SUMMARY
My work currently focuses on finding out what materials the minor planets of the outer Solar System are made of. Specifically, the objects I study are Trans-Neptunian Objects (TNOs) that orbit the Sun mostly beyond Neptune, and centaurs which orbit the Sun in the same region as the giant planets. It’s known that many of these small objects are partly made of water ice, and it’s suspected they should also contain some rocky material and more complex molecules like hydrocarbons. We search for the signatures of these materials in the sunlight that reflects off an object’s surface by examining its colour and how much light is absorbed by surface materials at different wavelengths. Uncovering the signatures of the materials that make up TNOs and centaurs informs us about their formation environment and how their surfaces have evolved since the Solar System formed. From this we can infer details about planet formation and the evolution of planetary systems over time.
Reports on my latest research can be found below. V
Older research highlights are archived here >
I mainly work with spectroscopic data observed with spectrographs on 8 meter telescopes. In the past I have used the X-Shooter and FORS2 spectrographs at the European Southern Obseravtory’s Very Large Telescope, which is sited at the peak of Cerro Paranal in Chile’s Atacama Desert (pictured above). Recently I have been making extensive use of the GMOS spectrographs at the Gemini Observatory telescopes to study minor planets and search for stars which have spectra that look similar to the spectrum of the Sun. Gemini North (pictured below) is situated close to the summit of Maunakea on the Big Island of Hawai’i, and its southern twin is sited at Cerro Pachón in the Chilean Andes mountains.
LATEST
2022 January – Null detection of unusual surface properties for a Jupiter Trojan asteroid
Jupiter Trojan asteroids are asteroids that are dynamically synchronized with Jupiter; they form two swarms of objects around Jupiter’s meta-stable L4 and L5 Lagrange points, where the combined gravity of Jupiter and the Sun loosely binds them and slows down their orbital evolution. Jupiter’s L4 point is situated on the orbit of Jupiter, 60 degrees ahead of the giant planet itself, while the L5 point similarly trails 60 degrees behind it. In early 2021 it was reported that 128383 (2004 JW52), a Jupiter Trojan asteroid, had colours in the Sloan Digital Sky Survey Moving Object Catalog (SDSS MOC1) that set it apart from all other dynamically similar objects recorded there2. Based on these colours, 2004 JW52 appeared to be a rocky S-type asteroid similar to those that orbit the Sun in the inner asteroid belt. This is strange, as the Jupiter Trojans are thought to have formed in the outer Solar System3 from icy and carbon-rich materials that make them dark and slightly red in colour4. It is also unlikely for any S-types that get kicked out of the inner Solar System to become captured into stable 1:1 orbital resonance with Jupiter over long timescales5, so the very long term stability estimated for the resonant capture of 2004 JW52 from dynamical simulations of its orbit6 would also be surprising if it turned out to be rocky instead of carbon-rich.
To double-check the colour properties of 2004 JW52, I was able to get some time to observe it with the GMOS-N spectrograph at the Gemini North telescope in October 2021. These new observations, when combined with more recently observed SDSS MOC colours7, showed that 2004 JW52 was in fact just a normal Jupiter Trojan asteroid that had been mis-characterised. Ultimately it turns out that the initial unusual colours were observed when 2004 JW52 was very close to a background star on the sky, which likely contaminated the colour measurement of 2004 JW52 in that observation, leading to spurious colours making it into the earlier SDSS catalog. Making sure that minor planets are observed when they clear of background stars and galaxies is very important to obtaining accurate colours and spectra of their surfaces.
