Exiled Asteroid Discovered in Outer Reaches of Solar System
ESO telescopes find first confirmed carbon-rich asteroid in Kuiper Belt
9 May 2018
An international team of astronomers has used ESO telescopes to investigate a relic of the primordial Solar System. The team found that the unusual Kuiper Belt Object 2004 EW95 is a carbon-rich asteroid, the first of its kind to be confirmed in the cold outer reaches of the Solar System. This curious object likely formed in the asteroid belt between Mars and Jupiter and has been flung billions of kilometres from its origin to its current home in the Kuiper Belt.
The early days of our Solar System were a tempestuous time. Theoretical models of this period predict that after the gas giants formed they rampaged through the Solar System, ejecting small rocky bodies from the inner Solar System to far-flung orbits at great distances from the Sun . In particular, these models suggest that the Kuiper Belt — a cold region beyond the orbit of Neptune — should contain a small fraction of rocky bodies from the inner Solar System, such as carbon-rich asteroids, referred to as carbonaceous asteroids .
Now, a recent paper has presented evidence for the first reliably-observed carbonaceous asteroid in the Kuiper Belt, providing strong support for these theoretical models of our Solar System’s troubled youth. After painstaking measurements from multiple instruments at ESO’s Very Large Telescope (VLT), a small team of astronomers led by Tom Seccull of Queen’s University Belfast in the UK was able to measure the composition of the anomalous Kuiper Belt Object 2004 EW95, and thus determine that it is a carbonaceous asteroid. This suggests that it originally formed in the inner Solar System and must have since migrated outwards .
The peculiar nature of 2004 EW95 first came to light during routine observations with the NASA/ESA Hubble Space Telescope by Wesley Fraser, an astronomer from Queen’s University Belfast who was also a member of the team behind this discovery. The asteroid’s reflectance spectrum — the specific pattern of wavelengths of light reflected from an object — was different to that of similar small Kuiper Belt Objects (KBOs), which typically have uninteresting, featureless spectra that reveal little information about their composition.
“The reflectance spectrum of 2004 EW95 was clearly distinct from the other observed outer Solar System objects,” explains lead author Seccull. “It looked enough of a weirdo for us to take a closer look.”
The team observed 2004 EW95 with the X-Shooter and FORS2 instruments on the VLT. The sensitivity of these spectrographs allowed the team to obtain more detailed measurements of the pattern of light reflected from the asteroid and thus infer its composition.
However, even with the impressive light-collecting power of the VLT, 2004 EW95 was still difficult to observe. Though the object is 300 kilometres across, it is currently a colossal four billion kilometres from Earth, making gathering data from its dark, carbon-rich surface a demanding scientific challenge.
“It’s like observing a giant mountain of coal against the pitch-black canvas of the night sky,” says co-author Thomas Puzia from the Pontificia Universidad Católica de Chile.
“Not only is 2004 EW95 moving, it’s also very faint,” adds Seccull. “We had to use a pretty advanced data processing technique to get as much out of the data as possible.”
Two features of the object’s spectra were particularly eye-catching and corresponded to the presence of ferric oxides and phyllosilicates. The presence of these materials had never before been confirmed in a KBO, and they strongly suggest that 2004 EW95 formed in the inner Solar System.
Seccull concludes: “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 reports of other ‘atypical’ Kuiper Belt Object spectra, none were confirmed to this level of quality,” comments Olivier Hainaut, an ESO astronomer who was not part of the team. “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.”
 Current dynamical models of the evolution of the early Solar System, such as the grand tack hypothesis and the Nice model, predict that the giant planets migrated first inward and then outward, disrupting and scattering objects from the inner Solar System. As a consequence, a small percentage of rocky asteroids are expected to have been ejected into orbits in the Oort Cloud and Kuiper belt.
 Carbonaceous asteroids are those containing the element carbon or its various compounds. Carbonaceous — or C-type — asteroids can be identified by their dark surfaces, caused by the presence of carbon molecules.
 Other inner Solar System objects have previously been detected in the outer reaches of the Solar System, but this is the first carbonaceous asteroid to be found far from home in the Kuiper Belt.
This research was presented in a paper entitled “2004 EW95: A Phyllosilicate-bearing Carbonaceous Asteroid in the Kuiper Belt” by T. Seccull et al., which appeared in The Astrophysical Journal Letters.
The team was composed of Tom Seccull (Astrophysics Research Centre, Queen’s University Belfast, UK), Wesley C. Fraser (Astrophysics Research Centre, Queen’s University Belfast, UK) , Thomas H. Puzia (Institute of Astrophysics, Pontificia Universidad Católica de Chile, Chile), Michael E. Brown (Division of Geological and Planetary Sciences, California Institute of Technology, USA) and Frederik Schönebeck (Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Germany).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 15 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a strategic partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.
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Wesley C. Fraser
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