Centaurs and their Implications

by Paul Gilster on June 24, 2013

phoebe

One of the themes I often use in my talks is the ‘filling out’ of our picture of the Solar System. In addition to the asteroid belt, we’ve added the icy bodies of the Kuiper Belt and the vast expanse of the Oort Cloud into what once seemed a relatively simple, nine-planet solar system. I could easily add to the ranks the population of so-called ‘Centaurs,’ small bodies that populate the space between the giant planets and show characteristics of both comets and asteroids.

10199 Chariklo is the largest Centaur yet discovered (260 kilometers in diameter), and Saturn’s moon Phoebe may be a captured Centaur, in which case images of it from the Cassini orbiter offer us our first detailed view of such an object. Both Chiron (discovered in 1977) and 60558 Echeclus show signs of a cometary coma; both are classified as asteroids and comets (as is 166P/NEAT). Although differences in definition exist, most agree that Centaurs orbit the Sun between Neptune and Jupiter and eventually cross the orbits of one or more of the gas giants.

Image: Saturn’s moon Phoebe, possibly a captured Centaur. Credit: NASA.

Now we have news from Universidad Complutense of Madrid (UCM) that Crantor, a large asteroid with a diameter of 70 kilometers orbits the Sun in exactly the same time period as Uranus. The asteroid’s orbit, says Carlos de la Fuente Marcos, one of the study’s authors, “…is controlled by the Sun and Uranus but is unstable due to disturbances from nearby Saturn.” Two other asteroids — 2010 EU65 and 2011 QF99 — are also associated with Uranus. The latter is in a stable Trojan orbit, moving 60 degrees in front of Uranus, while Crantor and 2010 EU65 show ‘horseshoe’ orbits that result periodically in close encounters with the planet.

About Crantor itself we have much to learn:

(83982) Crantor is remarkable in several respects: it is the first known minor body to be trapped in a 1:1 mean motion resonance with Uranus; it currently moves in a complex, horseshoelike orbit when viewed in a frame of reference co-rotating with Uranus; and it could be the ”Rosetta Stone” for understanding why the overall number of Uranus co-orbitals appears to be significantly below that of Jupiter or Neptune. The object is placed and removed from its horseshoe orbit by the mechanism of the precession of the nodes. This precession is accelerated by the perturbative effects of Saturn. The chaotic nature of the orbit of this object constraints the degree of predictability of its dynamical evolution on timescales longer than a few 10 kyr.

The horseshoe shape of a ‘horseshoe orbit’ appears when we map the movement of the asteroid in relation to the Sun and, in this case, Uranus. While the asteroid always orbits the Sun in the same direction, it periodically catches up with the planet and falls behind again, tracing out the horseshoe outline in relation to Sun and planet. 3753 Cruithne was the first object confirmed to follow a horseshoe orbit, in this case near the Earth, while multiple objects in such orbits have been found around Jupiter, with others identified in association with Venus and Mars.

Lagrange_Horseshoe_Orbit

Image: A NASA illustration of a horseshoe orbit, in this case showing the orbit in relation to the Earth.

I don’t want to leave the topic of Centaurs behind without mentioning an interesting 2010 paper, although its emphasis is on Neptune rather than Uranus. Jonathan Horner (University of Durham) and Patryk Sofia Lykawka (Kinki University, Japan) make the case that a significant fraction, if not more, of the Centaur population comes from the planetary Trojan clouds, considered here as ‘stable reservoirs of objects moving in 1:1 mean-motion resonance with the giant planets…’ The researchers’ simulations show that there should be an ongoing movement of objects into the Centaur population. They go on to suggest that the Neptune Trojans could be the main source of new Centaurs. And I thought this was interesting (from their paper):

We suggest that further observational work is needed to constrain the contribution made by the Neptune Trojans to the ongoing flux of material to the inner Solar system, and believe that future studies of the habitability of exoplanetary systems should take care not to neglect the contribution of resonant objects (such as planetary Trojans) to the impact flux that could be experienced by potentially habitable worlds.

The de la Fuente Marcos paper is “Crantor, a short-lived horseshoe companion to Uranus,” Astronomy & Astrophysics 551: A114, March 2013 (abstract). The Horner/Lykawka paper is “Planetary Trojans – the main source of short period comets?,” International Journal of Astrobiology 9, 227-234 (2010). Preprint online.

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andy June 24, 2013 at 15:21

Maintenance note: the RSS feed is broken due to an illegal character in the XML: looks like a miscopied “ff” ligature in the word “effects” after “This precession is accelerated by the perturbative…”.

Back on subject, the outer solar system certainly appears to be a very dynamic place, full of surprises. (Yet we keep focussing all our space exploration on Mars to a disproportionate degree.) We may have lost a planet but what has been revealed in its place is perhaps far more interesting.

Paul Gilster June 24, 2013 at 16:52

Re the RSS, thanks! That XML problem slipped right past me but I’ve corrected it and assume RSS is OK now. I appreciate the tip!

