A Moon For A Small World
By Judith E Braffman-Miller
In our Solar System’s outer limits, far from the melting heat and warm, welcoming fires of our Sun, there is a mysterious swath of space situated in the frigid darkness of a perpetual twilight. This is the Kuiper Belt, a remote region where a multitude of icy comet nuclei and other frozen objects orbit our distant Star. Here, in the deep freeze of our Solar System’s far suburbs, the ice dwarf planet Pluto and its quintet of moons do their strange dance along with a treasure trove of others of their frozen kind. Indeed, the Kuiper Belt is so far away that astronomers are only now beginning to explore this shadowy region–unveiling, at last, many of its well-kept secrets. In May 2017, astronomers announced that they had uncovered yet another of this dimly lit domain’s many mysteries–there is a little frozen moon in orbit around 2007 OR10, which is the third-largest known dwarf planet in the Kuiper Belt.
Moons orbiting distant planets can play some very successful games of hide-and-seek. For this reason, they are notoriously elusive. Indeed, the dwarf planet Pluto’s large moon Charon wasn’t discovered until the mid 1970s. However, in May 2017, astronomers made their announcement that they had detected yet another small, elusive moon circling a different dwarf planet. The astronomers used the combined power of three space observatories, including archival images obtained from the Hubble Space Telescope (HST), to find the little moon where it has been hiding. Because of the discovery of this moon, many astronomers think that most of the known dwarf planets inhabiting the Kuiper Belt–that are over 600 miles across– have companion moon-worlds. These frozen little moons could provide astronomers with valuable new insight into how moons were born in the distant, dim outer regions of our Solar System. Indeed, there is an emerging new viewpoint that collisions between planetary bodies can trigger the birth of moons. Based on lunar rock samples collected by NASA’s Apollo mission astronauts, many astronomers now propose that Earth’s solitary, lovely, large Moon was born as the result of a horrific collision between our still-forming planet and an ill-fated Mars-sized protoplanet named Theia about 4.4 billion years ago.
“The discovery of satellites around all of the known large dwarf planets–except for Sedna–means that at the time these bodies formed billions of years ago, collisions must have been more frequent, and that’s a constraint on the formation models. If there were frequent collisions, then it was quite easy to form these satellites,” explained Dr. Csaba Kiss in a May 18, 2017 Hubblesite Press Release. Dr. Kiss is of the Konkoly Observatory in Budapest, Hungary, and the lead author of a research paper announcing the little moon’s discovery.
The primordial planetary bodies, called planetesimals–the building blocks of planets–likely blasted into each other frequently. This is because they lived together in close quarters, due to the crowded conditions of their frozen distant home, located in our Solar System’s outer limits. “There must have been a fairly high density of objects, and some of them were massive bodies that were perturbing the orbits of smaller bodies,” explained team member Dr. John Stansberry in the same Hubblesite Press Release. Dr. Stansberry is of the Space Telescope Science Institute (STSI) in Baltimore, Maryland. He added that “This gravitational stirring may have nudged the bodies out of their orbits and increased their relative velocities, which may have resulted in collisions.”
However, the speed of the colliding planetesimals could not have been too fast or too slow, according to the astronomers. If the impact speed was too fast, the crash would have produced a large quantity of debris that could have escaped–screaming its way out of the Solar System altogether. Conversely, if the impact speed was too slow, the smash-up of primordial objects would have created only an impact crater left behind to tell the ancient tragic story.
For example, smash-ups in the Main Asteroid Belt, situated between Mars and Jupiter, are destructive events because asteroids are zipping around at a fast pace when they crash into each other. The Main Asteroid Belt is a region inhabited by rocky chunks where Jupiter’s powerful gravity can speed up the orbits of asteroids–producing some very violent impacts. The asteroids are lingering relics of the era of planet formation–the left over planetesimals that long ago built up the quartet of small, rocky inner planets: Mercury, Venus, Earth, and Mars. The icy comets of the distant Kuiper Belt–and the even more remote Oort Cloud –are the remains of the icy planetesimals that went into the construction of the four giant gaseous outer planets: Jupiter, Saturn, Uranus, and Neptune.
The Distant Denizens Of The Kuiper Belt
The remote Kuiper Belt is located beyond the orbit of the blue, banded, and beautiful ice giant Neptune–the outermost of the eight major planets of our Solar System. Pluto, the little world with a big heart, is a relatively large constituent of this region, and it was–originally–classified as the ninth major planet from our Sun, after its discovery by the American astronomer Clyde Tombaugh (1906-1997) in 1930. However, the eventual realization among astronomers that this distant little “oddball” ice ball is really only one of a host of other similar icy bodies dwelling in this frozen place, forced the International Astronomical Union (IAU) to formally define the term “planet” is in 2006. As a result, poor Pluto was booted out of the pantheon of major planets, only to be re-classified as a mere dwarf planet. This situation is still a topic of considerable debate among astronomers, because many of them are not ready to demote the distant small world.
Comets are really frozen invaders from far away, and they hold deep within their secret, frozen hearts the most pristine of primordial elements that contributed to the formation of our Solar System about 4.6 billion years ago. This ancient cold blend of icy material has been preserved in the “deep freeze” of our Solar System’s darkest, most distant domains. These frigid invaders come screeching into the inner Solar System, closer to the melting-heat and brilliant light of our Sun, from three domains: the Kuiper Belt, Scattered Disc, and Oort Cloud. Many planetary scientists think that by gaining an understanding of the composition of the well-preserved, frozen ingredients that make up these fragile, ephemeral visitors, a scientific comprehension of the precious recipe that formed our Solar System can be obtained.
