solar system evolved early in its history.

She and Benoit Carry of the Paris Observatory have published details of the map in Nature.

The compositional diversity seen in this new asteroid map may add weight to a theory of planetary migration called the Grand Tack model.

This model lays out a scenario in which Jupiter, within the first few million years of the solar system's creation, migrated as close to the sun as Mars is today.

During its migration, Jupiter may have moved right through the asteroid belt, scattering its contents and repopulating it with asteroids from both the inner and outer solar system before moving back out to its current position—a picture that is very different from the traditional, static view of a solar system that formed and stayed essentially in place for the past 4.5 billion years.

"That [theory] has been completely turned on its head," DeMeo says. "Today we think the absolute opposite: Everything's been moved around a lot and the solar system has been very dynamic."

DeMeo adds that the early pinballing of asteroids around the solar system may have had big impacts on Earth.

For instance, colder asteroids that formed further out likely contained ice. When they were brought closer in by planetary migrations, they may have collided with Earth, leaving remnants of ice that eventually melted into water.

"The story of what the asteroid belt is telling us also relates to how Earth developed water, and how it stayed in this Goldilocks region of habitability today," DeMeo says.

More information: Paper: dx.doi.org/10.1038/nature12908