Doug M. June 25, 2013 at 4:28

Yeah, the New Solar System really gets interesting in its outer reaches. Vast and slow and cold, yet active and diverse. (Readers of Mike Brown’s Planets will know that the KBOs alone seem to vary wildly in density, surface composition, and appearance.)

We’re not going to get a lot of flybys — New Horizon will probably be the last for at least the next 30 years, and probably longer. So, it’ll be a lot of research done the old-fashioned way (i.e., by fighting with other astronomers for telescope time).

Doug M.

Adam June 25, 2013 at 7:27

Patryk Lykawka’s other ideas from a different paper (http://adsabs.harvard.edu/abs/2012MEEP….1..121L) deserve some more interest from the astronomers surveying the outer reaches – there might be a planet(oid) out there too massing 0.3-0.7 Earth masses.

andy June 25, 2013 at 14:41

Yes, the RSS is working again. Thanks!

ljk August 2, 2013 at 20:27

http://scitechdaily.com/lazarus-comets-return-to-life-after-being-inactive-for-millions-of-years/

‘Lazarus Comets’ Return to Life after Being Inactive for Millions of Years

August 2, 2013 by StaffSpace

Astronomers Have Discovered a Graveyard of Comets

These illustration show the asteroid belt in the present day and in the early Solar System, located between the Sun (at center) and four terrestrial planets (near the Sun) and Jupiter (at bottom left). The top image shows the conventional model for the asteroid belt; largely composed of rocky material. The middle image shows the proposed model, with a small number of active comets and a dormant cometary population. The lower diagram shows how the asteroid belt might have looked in the early Solar System, with vigorous cometary activity. Credit: Ignacio Ferrin / University of Antioquia

Scientists have discovered a graveyard of comets where some of the dormant objects, inactive for millions of years, have returned to life from an increase in the energy they received from the Sun.

A team of astronomers from the University of Anitoquia, Medellin, Colombia, have discovered a graveyard of comets. The researchers, led by Antioquia astronomer Prof. Ignacio Ferrin, describe how some of these objects, inactive for millions of years, have returned to life leading them to name the group the ‘Lazarus comets’. The team publish their results in the Oxford University Press journal Monthly Notices of the Royal Astronomical Society.

Comets are amongst the smallest objects in the Solar System, typically a few km across and composed of a mixture of rock and ices. If they come close to the Sun, then some of the ices turn to gas, before being swept back by the light of the Sun and the solar wind to form a characteristic tail of gas and dust.

Most observed comets have highly elliptical orbits, meaning that they only rarely approach the Sun. Some of these so-called long period comets take thousands of years to complete each orbit around our nearest star. There is also a population of about 500 short period comets, created when long period comets pass near Jupiter and are deflected in orbits that last anything between 3 and 200 years. Although uncommon events, comets also collide with the Earth from time to time and may have helped bring water to our planet.

The new work looked at a third and distinct region of the Solar System, the main belt of asteroids between the orbits of Mars and Jupiter. This volume of space contains more than 1 million objects ranging in size from 1 m to 800 km. The traditional explanation for asteroids is that they are the building blocks of a planet that never formed, as the movement of the pieces was disrupted by the strong gravitational field of Jupiter.

In the last decade 12 active comets have been discovered in the asteroid main belt region. This was something of a surprise and the Medellin team set out to investigate their origin. The team, made up of Prof. Ferrin and his colleagues Profs. Jorge Zuluaga and Pablo Cuartas, now think they have an explanation.

“We found a graveyard of comets”, exclaims Professor Ferrín. He adds: “Imagine all these asteroids going around the Sun for aeons, with no hint of activity. We have found that some of these are not dead rocks after all, but are dormant comets that may yet come back to life if the energy that they receive from the Sun increases by a few percent.”

Surprisingly, this can happy fairly easily, as the orbits of many objects in the asteroid belt are nudged by the gravity of Jupiter. The shape of their orbits can then change, leading to a decrease in the minimum distance between the object and the Sun (perihelion) and a slight increase in average temperature.

According to this interpretation, millions of years ago the main belt was populated by thousands of active comets. This population aged and the activity subsided. What we see today is the residual activity of that glorious past. Twelve of those rocks are true comets that were rejuvenated after their minimum distance from the Sun was reduced a little. The little extra energy they received from the Sun was then sufficient to revive them from the graveyard.

Prof. Ferrin describes the 12 active comets. “These objects are the ‘Lazarus comets’, returning to life after being dormant for thousands or even millions of years. Potentially any one of the many thousands of their quiet neighbours could do the same thing.”

Publication: Ignacio Ferrín, et al., “The location of Asteroidal Belt Comets (ABCs), in a comets’ evolutionary diagram: The Lazarus Comets,” MNRAS, (2013); doi:10.1093/mnras/stt839

PDF Copy of the Study: The location of Asteroidal Belt Comets (ABCs), in a comets’ evolutionary diagram: The Lazarus Comets

Source: Royal Astronomical Society

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