Comets are migrating relic icy planetesimals. This means that they are the lingering left-overs of what was once an immense population of ancient dirty ice balls that went into the construction of the quartet of majestic giant gaseous planets. Comets come in two basic types: short-period and long-period. Short-period comets are evicted refugees from the Kuiper Belt that rampage into the inner Solar System more often than every 200 years. By comparison, the long-period comets invade the inner Solar System every 200 years–or more. Long-period comets fly towards our Sun from the extremely remote Oort Cloud, that is believed to form a remote shell around our entire Solar System, and is much farther away than the Kuiper Belt.
2007 OR10 (225088) is a trans-Neptunian object (TNO), circling our Sun in the scattered disc. At almost 1500 kilometers in diameter, it is the third-largest known object in the Solar System dwelling past the orbit of Neptune–as well as the largest body in our Solar System that is still unnamed.
A TNO is any minor planet in our Solar System that orbits our Star at a greater average distance (semi-major axis) than Neptune, 30 AU. A dozen minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are currently known. These objects are termed extreme trans-Neptunian objects (ETNOs).
Pluto was the first TNO to be discovered back in 1930. However, it was not until 1992 that a second TNO (1992 QB1) was detected to keep it company. As of February 2017, more than 2,300 TNOs appear on the Minor Planet Center’s List of Transneptunian Objects. Of these TNOs, 2,000 have a perihelion that carry them farther out than Neptune. As of November 2016, 242 of these have had their orbits sufficiently well-determined to give them a permanent minor planet designation. The largest known TNO is Pluto, followed by Eris, 2007 OR10, Makemake, and Haumea.
2007 OR10 was discovered by California Institute of Technology (Caltech) astronomers as part of the doctoral thesis of Dr. Megan E. Schwamb, who was then still a graduate student studying under Dr. Michael E. Brown. Dr. David Rabinowitz was also a member of the discovery team on the hunt for distant Solar System bodies using the Samuel Oschin Telescope at the Palomar Observatory in California. Caltech is located in Pasadena, California.
Dr. Brown informally dubbed the still-not-officially named 2007 OR10 “Snow White” because of its presumed white color. This means that it would have to be either very large or very bright to be discovered by their survey. It was also the “seventh dwarf” to be detected by Dr. Brown’s team, after Quaoar in 2002, Sedna in 2003, Haumea and Orcus in 2004, and Makemake and Eris in 2005. However, Snow White turned out to be one of the reddest objects inhabiting the Kuiper Belt, comparable only to Quaoar, so the inappropriate nickname was dropped.
As of February 2016 2007 OR10 was about 87.5 AU from the Sun, and traveling at the impressive speed of 6,000 miles per hour with respect to our Star. This makes it the third-farthest, as well as the third largest, body in our Solar System. The spectrum of 2007 OR10 shows signatures of both water ice and methane, which means that it is similar in composition to Quaoar. The presence of red methane frost on the surfaces of both TNOs indicates that a thin methane atmosphere may exist on both objects, and that this atmosphere slowly evaporates into space.
Even though 2007 OR10 travels closer to our Star than Quaoar, and is therefore toasty enough for its methane atmosphere to evaporate, its larger mass makes retention of an atmosphere a possibility. In particular, the comparatively large size of 2007 OR10 indicates that it could very well hold on to even its nitrogen, which almost all TNOs lose over the course of their existence. The presence of water ice on the surface of 2007 OR10 suggests that a brief period of cryovolcanism (icy volcanism) occurred in its remote past.
A Moon For A Small World
Dr. Kiss and his international team of astronomers uncovered 2007 OR10’s moon in archival images taken of it with HST’s Wide Field Camera 3. Other observations taken of the small, frigid dwarf planet by NASA’s Kepler Space Telescope had provided the first clues that a moon might be orbiting it. Kepler showed that 2007 OR10 has a slow rate of rotation with a period of 45 hours. “Typical rotation periods for Kuiper Belt Objects are under 24 hours. We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon. The initial investigator missed the moon in the Hubble images because it is very faint,” Dr. Kiss explained in the May 18, 2017 Hubblesite Press Release.
The astronomers detected the dim, distant little moon in two separate HST observations spaced a year apart. The two images revealed that the moon is gravitationally bound to 2007 OR10 because it travels along with the dwarf planet, as observed against a background sea of stars. However, the two observations did not provide sufficient information for the astronomers to determine an orbit.
“Ironically, because we don’t know the orbit, the link between the satellite and the slow rotation rate is unclear,” Dr. Stansberry commented in the Hubblesite Press Release.
However, the astronomers were able to calculate the diameters of both the dwarf planet and its tiny moon. Their calculations were based on observations in far-infrared light conducted by the Herschel Space Observatory, which measured the thermal emission of the two remote small icy worlds. The Herschel Space Observatory, operated by the European Space Agency (ESA), was an infrared observatory that operated from 2009 to 2013. Herschel’s observations indicated that 2007 OR10 is about 950 miles across, while its moon is estimated to be approximately 150 miles to 250 miles in diameter. The smaller dwarf planet, like Pluto, travels along an eccentric orbit, but it is currently three times farther away from our Star than Pluto.
2007 OR10 is a member of a group of only nine known dwarf planets. Of these brave new worlds, only Pluto and Eris are larger than 2007 OR10.
The team’s results appeared in the March 20, 2017 issue of The Astrophysical Journal Letters.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. 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.