UBC logo The first known Uranian Trojan and the frequency of temporary giant-planet co- orbitals

UBC researchers unveil Uranus' first known Trojan companion

2013, August 29th.

Canadian and French scientists have announced the discovery of the first celestial object known to share its orbit with Uranus in a Trojan configuration.
The object, a 60-km wide ball of rock and ice temporarily known by the code 2011 QF99, shares an orbit with the planet Uranus at a distance 19 times as far from the Sun than Earth is. This previously-unknown object was discovered in October 2011 during the team's search of the outer Solar System for even more-distant transneptunian objects beyond the planet Neptune. The comparatively fast-moving 2011 QF99 was re-observed many times during 2011 and 2012 to track it across the sky; these observations securely determined that it was indeed on a Uranian Trojan orbit. In such a Trojan state, the object always manages to remain 10-170 degrees ahead of the planet Uranus, never advancing too far ahead of, nor being caught up to by, the planet. Although the scientist showed that the newly discovered object remain in this state for many hundreds of thousands of years, they also show that within millions of years the object will certainly escape this state and be thrown around the outer Solar System. Other planets (like Earth and Neptune) have recently been shown to host similar temporarily co-orbital populations; 2011 QF99 is the first Trojan object identified for Uranus, which (at least, for now) does not have any known permanent co-orbital companions. The team then used extensive computer simulations to prove that the number of non-permanent co-orbital companions of Uranus and Neptune made sense if distant transneptunian objects are leaking inward towards the Sun.

Information on the discovery, the tracking effort, and the orbital simulations are provided below. A paper by Alexandersen et al. detailing the discovery and simulation results was published in the journal Science on the 30th of August 2013. The full published paper is available on the Science webpage (subscription required).


2011 QF99 was discovered on images taken at the Canada-France-Hawaii Telescope on the 24th of October, 2011, by a Canadian/French team consisting of (University of British Columbia astronomy PhD students Mike Alexandersen, Sarah Greenstreet, and professor Brett Gladman, along with National Research Council of Canada astronomers JJ Kavelaars and Stephen Gwyn, and Observatoire de Besancon astronomer Jean-Marc Petit).
These observations (part of a much larger survey) consisted of a series of three images each with 5-minute exposure of the same patch of sky, taken one hour apart, which is sufficiently spaced to see the motion of Solar System objects relative to background stars and galaxies. The discovery images can be found at the bottom of this page. Computer programs written by the team scan the images for moving objects, which the astronomers then review. 2011 QF99 was easily detected in the discovery triple.

Follow-up observations

Discovery of objects in the outer Solar System is only a small fraction of the work needed to do the science because, with only the discovery data, an object's orbital path around the Sun is so uncertain that it could easily be lost. While the team immediately knew the object was at the distance of Uranus, it was not possible to know that it shared the planet's orbit and was not simply one of the numerous unprotected planet-crossing objects (known as Centaurs) found in the outer Solar System. The team thus dedicated additional observing time to track 2011 QF99 during August, October, and November 2011, as well as January, February, August and October 2012. These additional observations, along with a forefront technique the team uses to measure the object's position relative to the stars, allowed a high-precision orbit to be calculated, proving that this object was the first object caught in a Trojan configuration with Uranus. "We were surprised, and excited" says Alexandersen. The position measurements were thus submitted to the IAU Minor Planet Center in March 2013, gaining the object the official IAU temporary designation 2011 QF99 (discovery announcement from Minor Planet Center here). More follow-up observations in January and February 2013 further fine-tuned the details of the Trojan orbit.

Characteristics and origin

Based on its brightness on the telescopic images and its distance, the size of 2011 QF99 is estimated to be ~60 km in diameter, assuming a typical surface reflectivity for the feeble solar light which bounces off the object and allows it to be seen. While there is no direct information on the composition of 2011 QF99, it is virtually certain to be a mix of rocks and ices that is typical for outer Solar System objects.

