Asteroids and their comet-cousins are the relics of a bygone era when our Sun and its familiar family of objects were still in the process of forming. During this very ancient time, our Solar System was a “cosmic shooting gallery”, where relatively small and primitive primordial bodies–called planetesimals–crashed violently into one another; sometimes merging to create larger bodies; sometimes breaking apart into smaller chunks of rubble and clouds of dust. The asteroids that inhabit our Solar System are similar to the rocky and metallic planetesimals that merged together to create the quartet of small solid planets–Mercury, Venus, Earth and Mars–that orbit our Star close to its brilliant light and heat. In contrast, the comets are the distant, relic icy and dusty remains of the frozen planetesimals that merged together long ago to create the four gaseous giant planets of our Solar System’s outer limits: Jupiter, Saturn, Uranus and Neptune. In May 2018, an international team of astronomers announced that they have used the European Southern Observatory’s (ESO’s) telescopes to investigate a relic of the primordial Solar System–a carbon-rich asteroid that was born in the Main Asteroid Belt between Mars and Jupiter, only to be unceremoniously evicted from its birthplace, to dwell within its adopted home in the distant, cold, and dark Kuiper Belt.
This unusual small body, named Kuiper Belt Object 2004 EW95, is rich in carbon. It is also the first of its strange kind to be confirmed as an inhabitant of the frozen outer limits of our Sun’s kingdom.
Our Sun and its family formed approximately 4,456,000,000 years ago, and had a tempestuous youth. Theoretical models of this ancient era predict that, after the quartet of outer gaseous giant planets were born, they rampaged through the baby Solar System. During their invasion, they tossed small rocky bodies out of the inner realm, to migrate to remote orbits at immense distances from our Star. These models propose that the Kuiper Belt–a frigid twilight region beyond the orbit of the outermost giant planet, Neptune–should be the new home of a small percentage of rocky bodies evicted from the inner Solar System. Among this population of unfortunate rocky objects would be carbon-rich asteroids, such as 2004 EW95, that are designated carbonaceous asteroids.
A Tale Of Two Celestial Cradles
The Kuiper Belt is a distant region of perpetual twilight, located in our Solar System’s outer fringes, where our Sun shines with only a feeble, faint fire. Here, a multitude of icy objects do a mesmerizing dance around our Star. These icy objects are the nuclei of comets that travel a treacherous path around our Sun in long elliptical orbits. Comets are famous for their thrashing tails that stream out behind them, flashing brightly, whenever their orbits take them sufficiently close to our Star. These alien invaders from far, far away, come screaming into Earth’s warm, inner, well-lit region from two distant, dark, and frozen regions: the faraway Kuiper Belt and the even more remote Oort Cloud.
Every time an invading comet makes its long, treacherous journey into the inner Solar System, flying closer and closer to our Sun’s light and melting heat, it loses some of its own substance by way of sublimation of its surface ices and gas. The frozen surface of the comet’s nucleus changes to a gas and then creates a glowing cloud called a coma. Radiation originating from our Star pushes motes of dust away from the coma–thus creating the comet’s bright tail.
In contrast, asteroids–that primarily inhabit the Main Asteroid Belt between Mars and Jupiter–are not icy like comets. Asteroids are rocky and metallic, and they are mostly found in the region around our Star where temperatures are warmer than the dimly-lit deep freeze of the distant Kuiper Belt.
The leftover primordial planetesimals–both rocky and icy–are all that remain of what was once a vast population of kindred objects. The planetesimals were the building blocks of the major planets of our Solar System, which was born when a relatively small, very dense blob–embedded within a frigid, cold, dark and enormous molecular cloud–collapsed under the merciless pull of its own gravity. Most of the material that composed the dense, collapsing blob collected at the center, and finally caught furious fire as a result of the process of nuclear fusion, forming a new baby star–or, frequently, forming multiple stellar siblings. What is left over from the process of star-birth gathers in a mass that is flattened into a pancake-shaped protoplanetary accretion disk, out of which the eight major planets of our Star’s family, along with their many moons were born. The asteroids and comets also congealed out of the primeval protoplanetary accretion disk.
Similar protoplanetry accretion disks have been detected surrounding stars beyond our Sun, and these stars dwell within young stellar clusters. The protoplanetary accretion disks form at approximately the same time as the brilliant neonatal star is born, and they serve the function of feeding a nourishing formula of gas and dust to the hot and hungry baby star (protostar). The accretion disk is both extremely massive and searing-hot, and it can hang around the youthful star for 10,000,000 years–or longer.
