An Exomoon For A Lonely World?

The discovery of alien worlds–exoplanets that dwell beyond our Sun’s family–has now become just “business as usual” for astronomers who hunt for these distant planets throughout our Milky Way Galaxy. The first alien world circling a Sun-like star was discovered back in 1995, and today the list of confirmed exoplanets has grown to approximately 1,000–with many more candidates awaiting confirmation.

But what about exomoons–moons that orbit these remote alien worlds?

In December 2013, a team of astronomers said that they may have spotted a weird system composed of an alien planet and its moon, floating freely in interstellar space, rather than inhabiting the close-knit family of a parent star.

The strange and remote system revealed its presence in a study that made use of a technique termed gravitational microlensing, which searches for the bending of starlight that results from the gravitational pull of an unseen body existing between a distant star and our planet. In this particular case, the massive body could conceivably be an alien world and its bewitching, circling moon. However, the astronomers admit that the signal is not sufficiently clear, and it might instead be coming from a faint star and a lightweight exoworld in orbit around it.

“An alternate star-plus-planet model fits the data almost as well” as an alien world and its moon-companion scenario, the astronomers reported in a study that was published in December 2013 on the preprint site arXiv. The paper has not yet been peer-reviewed.

We have known since 1995 that our Solar System is far from unique in the Cosmic scheme of things. In fact, astronomers think that our Milky Way Galaxy could be literally swimming with billions upon billions of planets–and an even greater number of moons. Some of these moons could even possess the mysterious and precious recipe that permits them to harbor living things.

As far back as ancient Greece, humanity has wondered if there are other solar systems beyond our own. This idea has not always been greeted with joy by the powers-that-be. For example, in 1584, when the Catholic monk Giordano Bruno declared (among other things) that there are “countless suns and countless earths all rotating around their suns,” he was accused of heresy by the Church and burned at the stake. Our planet was unceremoniously evicted from its exalted status as the most important body in the Universe early in the 16th century, when Nicolaus Copernicus calculated that Earth orbits the Sun, rather than the other way around. His revolutionary concept, although reluctantly and bitterly swallowed at the time, shattered the traditional Judeo-Christian dogma that we and our Earth hold a central, special place in the Universe.

During the past generation, alien worlds have been discovered at a frenetic pace. Since the 1995 discovery of 51 Pegasi b–the first exoplanet to be spotted in orbit around another Sun-like star–astronomers have been finding worlds upon worlds upon worlds dwelling in our majestic, barred-spiral Galaxy.

But What About Moons?

Our own Solar System’s eight major planets possess (at last count) 170 moons. Most of these moons are barren, icy, bodies circling the quartet of frigid, gaseous, giant planets dwelling in the outer limits of our Sun’s family: Jupiter, Saturn, Uranus, and Neptune. However, a few of these icy jewels may hide living tidbits. For example, Europa of Jupiter, may sport a global subsurface ocean secreted beneath its shattered icy shell, blissfully warmed by tidal flexing into a life-loving, liquid water state. It is possible that primitive tidbits of aquatic life swim around in Europa’s subsurface, salty sea. Titan, of Saturn, possesses an environment eerily similar to that of our Earth before life developed here (prebiotic). Lazy, large raindrops of liquid hydrocarbons float slowly onto the surface of this frigid, tortured moon, forming seas and lakes composed of liquid methane and ethane that play the same role as water does on our planet. It is possible that life, as we do not know it, has evolved there using liquids other than water.

In our own Solar System, moons are becoming major targets for future space missions. Yet, when astronomers peer far beyond our own extended solar family, they are apparently more focused on spotting alien planets than their alien moon-companions. However, as exomoon hunter Dr. David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts has noted: “I think exomoons are just as interesting as exoplanets.”

Indeed, now that astronomers know that a multitude of exoplanets are dancing around our Galaxy, exomoons should also abound. However, moons that circle alien worlds are extremely difficult to spot because of their small size and lack of brightness.

If Planets Come, Can Moons Be Far Behind?

The authors of the December 2013 paper, titled A Sub-Earth Mass Moon Orbiting a Gas Giant Primary or a High Velocity System in the Galactic Bulge, explain in their study that gravitational microlensing is a promising new technique for astronomers to use in order to spot moons dwelling beyond the close-in satellites that transit searches are designed to discover. The transit technique–another technique used to spot exoworlds–searches for the tattle-tale dimming of a star’s light that results from a planet or moon passing in front of its fiery, brilliantly incandescent face from Earth’s vantage point.