The plot above shows reflectance spectra of 2004 JW52 observed with the Gemini North telescope, as well as images and colours of the same object respectively observed and catalogued by the Sloan Digital Sky Survey. The spurious colours of 2004 JW52, which were measured from the images shown in the top panel from SDSS images taken in 2005, are plotted in pink in the bottom panel. In the images a white plus symbol marks the position of a background star that 2004 JW52 crosses during the observation (N, E, and v arrows in the images respectively show the directions of North, East, and the on-sky motion of the asterooid). The presence of a star so close to 2004 JW52 during the observation, and the resulting contamination of the photometry, is what causes the colours from 2005 to be so different when they are directly compared to other datasets obtained at different times. The middle set of images was taken by SDSS in 2008 when 2004 JW52 was clear of contaminating background sources, and the resulting colours are plotted in yellow in the bottom panel. These newer colours match extremely well with the reflectance spectra from 2021 which are plotted in black, which is a nice confirmation that the 2008 and 2021 datasets are accurate, and that the surface of 2004 JW52 is totally normal for a Jupiter Trojan asteroid. Each of the two reflectance spectra (where one has been shifted by +0.25 for clarity in the plot), have been calibrated with a different Solar calibrator star, but that is the only difference between them.
2021 December – Near-UV Reddening in the Spectrum of a High-Inclination Centaur
Small Solar System objects with perihelia above 15 astronomical units (au), and inclinations to the ecliptic of greater than around 60 degrees are known as High-inclination High-perihelion (HiHq) centaurs. The origin of this small population of dynamically unstable objects isn’t well understood. It is generally accepted that they are unlikely to come from the Kuiper Belt or the Trans-Neptunian scattered disk, and hypotheses about their origins generally converge toward a likely origin in the inner Oort Cloud, but there have been suggestions that an inner Solar System origin is also possible.
2012 DR30 is the largest HiHq centaur that we know of with an estimated diameter of around 185 km; it is also one of the most dynamically extreme. Its orbit has a perihelion of 14.5 au, an inclination of 78 degrees to the plane of the Solar System, an eccentricity of 0.987, and a semimajor axis of 1130 au, which puts its aphelion into the inner Oort Cloud and local interstellar space at 2260 au (over 75x further from the Sun than Neptune). Our team observed 2012 DR30 in 2015 and 2017 respectively with the X-Shooter and FORS2 spectrographs at the ESO VLT in an effort to better understand the materials on its surface.
Like previous observers, we observed a signature of water ice in the spectrum of 2012 DR30. Comparison of our own new data to that already published seems to suggest that the colour of 2012 DR30 does not significantly vary from one observation to the next as previously suggested. This finding lines up well with the observation that the brightness of 2012 DR30 does not appear to vary as it rotates, and the following inference that the reflectivitity, or albedo, of the material on 2012 DR30 is fairly constant across the entire surface.
Interestingly we detect a distinct increase in the redness of the surface of 2012 DR30 at Near-Ultraviolet wavelengths. Such a feature is rare in the spectra of TNOs, so our finding supports the dynamical argument that HiHq centaurs do not come from the Kuiper Belt. The feature could potentially be caused by the presence of aromatic hydrocarbons, which are produced on icy minor planets when they are irradiated by ultraviolet light and particles present in deep space. Alternatively the feature could be caused by iron oxides on the surface of 2012 DR30, which can be produced when certain kinds of iron-containing silicates are chemically altered by liquid water that they are in contact with; evidence of such a process may indicate the 2012 DR30 was heated to greater than 270 Kelvin at some point in its history, which is significantly hotter than the 90 K temperatures it can reach today when it passes closest to the Sun in its orbit. Confirmation of either of these hypotheses, however, will require observations to be done at thermal infrared wavelengths, where aromatic hydrocarbons and aqueously altered minerals both have diagnostic signatures that can be observed. The James Webb Space Telescope would be the perfect tool to carry out these observations.
The plot above shows the reflectance spectra we obtained from our observations of 2012 DR30 (yellow and black) alongside a previously published spectrum from a different team (pink). The spectra are all quite similar at short wavelengths, but in the near infrared (above 1.0 micron) there is a big difference between our data and that published previously. Further observations will likely be required to figure out if the difference is real and what might be causing it. Despite the difference there are signatures of water ice in the data from both teams, at 1.55 microns in our black spectrum, and at 2.0 microns in the pink one. We can therefore be pretty confident that there is water ice on the surface of 2012 DR30, but further observations will be needed to uncover the signatures of other materials. Note that both the yellow and black spectra curve downward to the left from about 0.6 microns; this is the Near-UV reddening that is discussed above.
Tom's Planetary Science Site 