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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Robo-AO laser: Laser-wielding robot probes exoplanet systems

The ultraviolet Robo-AO laser originating from the Palomar 1.5-meter Telescope dome. 

Although the laser is invisible to the human eye, it shows up in digital SLR cameras once their internal UV blocking filters are removed. 

The apparent colour of the laser beam is a result of the UV light leaking through the camera's red, green and blue pixel filters by slightly different amounts.

An international team, including Dr. Christoph Baranec of the University of Hawaii at Manoa's Institute for Astronomy, is using the world's first robotic laser adaptive optics system, Robo-AO, to explore thousands of exoplanet systems (planets around other stars) at resolutions approaching those of the Hubble Space Telescope.

The results, which shed light on the formation of exotic exoplanet systems and confirm hundreds of exoplanets, have just been published in the Astrophysical Journal.

The design and operation of the unprecedented instrument has just been published in the Astrophysical Journal Letters.

Laser adaptive optics systems are used by terrestrial telescopes to remove the image-blurring effects of Earth's turbulent atmosphere, thereby capturing much sharper images than are otherwise possible from the ground.

Baranec, Robo-AO's principal investigator and lead author of the Astrophysical Journal Letter, led the development of the innovative Robo-AO system on the Palomar 1.5-meter Telescope.

It is the world's first instrument that fully automates the complex and often inefficient operation of laser adaptive optics.

"We're using Robo-AO's extreme efficiency to survey in exquisite detail all of the candidate exoplanet host stars that have been discovered by NASA's Kepler mission," said Baranec.

"While Kepler has an unrivaled ability to discover exoplanets that pass between us and their host star, it comes at the price of reduced image quality, and that's where Robo-AO excels."


In fact, analysis of the first part of the Robo-AO/Kepler exoplanet host survey is already yielding surprising results.

Nicholas Law

"We're finding that "hot Jupiters," rare giant exoplanets in tight orbits, are almost three times more likely to be found in wide binary star systems than other exoplanets, shedding light on how these exotic objects formed," said Prof. Nicholas Law (University of North Carolina at Chapel Hill's College of Arts and Sciences), Robo-AO's project scientist and lead author on the Astrophysical Journal paper.

"Going further, Robo-AO's unique capabilities have allowed us to discover even rarer objects: binary star systems where each star has a Kepler-detected planetary system of its own.

These systems will be uniquely interesting for studies of how the planets formed, and for science fiction about what life would be like with another planetary system right next door," continued Law.

Indeed, the first Robo-AO survey, covering 715 Kepler candidate exoplanet hosts, is the single largest scientific adaptive optics survey ever.

That record won't stand for very long, as the Robo-AO team is extending the survey to image each and every of the 4,000 Kepler candidate exoplanet hosts, and is ready to observe exoplanet hosts from Kepler's new K2 mission as they are discovered.

The Robo-AO laser being used to probe exoplanet host stars in the Kepler field. Images of the stars on the Robo-AO science camera (inset) are the same size as a single Kepler pixel. Credit: Robo-AO Collaboration.

The key to Robo-AO's success is its efficiency, allowing it to observe hundreds more targets per night than conventional adaptive optics systems.

So far, the Robo-AO system has already been used to make over 13,000 observations. "The automation of laser adaptive optics has allowed us to tackle scientific questions that were unimaginable just a few years ago.

We can now observe tens of thousands of objects at Hubble-Space-Telescope-like resolution in short periods of time," Baranec said.

"Now that the technology has been proven, we're looking to bring it to the pristine skies of Maunakea, Hawaii, where it will be even more powerful."

More information: Christoph Baranec et al. "HIGH-EFFICIENCY AUTONOMOUS LASER ADAPTIVE OPTICS." The Astrophysical Journal Letters Volume 790 Number 1, 2014 ApJ 790 L8 DOI: 10.1088/2041-8205/790/1/L8

Nicholas M. Law et al. "ROBOTIC LASER ADAPTIVE OPTICS IMAGING OF 715 KEPLER EXOPLANET CANDIDATES USING ROBO-AO" The Astrophysical Journal Volume 791 Number 1. 2014 ApJ 791 35 DOI: 10.1088/0004-637X/791/1/35

Most precise measurement of an alien world's size: Kepler-93b

Using data from NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist's conception.

Credit: NASA/JPL-Caltech

The size of the exoplanet, dubbed Kepler-93b, is now known to an uncertainty of just 74 miles (119 kilometers) on either side of the planetary body.

The findings confirm Kepler-93b as a "super-Earth" that is about one-and-a-half times the size of our planet. Although super-Earths are common in the galaxy, none exist in our solar system.

Exoplanets like Kepler-93b are therefore our only laboratories to study this major class of planet.

With good limits on the sizes and masses of super-Earths, scientists can finally start to theorize about what makes up these weird worlds.

Previous measurements, by the Keck Observatory in Hawaii, had put Kepler-93b's mass at about 3.8 times that of Earth.

The density of Kepler-93b, derived from its mass and newly obtained radius, indicates the planet is in fact very likely made of iron and rock, like Earth.

