Exomoons: When Parent-Planets Go Their Separate Ways
By Judith E Braffman-Miller
Restless young planets like to roam through their solar systems–usually disrupting their environment, during the course of their reckless rampages, as they travel chaotically around their stars. Such planetary migration occurs when a planet interacts with either the natal disk of gas and dust whirling around their stellar hosts, or with primordial planetary building blocks called planetesimals. But what happens to orbiting moons when their young parent-planets go their separate ways? Exomoons are natural satellites of exoplanets, or other non-stellar extrasolar bodies, and they are frequently disrupted because planet migration is thought to be common as young solar systems are just beginning to settle down. In March 2018, astronomers announced that their new research shows that migrating planetary encounters of the worst kind could have a significant impact on the moons of giant exoplanets–and they may generate a large population of orphan, free-floating exomoons, that have no parent-planet to call their own.
Indeed, it has been inferred from empirical studies of the multitude of moons inhabiting our own Solar System, that moons are likely to be common denizens of planetary systems belonging to stars beyond our own Sun. Most of the exoplanets that have been discovered so far are giant planets–such as our own Solar System’s quartet of outer behemoth planets: Jupiter, Saturn, Uranus, and Neptune. Indeed, the four jumbo outer gaseous worlds of our Sun’s family have large families of mostly icy moons, that dance around them in a mesmerizing, sparkling ballet. For this reason, it is reasonable to assume that exomoons are just as common in the families of other stars.
Even though exomoons are very faint and hard to find, making them difficult to confirm using current techniques, observations from missions such as NASA’s planet-hunting Kepler Space Telescope have discovered a number of intriguing candidates. Some of these exomoons can possibly be habitats for extraterrestrial life–and one may even be an orphaned rogue free-floater.
During the “scattering” process, any exomoons that are in orbit around giant parent-planets, can be shoved into unstable orbits that result from tragic close encounters with perturbing planets. Exomoons can also be shaken up if their parent-stars’ properties or orbits change as the result of their family of migrating, rampaging young planets.
A team of astronomers, led by Dr. Yu-Cian Hong of Cornell University in Ithaca, New York, have explored the fate of exomoons in planet-planet scattering situations, using a suite of N-body numerical simulations.
Many Moons, Migrating Planets, And Their Stellar Systems
Moons come in a rich assortment of differing sizes, shapes, and types. Although moons are usually very small, solid, and airless worlds, a few of them are known to possess atmospheres. For example, in our own Solar System, Saturn’s large moon Titan is enshrouded by a dense orange hydrocarbon atmosphere that is so thick that it hides the tormented surface of this moon-world.
Most of the moons dwelling in our own Solar System formed from ancient, whirling accretion disks surrounding young planets when our Sun was still in its flaming youth–about 4.5 billion years ago. There are at least 150 moons known to be orbiting the planets of our Solar System–but there are probably many more that still wait to be confirmed.
What astronomers have known for more than a generation is that our own Solar System is far from unique. In fact, there are a billions of planets in orbit around stars far beyond our own Sun. Some of these distant alien planets might very well host alien moons–just like most of the planets that orbit our own Star. These distant moon-worlds that form around remote exoplanets sing a haunting sirens’ song to those astronomers who are on the hunt for them.
Most of the stellar inhabitants of our barred-spiral Milky Way Galaxy are much dimmer and smaller than our Sun. These red dwarf stars are both the most numerous, as well as the tiniest, true stars known–and because they are so cool, they take “life” easy, leading very long, peaceful “lives”. Red dwarfs “live” for an extraordinarily long time on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. Stars, like our Sun, are small stars–but they are not nearly as small as their red dwarf cousins. Our Sun is a middle-aged star of about 4.5 billion years of age–and it has another 5 billion years to go before it perishes, after having run out of its necessary supply of hydrogen-burning nuclear fuel. Stars of our Sun’s mass live for about 10 billion years. In dramatic contrast, it is thought that red dwarfs can lazily burn their needed supply of nuclear-fusing hydrogen fuel for trillions of years. This means that no red dwarf star has had enough time to perish because our Universe is “only” about 14 billion years old.
Low-mass, red dwarf stars–that are about 80 times the mass of our own Solar System’s banded behemoth, the gas-giant planet Jupiter–possess core temperatures that are just barely high enough to fuse hydrogen into helium. The ability to fuse hydrogen into helium separates true stars from stellar “failures”–called brown dwarfs–that lack sufficient mass to perform this feat. The brightness of small “true” red dwarf stars is less than one thousandth that of our Sun.
As of March 8, 2018, there are 3,743 confirmed exoplanets inhabiting 2,796 systems, with 625 systems hosting more than one planet. Assuming there are 200 billion stars in our Galaxy, one can go on to estimate that there may be as many as 11 billion habitable planets in our Milky Way–rising to 40 billion if red dwarfs are also taken into account.
Are we alone? The current scientific quest to discover life elsewhere in the Cosmos can finally answer this profound question. Some of the distant exomoons, belonging to the planets of faraway stars, may be precious abodes of life.