"Uranus is not predicted to have many stable Trojans, or perhaps any" says Alexandersen and continued "However, once we had enough observations to determine the orbit well, we proceeded to use computers to predict its future path." These calculations revealed that 2011 QF99 is indeed a leading (or "L4") Trojan, meaning that it has an orbit very similar to that of Uranus and always stays "ahead" of Uranus in its orbit because they orbit at virtually the same rate (see figure below). If 2011 QF99 gets either too far ahead of Uranus, or if the planet catches up to the Trojan, then weak gravitational pulls from Uranus act to slow down or speed up the object and maintain the L4 Trojan state. Although such Trojan orbits (also known as 'tadpole' orbits) are known with Earth, Mars, Jupiter, and Neptune, this is the first Uranian Trojan.
Although 2011 QF99 bounces back and forth around the L4 point many times, the team's orbital determination and dynamical calculations are accurate enough to clearly show that about 70,000 years from now the Trojan state will end. At that point it will either immediately escape into the very unstable planet-encountering Centaur population, or (more likely) transit to another co-orbital state: either a L5 Trojan (trailing Uranus instead of leading it) or a so-called `horseshoe' state (a state already known to be occupied for Uranus by the object Crantor). "We explored every possibility but there is no way that this thing is primordial" says Gladman, meaning that there is no chance that 2011 QF99 has been a Trojan for the 4.5 billion year age of the Solar System. The future orbital calculations confirm that within time scales of millions of years 2011 QF99 will no longer share the Uranian orbit. The team concludes that the object is clearly a Centaur that got relatively-recently trapped into the Trojan state, and we happen to be seeing it about a million years before it leaves.
Astronomers used to think that objects in Trojan orbits have been there for 4 billion years (and it is clear that there are such primordial objects for Jupiter and Neptune). However, several other temporary Trojans and co-orbitals have been discovered in the inner and outer Solar System during the past decade. In fact, about 2% of all minor bodies between Jupiter and Neptune are in co-orbital motion with Uranus or Neptune. "It was completely unclear that there was a sensible explanation of this, because getting Centaurs temporarily trapped as Trojans is a low-probability event" says Sarah Greenstreet "and even 2% seemed like a lot". Because of this surprisingly-large presence of temporary co-orbitals, the team set out to try to explain their presence. Starting with a model of the transneptunian region from which Centaurs come, they simulated the orbital dynamics of 17,800 objects as they interacted with the planets for 1 billion years. They tracked the particles which got scattered into the giant planet region in order to get an estimate of the fraction that became co-orbital at any time. They found, to their surprise, that about 3% of the minor objects in the giant-planet region at any given instant should be in co-orbital motion with Uranus or Neptune. "We believe that estimate is correct to a factor of two accuracy" says Greenstreet, which is thus consistent with the available information. Thus, the team's work provides not just the first Uranian Trojan, but a consistent theoretical framework for all temporary co-orbitals of the giant planets.

Discovery image

Images, animations and videos

One of three discovery images of 2011 QF99 (.png) taken from CFHT on 2011 October 24 (2011 QF99 is inside the green circle). This is the first of three images of the same patch of sky, taken one hour apart, that were then compared to find moving light-sources.

Animation of 2011 QF99 (.gif) using the three CFHT discovery images from 2011 October 24, sped up by a factor of 2000. The actual time between each image in the animation is 60 minutes. 2011 QF99 is highlighted with a green circle. Stationary white objects are background stars and galaxies. The small, randomly occuring white dots are caused by charged cosmic rays hitting the camera's detector.

Motion in co-rotating frame, 59 kyr The motion of 2011 QF99 over the next 59 kyr (.png). Shown here is the trajectory of 2011 QF99, according to the best fit to the observations. The current position is marked by a red square, and the black line shows the trajectory 59 kyr into the future. L4 and L5 are the triangular Lagrange points, the points in space which Trojans hover near. Here, the co-ordinate frame co-rotates with Uranus, cancelling out the motion of Uranus, thus showing only the motion of 2011 QF99 relative to Uranus. The oval oscillations occur over one orbit around the Sun, while the larger angular extent around the Sun is due to a longer period libration. For clarity, see one of the following animations.

Short-term Animation showing the motion of 2011 QF99 (4.3 MB .avi, 6.4 MB .mpg, YouTube), as seen from above the north pole of the Solar System. The video initially shows the position of Uranus and 2011 QF99 at the time of discovery and then shows their motion for the next 1 kyr. The dotted lines are the average orbits of the planets. The left panel is in the stationary reference frame, while the right panel is in the "Uranian co-rotating frame", which spins at the same rate as Uranus, cancelling out the motion of Uranus and thus showing the motion of 2011 QF99 relative to Uranus.

Medium-term animation showing the motion of 2011 QF99 (3.6 MB .avi, 4.7 MB .mpg, YouTube), as seen from above the north pole of the Solar System, in the "Uranian co-rotating frame", which spins at the same rate as Uranus, cancelling out the motion of Uranus and thus showing the motion of 2011 QF99 relative to Uranus. The video initially shows the position of Uranus and 2011 QF99 at the time of discovery and then shows their motion for the next 10.8 kyr. The dotted lines are the average orbits of the planets.


1) Is the Trojan 2011 QF99 technically an asteroid? What is the most accurate description?
The terms 'asteroid' and 'comet' are used in different ways in the astronomical literature. For thousands of years of history, humans have seen comets when an icy object from the outer Solar System came close to the Sun (roughly inside the orbit of Jupiter) and developed a tail which was the hallmark of a comet. Asteroids (the first of which was discovered in 1801) were ojbects between the orbits of Mars and Jupiter, and in telescopes they looked like moving points of light. Thus, historically the distinction between 'comet' and 'asteroid' was made based on the apperance of the object in a telescope. Astronomers now understand that this visual difference is due to composition; asteroids have rocky surfaces that are inert (or inactive) and the Sun's heat does not alter them. Comets, on the other hand, are rock and ice mixtures that when they approach the Sun 'boil away' to generate the comet's tail. Therefore, because 2011 QF99 does not have a tail, it would be incorrect to call it a comet. On the other hand, it is almost certainly a mix of ices and rocks that, were it to approach the Sun and enter the inner Solar System, would activate and generate a cometary tail. We thus think of 2011 QF99 as an object with asteroidal appearance but likely cometary composition.