"With Kepler and Spitzer, we've captured the most precise measurement to date of an alien planet's size, which is critical for understanding these far-off worlds," said Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seattle and lead author of a paper on the findings published in the Astrophysical Journal.

"The measurement is so precise that it's literally like being able to measure the height of a six-foot tall person to within three quarters of an inch, if that person were standing on Jupiter," said Ballard.

Kepler-93b orbits a star located about 300 light-years away, with approximately 90 percent of the sun's mass and radius.

The exoplanet's orbital distance, only about one-sixth that of Mercury's from the sun—implies a scorching surface temperature around 1,400 degrees Fahrenheit (760 degrees Celsius).

Despite its new found similarities in composition to Earth, Kepler-93b is far too hot for life.

To make the key measurement about this toasty exoplanet's radius, the Kepler and Spitzer telescopes each watched Kepler-93b cross, or transit, the face of its star, eclipsing a tiny portion of starlight.

Kepler's unflinching gaze also simultaneously tracked the dimming of the star caused by seismic waves moving within its interior.

These readings encode precise information about the star's interior. The team leveraged them to narrowly gauge the star's radius, which is crucial for measuring the planetary radius.

Spitzer, meanwhile, confirmed that the exoplanet's transit looked the same in infrared light as in Kepler's visible-light observations.

These corroborating data from Spitzer, some of which were gathered in a new, precision observing mode, ruled out the possibility that Kepler's detection of the exoplanet was bogus, or a so-called false positive.

Taken together, the data boast an error bar of just one percent of the radius of Kepler-93b.

The measurements mean that the planet, estimated at about 11,700 miles (18,800 kilometers) in diameter, could be bigger or smaller by about 150 miles (240 kilometers), the approximate distance between Washington, D.C., and Philadelphia.

More Information: Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode - Authors: Sarah Ballard et al. 2014 ApJ 790 12. doi:10.1088/0004-637X/790/1/12

Kepler-186f: First potentially habitable Earth-sized planet confirmed - water

The artist's concept depicts Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone, a range of distances from a star where liquid water might pool on the surface of an orbiting planet.

The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth.

The artistic concept of Kepler-186f is the result of scientists and artists collaborating to help imagine the appearance of these distant worlds. 

Credit: Danielle Futselaar.

The first Earth-sized exoplanet orbiting within the habitable zone of another star has been confirmed by observations with both the W. M. Keck Observatory and the Gemini Observatory.

The initial discovery, made by NASA's Kepler Space Telescope, is one of a handful of smaller planets found by Kepler and verified using large ground-based telescopes.

It also confirms that Earth-sized planets do exist in the habitable zone of other stars.

"What makes this finding particularly compelling is that this Earth-sized planet, one of five orbiting this star, which is cooler than the Sun, resides in a temperate region where water could exist in liquid form," says Elisa Quintana of the SETI Institute and NASA Ames Research Center who led the paper published in the current issue of the journal Science.

The region in which this planet orbits its star is called the habitable zone, as it is thought that life would most likely form on planets with liquid water.

Steve Howell, Kepler's Project Scientist and a co-author on the paper, adds that neither Kepler (nor any telescope) is currently able to directly spot an exoplanet of this size and proximity to its host star.

"However, what we can do is eliminate essentially all other possibilities so that the validity of these planets is really the only viable option."

With such a small host star, the team employed a technique that eliminated the possibility that either a background star or a stellar companion could be mimicking what Kepler detected.

To do this, the team obtained extremely high spatial resolution observations from the eight-meter Gemini North telescope on Mauna Kea in Hawai`i using a technique called speckle imaging, as well as adaptive optics (AO) observations from the ten-meter Keck II telescope, Gemini's neighbour on Mauna Kea.

Together, these data allowed the team to rule out sources close enough to the star's line-of-sight to confound the Kepler evidence, and conclude that Kepler's detected signal has to be from a small planet transiting its host star.

The diagram compares the planets of the inner solar system to Kepler-186, a five-planet system about 500 light-years from Earth in the constellation Cygnus. 

The five planets of Kepler-186 orbit a star classified as a M1 dwarf, measuring half the size and mass of the sun. 

The Kepler-186 system is home to Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distances from a star where liquid water might pool on the surface of an orbiting planet. 

The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth. 

Kepler-186f is less than ten percent larger than Earth in size, but its mass and composition are not known. 

Kepler-186f orbits its star once every 130-days and receives one-third the heat energy that Earth does from the sun, placing it near the outer edge of the habitable zone. 

The inner four companion planets all measure less than fifty percent the size of Earth. Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e, orbit every three, seven, 13, and 22 days, respectively, making them very hot and inhospitable for life as we know it. 

The Kepler space telescope, which simultaneously and continuously measured the brightness of more than 150,000 stars, is NASA's first mission capable of detecting Earth-size planets around stars like our sun. 

Kepler does not directly image the planets it detects. The space telescope infers their existence by the amount of starlight blocked when the orbiting planet passes in front of a distant star from the vantage point of the observer. 

The artistic concept of Kepler-186f is the result of scientists and artists collaborating to help imagine the appearance of these distant Credit: Credit: NASA Ames/SETI Institute/JPL-CalTech.

