From Forbes: “The Hunt For Extrasolar Moons Heats Up”


Forbes Magazine

Nov 21, 2016
Bruce Dorminey

Super-earths have been the exoplanet-hunting flavor of the last decade, but the moons of extrasolar planets could still be a good bet for finding extant life around some far-flung star. Problem is, to date, such moons have been observationally out of reach.

A hypothetical rendition of the Blue Moon created by “Frizaven” on the 3D Space Simulator Celestia, via Wikipedia.

Yet if the European Space Agency (ESA) launches its ARIEL (Atmospheric Remote-sensIng Exoplanet Large-survey) mission by 2025, there remains a small chance that the spacecraft might actually spot a moon around a relatively hot, giant extrasolar planet. That is, one circling one of the roughly 500 stars the mission would survey. Or so says David Waltham, a University of London geophysicist.

Despite years of sifting through data for their signature, to date, no exomoons have been confirmed. That’s arguably more of a testament to the rudimentary nature of our planet-hunting technology than a dearth of earth-sized moons around giant planets circling other sun-like stars.

“At the moment, we simply don’t know whether earth-sized moons exist,” Waltham, author of Lucky Planet: Why Earth is Exceptional and What that Means for Life in the Universe, told me.

The current search for exomoons, uses data from planets transiting across the face of their parent stars, the so-called transit method; as well as data from surveys to look for the telltale stellar wobble caused by an exoplanet in orbit around its host star.

Planet transit. NASA/Ames
Planet transit. NASA/Ames

Such surveys sometimes even look for the wobble of an exoplanet caused by an orbiting exomoon.

ARIEL will look for “transit time variations” (TTVs) in the exoplanets it finds, says Waltham. That is, small variations of less than a minute in the exact timing of a planet as it transits across the face of its parent star. Such transit variations could, in theory, be caused by the gravitational effect of a moon on its host planet; causing the planet to wobble around the planet-moon center of gravity.

“Attempts to spot moon-generated TTVs in Kepler data have so far failed because the data is too noisy to allow such small effects to be seen,” said Waltham. “My hope had been that since ARIEL data will be from bright stars, the data would be less noisy and allow moons to be found.”

But Waltham plans on looking for ARIEL’s transit time variations anyway.

“Exomoons are unlikely around the majority of target planets for ARIEL but, if a few of the targeted planets orbit sun-sized stars, I might get lucky,” said Waltham.

On another front, using data from NASA’s Kepler Space Telescope, the ongoing “Hunt for Exomoons with Kepler” (HEK) Project, led by Columbia University astronomer David Kipping, is continuing. Kipping and colleagues note that our own moon — only about 1% of earth’s mass — would be very hard to detect from light years away. But their search is still sensitive to statistically detecting an Earth-Moon combination for about one in every eight extrasolar planets they study.

As for an earth-sized moon around a large planet?

The project notes that statistically they would be sensitive to detecting one such exomoon for every three extrasolar planets surveyed.

Our own anomalously large moon is about as inhospitable as they come. Most habitable planetary bodies require some sort of active geophysical tectonics or atmospheric recycling, which are usually not a feature of smaller bodies. And the only known moon with a substantial atmosphere, remains the Saturn’s moon of Titan, some 0.4 earth radii in size.

Thus, what’s the next ground- or space-based effort to detect exomoons?

Waltham says exomoon detection using transit timing variations will be very hard from the ground but it may be possible to see their direct transit signature using the next generation of very-large telescopes. But he says that NASA’s planned James Webb Space Telescope (JWST) due for launch in 2018, has the best chance of finding the first exomoons.

NASA/ESA/CSA Webb Telescope annotated
NASA/ESA/CSA Webb Telescope annotated

As for the percentage of known exoplanetary systems that might also harbor habitable exomoons?

“The statistics suggest that if earth-sized exomoons were really common, we’d probably have spotted one by now ,” Duncan Forgan, an astronomer at the U.K.’s University of St. Andrews, told me.

Getting anything like a spectrum — which might tell us about the moon’s atmosphere or surface composition — will require much bigger telescopes than anything currently planned, Waltham says.

However, Forgan says the biggest issue with exomoon bio-signatures is that if astronomers do get an exomoon spectrum, for the foreseeable future, they’ll probably be looking at it combined with its host planet’s spectrum as well.

“There’s actually a worrying feature of this blending, which might convince us of habitability when the opposite is true,” said Forgan.

Waltham says the best chance of detecting an exomoon is to look at a long-period, medium-sized planet orbiting a bright star. However, he cautions, astronomers would need to look for many decades to see enough transits to detect such long-period planets.

Meanwhile, Kipping told me that there may still be some exomoon candidates in Kepler data of hundreds of extrasolar planets that he and colleagues are currently analyzing. But he says he is holding off on revealing more until their work is further along.

See the full article here .

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