2) What's unique about this object trailing Uranus compared to other Trojans following other planets?
Trojan objects share the behaviour that they share the orbit of the planet around the Sun and do not stray too far from the leading (L4) Lagrange point or the trialing (L5) Lagrange point. 2011 QF99 is a leading Trojan of Uranus. In the last two decades astronomers have understood that there are actually both 'permanent' and temporarily-stable Trojans. Jupiter hosts a population of roughly 100,000 (larger than 1 km diameter) Trojans, of which many hundreds are known; these objects have remained in their current state for billions of years. On the other hand, Earth and Neptune have in the last few years been shown to have 'temporary' Trojans which, while they remain near their planet's L4 or L5 point for many many orbits around the Sun, ultimately escape. Such bodies must have been 'recently' (in astronomical terms) been planet crossing objects that have been perturbed by weak gravitational interactions into the Trojan state and will eventually escape back into the planet-crossing state. While computer models indicate that Uranus may have a tiny region of orbital space where 4-Gyr stability might be possible, 2011 QF99 is not in this region. In fact, the discovery team's study show conclusively that in at most 1 million years this object will certainly escape from the state of sharing the orbit of Uranus and rejoin the flood of planet-crossing Centaurs from which it came.

3) What were the circumstances that allowed this discovery?
We were performing a survey of the outer Solar System, searching for objects beyond Neptune and Neptunian Trojans near the Neptunian L4 region. Uranus was located such that part of the Uranian L4 region was within the survey, allowing for the detection of Uranian Trojans as well. Our computer software had been programmed to search for objects moving even faster than (and thus closer to the Sun than) 2011 QF99.

4) What planets have Trojans and how many are there for each of the planets?
Earth is known to have one temporary Trojan; Mars has eight long-term stable Trojans; Jupiter has almost 6000 long-term stable Trojans; Neptune has nine known Trojans, six of which are long-term stable, one is temporary, and two are still not well-studied enough to determine stability.

5) Why is it important to study Trojans?
Trojans on stable orbits are an important population to study, because they have been on their current orbits since the Solar System formed 4.5 billion year ago. These remnants from Solar System formation can provide us with clues as to what the early Solar System was like. On the other hand, temporarily trapped Trojans are important because have been more recently captured into their current orbits as they migrate from the outer regions of the Solar System in closer to the Sun. One way to think of this process is to consider a fast-flowing river where most of the water flows quickly and freely down the river, however pools and eddies along the river can temporarily slow down the flow and for a short time trap the water from continuing down the river. Similarly, objects from the outer Solar System, beyond the reach of the planets, travel into the planetary region, where it is possible for objects to become temporarily trapped into Trojans orbits before they continue on their path slowly getting closer to the Sun. By studying the process by which Trojans become temporarily captured, one can better understand how objects migrate from the transneptunian region (beyond the orbit of Neptune) into the planetary region of the Solar System.

6) Can you provide more technical information about 2011 QF99?
2011 QF99 was discovered in October 2011. After several follow-up observations during August, October, and November 2011 as well as January, February, August, and October 2012, cosisting of 29 astrometric measurements for a total arc of 419 days, the following orbital elements were determined:
a=19.090 +/- 0.004 AU
e=0.1765 +/- 0.0007
i=10.811 +/- 0.001 deg
longitude of ascending node=222.498 +/- 0.001 deg
argument of pericenter=287.51 +/- 0.11 deg
julian day of pericenter=246 4388 +/- 11 JD
2011 QF99 has an apparent magnitude of 22.6 in r-band; at its distance of 20.3 AU this corresponds to an absolute magnitude H_r=9.6. For a 5% albedo this corresponds to a diameter of ~60 km. There is no color or spectral information available yet.

7) Is it "Uranus Trojan" or "Uranian Trojan"? The adjective associated with Uranus is Uranian. Just as the Uranian satellites and Uranian rings are not called "Uranus satellites" and "Uranus rings", we must too use the adjective form for the Trojans. So the answer is that "Uranian Trojan" is the correct term. Unfortunately, the term "Neptune Trojan" and "Jupiter Trojan" are already in use, so it might be difficult to correct this gramatical error.

Related links

  • The Canada-France-Hawaii Telescope in Hawaii.
  • Paul Wiegert's home page, which countains several pages about Trojans and co-orbitals.
  • 2010 TK7, Earth's first know Trojan.
  • Crantor, the first Uranian horseshoe
  • The Department of Physics and Astronomy at the University of British Columbia in Canada

    Contact information

    Mike Alexandersen
    mikea (*) astro.ubc.ca
    Office: (604) 822-3657
    Lab: (604) 827-5319

    Brett Gladman, office (604) 822-6244
    Sarah Greenstreet, office (604) 822-2853

    Mike Alexandersen's home page (UBC).