More information: "An Earth-Sized Planet in the Habitable Zone of a Cool Star," by E.V. Quintana et al. Science, 2014.

Lick Observatory: APF - First robotic telescope for planet hunters

The Automated Planet Finder is the newest telescope at University of California's Lick Observatory on Mt. Hamilton. 

Credit: Laurie Hatch

Lick Observatory's newest telescope, the Automated Planet Finder (APF), has been operating robotically night after night on Mt. Hamilton since January, searching nearby stars for Earth-sized planets.

Every night the fully autonomous system checks the weather, decides which stars to observe, and moves the telescope from star to star throughout the night, collecting measurements that will reveal the presence of planets.

Its technical performance has been outstanding, making it not only the first robotic planet-finding facility but also one of the most sensitive.

The search for planets beyond our solar system (called "extrasolar planets" or "exoplanets") has yielded a huge number of discoveries in recent years, especially since NASA's Kepler spacecraft joined ground-based telescopes in the effort.

Unlike Kepler, however, which focused on distant stars in one small patch of sky, the APF focuses on nearby stars and covers the entire sky.

"The planetary systems we're finding are our nearest neighbours. Those are the ones that will matter to future generations," said Steve Vogt, professor of astronomy and astrophysics at UC Santa Cruz, who led the $12 million APF project and designed the Gloria and Kenneth Levy spectrometer at the heart of the system.

Vogt said it was a long and difficult road to get the APF facility completed, but now that it's up and running he couldn't be happier with its performance.

Matthew Radovan

Project manager and principal engineer Matthew Radovan of UC Observatories (UCO) oversaw a large team of UCO technical staff who provided crucial support for the construction and commissioning of the APF facility.

Sandra Faber, interim director of UC Observatories (UCO), said she was "absolutely thrilled" at the success of the APF.

"Our UCO staff worked incredibly hard during the past year to get the new facility on the air.

Sandra Faber

The APF is living proof that older observatory sites like Lick Observatory, which has been working for over 125 years, can still produce cutting-edge science," said Faber, a University professor of astronomy and astrophysics at UC Santa Cruz.

Vogt's team has submitted two papers describing new planetary systems recently discovered by the APF.

A paper on the APF itself, with a full description of the system and details on how well it is performing, will be published April 1 in Publications of the Astronomical Society of the Pacific (available online at arXiv.org).

Lobster-shaped extrasolar oceans

What the day side of a tidally locked exoplanet orbiting a red dwarf might look like, given atmospheric carbon dioxide levels similar to modern-day Earth. 

On the top frame, white represents ice while blue represents open water; on the bottom frame, colors represent surface air temperatures. 

The top image in each frame represents a computer model that does not take ocean heat flow into account; the bottom image in each frame does take such heat flow into account. 

Credit: Yongyun Hu

Yongyun Hu

Alien planets circling the most common stars in the universe may often have strange lobster-shaped oceans on their surfaces, researchers in China now say.

These findings suggest the habitable zones where life as we know it might dwell around these stars is smaller than previously thought, scientists added.

The most common type of star in the universe is the red dwarf. These stars, also known as M dwarfs, are small and faint, about one-fifth as massive as the sun and up to 50 times dimmer.

They make up to 70 percent of the stars in the cosmos, a vast number that potentially makes them valuable places to look for extraterrestrial life.

Indeed, recent findings from NASA's Kepler space observatory reveal that at least half of these stars host rocky planets that are one-half to four times the mass of Earth.

Artistic representation of five known potential habitable worlds including Gliese 581g. 

Credit: The Habitable Exoplanets Catalogue, PHL @ UPR Arecibo

Research into whether a distant planet might host life as we know it usually focuses on whether or not it has liquid water, since there is life virtually everywhere there is liquid water on Earth, even miles underground.

Scientists typically concentrate on habitable zones, also known as Goldilock zones—the area around a star where it is neither too hot nor too cold enough for a planet to possess liquid water on its surface.

The habitable zones around red dwarfs are close to such stars because of how dim they are, often closer than the distance Mercury orbits the sun.

Artist's impression of the planetary system around the red dwarf Gliese 581. Credit: ESA

This makes it relatively easy for astronomers to detect worlds in a red dwarf's habitable zone; since the orbits of these exoplanets are small, they complete their orbits quickly and often, and scientists can in principle readily detect the way these worlds dim the light of these stars by passing in front of them.

When a planet orbits a star very closely, the star's gravitational pull can force the world to become "tidally locked" to it.

When a planet is tidally locked to its star, it will always show the same side to its star just as the moon always shows the same side to Earth, so that the planet will have one permanent day side and one permanent night side.

More information: Hu and Yang detailed their findings online Dec. 30 in the journal Proceedings of the National Academy of Sciences: www.pnas.org/content/111/2/629.full

NASA News from Planet-Hunting Kepler Space Telescope

NASA scientists will hold a public teleconference today to reveal the latest planet discoveries by the space agency's prolific Kepler Space Telescope and you can follow the major exoplanet news live online here.

The NASA Kepler mission teleconference will begin at 1 p.m. EST (1800 GMT) and will be webcast live via NASA.