Protoplanetary accretion disks surrounding youthful stars have lifetimes of a few million years. If planets that have masses of about one Earth-mass or greater form while the gas is still present, the planets can exchange angular momentum with the encircling gas of the protoplanetary accretion disk. When this happens, the orbits of the planets change gradually over time. Even though the direction of migration is usually inwards in locally isothermal disks, outward migration can sometimes occur in disks that possess entropy gradients.
Planets close to their stars in circular orbits have a tendency to stop spinning and, as a result, become tidally locked. This means that close-in planets show only one face to their roiling stellar hosts. As the planet’s rate of rotation slows down, the radius of a synchronous orbit of the planet travels outward from the planet. For those worlds that are tidally locked to their host stars, the distance from the planet at which a moon will be in synchronous orbit around its parent-planet is outside what is termed the Hill sphere of the planet. The Hill sphere of a planet is the region where its gravity is dominant over that of its host star, and so it can retain its grip on its attendant moon or moons. Moons that are situated inside the synchronous orbit radius of a planet will tend to spiral into their parent-planet. As a result, if the synchronous orbit is outside the Hill sphere, then all moons will spiral into their cannibalistic parent-planet. In dramatic contrast, if the synchronous orbit is not three-body stable then any moons outside this radius will escape orbit before they reach the synchronous orbit.
The existence of exomoons orbiting alien planets is still theoretical. Although there have been many successes, performed by planet-hunting astronomers using the Doppler spectroscopy of the roiling host star, exomoons elude this particular detection technique and cannot be discovered this way. This is because the resultant shifted stellar spectra, that results from the presence of an orbiting planet with its attendant moons, would behave in exactly the same way as a single point-mass traveling in orbit around its host star. Because of this, there have been several other methods devised by planet-hunting astronomers in their quest to find exomoons. These additional techniques include:
It has been proposed that the star, with the telephone-book-sounding name of J140747.93-394542.6, located in the constellation Centaurus, might possess a planet with a moon. The confirmed exoplanet, dubbed WASP-12b, may also be orbited by a moon of its own.
In December 2013, a candidate exomoon belonging to an orphan, free-floating planet, named MOA-2011-BLG-262, was announced. Alas, as a result of degeneracies in the modeling of the gravitational microlensing event, the observations can also be explained as a Neptune-mass exoplanet circling a low-mass red dwarf stellar host–a scenario that the researchers consider to be more probable. This exomoon was also featured in the news in April 2014.
Currently, there are several known candidate exomoons:
In 1995, the first exoplanet in orbit around a normal main-sequence star, like our Sun, was discovered by a team of Swiss astronomers and confirmed shortly thereafter by a very prolific team of planet-hunting astronomers from the U.S. However, the historic discovery of this bizarre, giant world, 51 Peg b, caused a considerable amount of confusion. This is because 51 Peg b circled its star, 51 Pegasi, fast and close in a “roasting” orbit. At the time, astronomers believed that giant, gas-laden worlds could only exist in orbits far from their host stars–similar to that of Jupiter’s whirl around our own Sun. So, why was 51 Peg b performing such a close dance with the roiling, searing-hot 51 Pegasi? 51 Peg b proved to be the first to be detected of a new class of previously unknown planets called hot Jupiters. The leading theory, explaining how hot Jupiters form, is that they originally are born far from their stars–like our own Jupiter–but ultimately migrate inward in the direction of the fiery furnaces of their searing-hot stellar hosts. During the course of their travels, these unfortunate giant worlds wreak havoc with the orbits of their sibling planets.
Where Are You Going My Little One?
Dr. Hong and her colleagues have found that the great majority–approximately 80 to 90%–of exomoons around giant planets are destabilized during migratory scattering, and do not survive in their original places of birth, within their own solar systems. The tragic fates of these distant moons, caused by these destabilized orbits, include:
–moon perturbation onto a new heliocentric orbit–thus, becoming a planet, and a moon no more.
–moon capture by the perturbing planet.
–moon ejection from its solar system.
–moon collision with the star or a planet.
–ejection of the entire planet-moon system from its solar system.
It is not particularly surprising that exomoons, that reside close to their parent-planets in close orbits–as well as those that circle larger planets–are the most likely to survive those dangerously close encounters. For example, exomoons on orbits similar to those of Jupiter’s quartet of Galilean moons–Io, Europa, Ganymede, and Callisto–have a 20-40% chance of survival.
One especially interesting result of Dr. Hong and her collaborators’ research is the prediction of a population of orphan, free-floating exomoons that were evicted from their solar systems during a planet-planet scattering. These lonely alien moons now wander through the cold Cosmos, with no star or parent-planet of their own to keep them company. According to the researchers’ new models, there could well be as many of these solitary exomoons as there are stars in the Universe.
Future surveys that hunt for objects using gravitational microlensing–like that planned with the Wide-Field Infrared Survey Telescope (WFIRST)–may have the ability to discover such objects down to masses of a tenth of an Earth-mass. Meanwhile, astronomers are getting ever closer and closer to a new understanding of the complex and mysterious dynamics that built up ancient solar systems.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, 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 some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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