Judith E. Braffman-Miller speculates in her recent blog about moons on Exoplanets and the search for habitable environments near Kepler-22b.
image from NASA
Moons are enchanting, mesmerizing objects dwelling in their orbits around planets both within and beyond our Solar System. Earth's own large Moon, a silver-golden world that shines in our starlit night sky with the reflected fires of our Star, the Sun, has long been the inspiration of haunting poems and tales of love, as well as myths of magic and madness. Most of the moons of our Sun's own bewitching family are glistening little icy worlds in orbit around the giant planets of the outer Solar System. In June 2013, astronomers announced their dedicated hunt for a habitable moon-world beyond our Sun's family, circling around the planet Kepler-22b, that dwells in the faraway family of a different star.
Moons can be found in a rich assortment of various sizes, shapes, and types. Although they are generally solid little worlds, a few of them are known to sport atmospheres. Indeed, the atmosphere of the second largest moon in our Solar System, Titan of Saturn, is so dense that it hides Titan's strange hydrocarbon-slashed surface beneath a thick orange veil.
Most of the moons dwelling in our Sun's family were probably born from primordial disks of dust and gas, orbiting around newly formed planets, when our Solar System was very young about 4.5 billion years ago. There are at least 150 moons circling the planets in our Solar System--and about 25 moons are currently awaiting official confirmation of their discovery.
Of the four terrestrial, rocky planets of the inner Solar System (Mercury, Venus, our Earth, and Mars), both Mercury and Venus are moonless. Earth possesses one lone Moon, but it is a very large one--the fifth largest moon in our entire Solar System, in fact. Mars, on the other hand, has two tiny misshapen moons that resemble rocky potatoes, and are lumpy and dark, as they travel in their nearly circular orbits close to the plane of the Martian equator. The Martian moons, Phobos and Deimos, are probably asteroids that were captured by Mars long ago.
The outer Solar System is more richly endowed with moons than the inner regions. The two enormous gas giant planets, Jupiter and Saturn, and the ice giant planets, Uranus and Neptune, have numerous moons of various sizes, shapes, and origins. As these enormous planets grew, during the early days of our Solar System, they were able to ensnare wandering objects with their mighty gravitational grips.
Jupiter, the largest planet in our Solar System, also has the largest moon--Ganymede. A large number of Jovian moons sport highly elliptical orbits and also circle backwards--that is, opposite to the spin of their planet. Saturn, Uranus, and Neptune also sport such so-called irregular moons, that orbit far from their respective parent planets.
Earth's bewitching large Moon was probably born as the result of an immense impact, when a Mars-size protoplanet named Theia smashed into Earth about 4.5 billion years ago. This cataclysmic collision is thought to have hurled a vast amount of Earth-stuff and Theia-stuff into orbit around our ancient planet. Debris from the two unfortunate bodies gradually accumulated to give birth to our Moon, as tumbling little newborn moonlets crashed into one another and melded together into one large object.
We have known since 1995 that our Solar System is far from unique in the Cosmic scheme of things, and that there are a vast number of planets that circle stars beyond our own Sun. Furthermore, some of these extrasolar planets probably have moons just like most of the planets in our Sun's family. These faraway exomoons are enticing little worlds of wonder and mystery--and possibly even life.
A Habitable Exomoon For Kepler-22b?
Kepler-22b is an extrasolar planet that circles Kepler-22, a G-type star that is situated about 600 light-years from our own planet in the constellation Cygnus. This intriguing new world, that resides beyond our Solar System, was first spotted by NASA's highly productive, though ill-fated, Kepler Space Telescope in 2011. Kepler-22b has the distinction of being the first known transiting extrasolar planet to reside within the so-calledhabitable zone of its star. The habitable zone is the term used to describe that Goldilocks region around a star where water can exist in its life-loving liquid state. Planets dwelling in this fortunate region are not too hot, not too cold, but just right for water and, hence, life to exist. A planet that circles its star in the habitable zone suggests that there is the possibility--though not the promise--of life as we know it to exist on that world.
Kepler-22b's initial transit in front of the face of its fiery star was seen by Kepler on its third day of scientific observations, back in May 2009. The third passage was spotted in late 2010. Still more confirming evidence was provided by the infrared Spitzer Space Telescope, as well as by ground-based 'scopes. The confirmation of the existence of this interesting extrasolar planet world was announced on December 5, 2011.
Kepler-22b sports an orbital period of approximately 200 days. Its inclination is about 90 degrees, and it transits in front of the face of its star as observed by Earth-based astronomers. The shape of Kepler-22b's orbit is unknown, but it is known that its average orbital distance is well within its parent star's habitable zone. Many extrasolar planets are known to sport highly elliptical (football-shaped) orbits, and if Kepler-22b also travels along such a path around its star, it would only spend a tiny fraction of its time within this Goldilocks zone. This would cause the planet to undergo such extreme variations in temperature that it would not be a pleasant place for delicate living things to evolve and flourish. Kepler-22b is approximately 2.4 times the radius of Earth.
With the discoveries of 132 confirmed extrasolar planets and more than three thousand planet candidates, the Kepler mission revolutionized scientific understanding of planets residing beyond our own Star. Much of the attention surrounding these discoveries has focused on identifying an Earth-analog--a planet about the size of our own world dwelling within the precious Goldilocks zone around its distant star. Now, for the first time, Dr. David M. Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and his team, have started to hunt for a habitable moon around Kepler-22b!
Moon-Hunters
Kepler-22b, with its radius almost two and a half times that of Earth, is too large to be considered a true Earth-analog. Nevertheless, if it possesses an Earth-sized moon, the planetary system could still host a habitable world like our own.
In order to spot such a remote exomoon, the authors of this new study, The Hunt for Exomoons with Kepler (HEK): III. The First Search for an Exomoon around a Habitable-Zone Planet, used a technique that models the dips and features of the parent star's light-curve (stellar brightness vs. time), which are caused by transits of the planet (and any accompanying moons) in front of the face of its star. This is a complicated and difficult endeavor because numerous and diverse models of planet-moon dynamics must be taken into consideration. Each one of these models possesses parameters that describe physical properties belonging to the planet or moon, as well as parameters describing the orbital system. The authors use what is termed Bayesian statistics to account for the fact that the true orbital model of this planetary system is still not known--and this enables them to calculate if a model with our without a moon fits the observed light-curve the best.
The team also considered whether it would be possible to determine, with an adequate degree of certainty, if a detected moon could bear life-loving liquid water. In their analysis, the "input" climate for the moon is habitable, which is identified with high probability. However, there still remains approximately a one in six failure rate.
With such intriguing results before them, the team of astronomers studied the data to determine if Kepler-22b actually has a moon. Unfortunately, their analysis reveals no evidence for the existence of an exomoon circling Kepler-22b. This non-detection suggests that the mass of any companion world around Kepler-22b must be less than 0.54 times the mass of our planet--with an impressive confidence rate of 95%! Therefore, it is very unlikely that Kepler-22b is circled by an Earth-like moon. Nevertheless, it is still too soon to give up hope. The Hunt for Exomoons with Kepler project has studied nine planetary systems in search of exomoons. Although none were detected, with the team's new results about the possibility of finding Earth-sized moons and the remaining treasure trove of Kepler data to sift through, large and possibly even habitable exomoons may start being spotted in the near future.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
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