By the time a young star–of approximately our own Sun’s mass–reaches what is termed the T Tauri stage of its childhood, the nourishing accretion disk has grown thinner–and much cooler. T Tauri stars are very young variable stars, and by the time the stellar tot has reached this phase, less volatile materials have begun to condense close to the center of the encircling protoplanetary accretion disk–forming very fine and smokelike motes of dust.
These very fine grains of dust possessed a natural “stickiness”, and they “glued” themselves to one another, thus creating ever larger and larger objects–from pebble size, to boulder size, to mountain size–and finally to the size of planetesimals. These planetary building blocks eventually merged together to become our Solar System’s major planets, along with their accompanying moons.
The international team of astronomers, who discovered the unusual Kuiper Belt Object 2004 EW95, presented powerful new evidence supporting theoretical models that describe our Solar System’s mysterious, violent, and turbulent youth. After taking careful and meticulous measurements, derived from multiple sources at ESO’s Very Large Telescope (VLT), the astronomers were able to measure the composition of the bizarre 2004 EW95. The team, led by Dr. Tom Seccull of Queen’s University Belfast (UK), then determined it to be a carbonaceous asteroid. Their discovery suggests that 2004 EW95 was born in the warm and well-lit inner Solar System, and must have since traveled outwards into the frigid domain of the Kuiper Belt beyond the planet Neptune.
The mysterious nature of 2004 EW95 first came to light as the result of routine observations with the NASA/European Space Agency (ESA) Hubble Space Telescope (HST). The HST observations were conducted by Dr. Wesley Fraser, an astronomer from Queen’s University Belfast, who was also a member of the team behind the discovery. The asteroid’s reflectance spectrum–which is the specific pattern of wavelengths of light reflected from an object–was different from that of similar small Kuiper Belt Objects (KBOs). KBOs are icy and dusty comet-nuclei that typically have unremarkable and featureless spectra providing only scanty information about their composition.
“The reflectance spectrum of 2004 EW95 was clearly distinct from the other observed outer Solar System objects. It looked enough of a weirdo for us to take a closer look,” explained Dr. Seccull in a May 9, 2018 ESO Press Release.
The astronomers observed 2004 EW95 using the X-Shooter Observatory. The sensitivity of these spectrographs enabled the scientists to collect more detailed measurements of the pattern of light reflected from the asteroid and from those measurements infer its composition.
Alas, even with the detailed light-collecting ability of the VLT, 2004 EW95 remained difficult to observe. Even though the weird object is 300 kilometers across, it is currently an extremely remote four billion kilometers from our planet. This means that collecting information from its dark, carbon-laden surface presents a great scientific challenge.
“It’s like observing a giant mountain of coal against the pitch-black canvas of the night sky,” commented study co-author Dr. Thomas Puzia in the May 9, 2018 ESO Press Release. Dr. Puzia is of the Pontificia Universidad Catolica de Chile.
“Not only is 2004 EW95 moving, it’s also very faint. We had to use a pretty advanced data processing technique to get as much out of the data as possible,” Dr. Seccull continued to explain in the same ESO Press Release.
There are two features that show themselves in the “weirdo” object’s spectra that were particularly interesting because they correspond to the presence of ferric oxides and phyllosilicates. The presence of the two materials had never before been confirmed in a KBO, and they strongly indicate that 2004 EW95 was born in the bright and toasty inner region of our Solar System.
Dr. Seccull continued to explain to the press that “Given 2004 EW95’s present-day abode in the icy outer reaches of the Solar System, this implies that it has been flung out into its present orbit by a migratory planet in the early days of the Solar System”.
“While there have been previous records of other ‘atypical’ Kuiper Belt Object spectra, none were confirmed to this level of quality. The discovery of a carbonaceous asteroid in the Kuiper Belt is a key verification of one of the fundamental predictions of dynamical models of the early Solar System,” Dr. Oliver Hainaut commented in the May 9, 2018 ESO Press Release. Dr. Hainaut is an ESO astronomer who was not part of the team.
The paper describing this research is published in the March 10, 2018 issue of The Astrophysical Journal Letters under the title: “2004 EW95 A Phyllosilicate-bearing Carbonaceous Asteroid in the Kuiper Belt”.