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A Centaur in Transformation

There was a time when the Solar System seemed relatively well defined, with nine planets including Pluto and an asteroid belt that orbited in a niche between Mars and Jupiter. These days, in addition to the Kuiper Belt and Oort Cloud, we have to factor in all the objects that move on unusual orbits. We have a mission in the works, called Lucy, to the Jupiter Trojans, those asteroids that share the giant planet’s orbit around the Sun. And today we’re looking at Centaurs, which cross the orbits of giant planets and are in rapid dynamical evolution.

The subject comes up because a newly discovered comet — 2019 LD2 (ATLAS) — is not only a Centaur, but a Centaur that is rapidly on its way to becoming another class of object, a Jupiter Family Comet (JFC). The latter are short-period comets with an orbital period of less than 20 years, largely under the influence of Jupiter. A paper by Jordan Steckloff (Planetary Science Institute) and team lays out the case: Centaurs are objects in transition.

Image: Comet 2019 LD2 (ATLAS) as seen by the Hubble Space Telescope. Credit: Hubble Space Telescope/Bryce Bolin.

According to the paper, the new comet is in the earliest parts of a transition that is common to Centaurs. Their orbits, between Jupiter and Neptune, are unstable. Their fate takes them in one of two directions: They may be ejected from the Solar System entirely, or they may evolve into Jupiter Family Comets. They first emerge beyond the orbit of Neptune, putting them into the category of trans-Neptunian Objects, with gravitational interactions in the outer system pulling them into the population of Centaurs.

The transition to Jupiter Family Comet is, in astronomical terms, a fleeting affair, lasting from a few million to a few tens of millions of years, according to the researchers. And tracing their evolution produces what the authors refer to as a ‘gateway’ that helps them make the transition into JFC status. Thus Steckloff:

“We find that 2019 LD2 is currently in the vicinity of a dynamical ‘Gateway’ that facilitates the majority of transitions from the Centaur population into the Jupiter Family of Comets. The dynamical gateway is a region beyond Jupiter, extending to just inside of Saturn’s influence. Our previous work (Sarid et al. 2019) found that the majority of JFCs first pass through this dynamical gateway as Centaurs immediately prior to transitioning into the JFC population; indeed this ‘Gateway Region’ facilitates the majority of transitions between the JFC and Centaur populations. Currently, there are a handful of objects in the gateway, including LD2, and the much more famous object 29P/Schwassmann-Wachmann 1.”

Let’s pull up short for a moment with that last reference. 29P/Schwassmann-Wachmann 1, known since the 1920s, displays frequent outbursts that cause a sudden change in magnitude, brightening in some cases as much as a thousand times. The outbursts are sudden and tend to fade within days, evidently the result of cryovolcanic activity. Like 2019 LD2, 29P/Schwassmann-Wachmann 1 is a Centaur in transition into a Jupiter Family Comet.

The presence of super-volatile ices on 2019 LD2 points to the pristine nature of the object, which according to Steckloff is unlikely ever to have been in the inner Solar System. Here we are looking at a comet whose ices are subliming for the first time, driving cometary activity. The astronomer says this will be a rapid process: “…this transition is likely to finish in only 40 years from now, which is a blink of an eye for astronomy. This means that people alive today will be able to follow this object all the way through its transition into the JFC population.”

As part of that change in perspective that has taught us so much about the wide range of outer system objects, we’re learning that our definitions have to be malleable enough to incorporate objects that, on the range from active comets to inactive asteroids, can take on characteristics of either, presenting different aspects as they proceed through this dynamical evolution. Fast-changing 2019 LD2 is a chance to witness such change within our lifetimes.

The paper is Steckloff et al., “P/2019 LD2 (ATLAS): An Active Centaur in Imminent Transition to the Jupiter Family,” Astrophysical Journal Letters. Abstract.


Comments on this entry are closed.

  • charlie November 30, 2020, 14:13

    What happened to last Friday’s comment which was so briefly mentioned with some type of Zoom connection ? I don’t exactly remember what it was referring to but I think it had something to do with some kind up coming conference. Are you intending to put up that link again, and does a zoom connection work on an ordinary personal computer at home or is it restricted to some type of handheld telephone connection ?

    • Paul Gilster November 30, 2020, 14:38

      I’ll be reposting a modified version of that short article within days. A few changes in the planned session made it necessary to pull the original until I have the correct link.

  • Alex Tolley November 30, 2020, 18:48

    If 2019 LD2 is also in pristine condition, wouldn’t this make a good target for a space probe as its orbit is well known, can be planned for, and is close enough to be reasonably quickly reached, even with electric engines or sails? (A beamed sail might be an interesting possibility too). I would like to see a good analysis of the organics as possible precursors for abiogenesis before they have been subjected to solar heating.

    • Paul Gilster November 30, 2020, 21:14

      I’d love to see a mission concept around a beamed sail as an early shakeout of this kind of technology.