The study was led by Dr. David Bennett of the University of Notre Dame in South Bend, Indiana. Regardless of whether or not the weird system does turn out to include a distant, dancing moon, “these results indicate the potential of microlensing to detect exomoons,” the authors wrote.

“I was excited by this paper,” said Dr. Jean Schneider in the December 23, 2013 Scientific American. Dr. Schneider is of the Paris Observatory, and was not part of the new research. He continued to note that exomoons have “become fashionable these days,” and are one of his personal “holy grails.” Dr. Schneider authored a paper in 1999 describing how to spot exomoons using the alternative transit technique.

Microlensing is a form of gravitational lensing, an effect on light that was predicted by Albert Einstein in his General Theory of Relativity. According to the General Theory of Relativity, massive objects bend–or warp–the Spacetime near them. This means that literally everything, even light, will take a curved path around such massive objects due to Spacetime warpage.

Imagine a child carrying a heavy bowling ball to her trampoline, and tossing it onto its stretchy surface. The bowling ball bends, or warps, the material of the trampoline. Now, the little girl takes a handful of marbles, and throws them onto the trampoline where the bowling ball has caused a dimpling in the fabric. The tossed marbles take curved paths around the bowling ball–instead of the straight paths that they would otherwise take if the bowling ball were not there to warp the material of the trampoline. The material of the trampoline represents Spacetime; the marbles are anything, even light, that must take a curved path, instead of a straight one, because of the warpage induced on Spacetime by a massive object–in this case, the bowling ball.

When light emanating from a brilliant background star wanders past a massive object on its way to the greeting telescopes of astronomers, it shows up as bright circles of warped light that are termed Einstein rings. If the massive object is composed of two objects, such as a planet and its companion moon, the circle will appear to be bulging in spots and broken. Sometimes the Einstein ring is too small for astronomers to resolve the details, but the general effect of gravitational microlensing can be determined from the way the star’s overall brightness changes in time.

Dr. Bennett and his team have observed a two-body system, which they have dubbed MOA-2011-BLG-262, from gravitational microlensing information collected at the Mount John University Observatory located in New Zealand, and the Mount Canopus Observatory in Tasmania.

However, at this time, the astronomers are not certain which of the two bodies is responsible for the brightness fluctuations. The explanation that best explains the observations is an enormous exoplanet, about four times the mass of Jupiter, circled by a companion moon. If this is correct, the exoplanet and its moon would be nearby–by cosmological standards–between 1,000 and 2,000 light years from our planet.

They would also be adrift, lost and alone, in our Milky Way Galaxy, with no star to guide them, rather than being part of the close-knit family of one of our Galaxy’s billions of stars!

Free-floating planets are outcasts–the rejected and ejected children of our Milky Way’s billions of stellar systems. These unlucky planetary orphans were cruelly tossed out to fend for themselves in the frigid blackness of interstellar space, far, far away from the delightful warmth and companionship of their parent stars and planetary siblings. Youthful stellar systems are unruly environments, where violent interactions between newly-forming planets and parent stars can eventually result in the tragic ejection of a doomed baby planet from the system that was its cradle. Our own Solar System may have given rise to many more planets than the eight planets that we now know, and these rejected siblings of our Earth may well be freely floating around in the space between stars with no stellar parent to call their own. Astronomers have predicted the real existence of such orphaned worlds for a long time.

Indeed, our Galaxy could be chock full of Jupiter-size free-floating, orphaned alien worlds–there may be billions of them, wandering their solitary way around the Galactic center, in a way similar to the manner in which our Sun and other stars do.

But some of these free-floating worlds may not be so lonely, after all. They may be orbited by companion moons.

If confirmed, this recent tentative discovery of an alien moon beyond our Solar System, will likely represent only the tip of an enormous iceberg! Furthermore, exomoons are just as alluring as exoplanets–and show just as much promise.

“It seems probable that many thousands, possibly millions, of habitable exomoons exist in the Galaxy and now we can start to look for them,” Dr. Kipping told the press in September 2009.