  • Robin Datta December 1, 2020, 5:15

    Centaurs originate in Greek mythology, but with such a creature, the possibilities are rife, and imagination runs wild.

    The solar system centaurs appear to be so named because of their changing orbits and varying properties.

  • Laintal December 1, 2020, 6:08

    “There was a time when the Solar System seemed relatively well defined, with nine planets including Pluto and an asteroid belt that orbited in a niche between Mars and Jupiter.”

    Yes I must be getting old, as this was the story I remember well reading Astronomy books as a kid in the 80s

    • Laurel Kornfeld December 2, 2020, 17:19

      Pluto is still a planet, as are all dwarf planets.

  • Michael Fidler December 2, 2020, 8:35

    A number of interesting and unusual details about the Centaurs.

    There Might Be an Entire Orbit, Filled with Asteroids that Came from Outside the Solar System.


    An interstellar origin for high-inclination Centaurs.


    Centaurs from the Oort cloud and the origin of Jupiter-family comets.



    A very good theory from Loeb and Siraj.

    Interstellar Objects Outnumber Solar System Objects in the Oort Cloud.
    Amir Siraj and Abraham Loeb.

    “Here, we show that the detection of Borisov implies that interstellar objects outnumber Solar system objects in the Oort cloud, whereas the reverse is true near the Sun due to the stronger gravitational
    focusing of bound objects. This hypothesis can be tested with stellar occultation surveys of the Oort cloud. Furthermore, we demonstrate that ∼ 1% of carbon and oxygen in the Milky Way Galaxy may
    be locked in interstellar objects, saturating the heavy element budget of the minimum mass Solar nebula model.”


    The interesting aspect is the amount of carbon and oxygen it would of brought into the solar system and the possibility of panspermia. This would have implications for the Jovian planets having also been seeded with life.

    Giving some thought as to what might happen if a large Centaur past close to Jupiter like comet Shoemaker Levy 9 did, made me wonder if the objects orbit could be stopped and drop straight toward the Sun? Breaking apart after the close encounter with Jupiter and within less then two years after, the earth could be in a similar situation as in the movie Greenland! What would be the conditions of such a comet with water ice, other ices, carbon dioxide and large amounts of dust entering the earths atmosphere over extended periods (days to months) plus large impactors. Could this be what happened to earth 12,900 years ago?

    On a second note, could Saturn’s rings have been created by a large Centaur entering the outer atmosphere and ejecting material to form the ring plus expanding the the now low density atmosphere?

    This brings to mind that maybe we should have a world wide dedicated Jupiter and Saturn monitoring telescopes system to track any objects like Shoemaker Levy 9 that pass near these planets 24 hours a day. The Vera C. Rubin Observatory, previously referred to as the Large Synoptic Survey Telescope (LSST) may capture many of these objects but may also be saturated by the brightness of the planets in the nightly surveys.

    • Michael Fidler December 4, 2020, 10:06

      DECEMBER 3, 2020
      Solid Phosphorus has been Found in Comets. This Means They Contain All the Raw Elements for Life.

      Did comets deliver the elements essential for life on Earth? It’s looking more and more like they could have. At least one comet might have, anyway: 67P/Churyumov–Gerasimenko.

      A new study using data from the ESA’s Rosetta mission shows that the comet contains the life-critical element phosphorous.

      Researchers from the University of Turku in Finland led this research. The lead author of the study is Esko Gardner, an astrophysicist and software engineer. The title of the work is “The detection of solid phosphorus and fluorine in the dust from the coma of comet 67P/Churyumov–Gerasimenko,” and it’s published in Monthly Notices of the Royal Astronomical Society.

      The raw elements for life are known as CHNOPS, which stands for Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous, and Sulfur. Combinations of these six chemical elements make up the large majority of biological molecules on Earth. Together they account for almost 98% of Earth’s living matter.


  • ljk February 16, 2021, 11:34

    Claim for giant ‘Planet Nine’ at Solar System’s edge takes a hit

    By Daniel Clery

    Feb. 15, 2021 , 3:32 PM

    For planetary scientists, it was the boldest claim in a generation: an unseen extra planet, as much as 10 times the mass of Earth, lurking on the Solar System’s frontier, beyond Neptune. But the claim looks increasingly shaky, after a team of astronomers reported last week that the orbits of a handful of distant lumps of rock are not bunched together by the gravity of “Planet Nine,” as its proponents believe, but only seem clustered because that’s where telescopes happened to be looking.

    Planet Nine supporters aren’t backing down yet but one skeptic not involved with the new work says she is “very happy” to see it. The study has carried out “a more uniform analysis” than done previously of the far-off rocky bodies known as known as Trans-Neptunian Objects (TNOs), says astronomer Samantha Lawler of the University of Regina, who has tried and failed to simulate the clustered orbits in computer models with an extra planet.

    Full article here:


  • ljk March 17, 2021, 15:26

    What if Planet X is a baby black hole…