By Duncan Steel 01/01/2019

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Looking out at the stars it would be easy to think that the solar system is mostly empty, bar the handful of planets circuiting the Sun and the occasional comet we see passing by. The reality, we now know, is that the edge of the solar system contains a vast population of substantial objects orbiting just beyond Neptune, one of which is currently being visited by NASA’s New Horizons spacecraft.


Sometimes the mechanics of the heavens lead to things happening at inopportune times. I am not talking about astrology. What I mean is that NASA’s New Horizons spacecraft is just flying past a minor planet out beyond the orbits of Neptune and Pluto, and it’s January 1st. That will have played havoc with the seasonal festivities of all involved – heck, I’ve been following the events all day – but somehow I don’t think they mind too much.

This flyby is a ‘bonus’, in that New Horizons was sent primarily to visit Pluto, having a successful encounter in July 2015. If you’d like to see the imagery and other data collected, take a look at the NASA JPL Photojournal (a wonderful resource for all interested in the exploration of the solar system). Shortly there will be more pictures there, this time of a minor planet currently labelled as (486958) 2014 MU69, but soon to receive a ‘proper’ name to replace the 2014… bit. Meanwhile it has been given the informal moniker Ultima Thule, for reasons explained on the relevant Wikipedia page.

Having whizzed past Pluto, it was recognised that it would be inevitable that the spacecraft would pass by other objects in the broad band known as the Kuiper Belt, beyond Neptune’s path. In consequence the Hubble Space Telescope was used to search for candidate targets, and several were soon found and their orbits determined. The 30-km-wide object that is the subject of the present flyby might not be the last, as New Horizons continues its voyage into deep space.

Although some graphics have been made available to show the orbital paths of the candidate targets, such as that here, these do not indicate to the viewer just how crowded it is out there. The existence of the Kuiper Belt was hypothesized many decades ago (separately, by Frederick Leonard, Kenneth Edgeworth, and Gerard Kuiper) but it was not until 1992 that the first object was spotted out there (if one does not accept Pluto as actually being a member of that belt).

Across the intervening 26 years astronomers have discovered over 3,000 large bodies in the outer solar system, mostly in the Kuiper Belt, and these are classed as being minor planets (synonym: asteroids) although by nature they appear to be icy objects more like comets. Indeed the Kuiper Belt, along with the more distant Oort Cloud, appears to be the source of most observed comets.

Apart from Kuiper Belt bodies beyond Neptune (collectively termed Trans-Neptunian Objects or TNOs) there are also numerous similar lumps of material on orbits that cross the paths of Neptune and the other outer planets. These are generally called Centaurs. A catalogue of the orbits of currently-known Centaurs is available; a similar listing of TNOs is also easily accessed.

In the map below I have plotted the orbits of the TNOs and Centaurs that have so far been named (that requiring their heliocentric orbits to be well-determined, the discoverers then being able to suggest a name to the International Astronomical Union, IAU). Pluto is now classified as a ‘dwarf planet’ by the IAU, and so I have shown its orbit as a white line. In turquoise are shown the orbits of the major planets. In red are depicted the current paths of the 23 named Centaurs; and in yellow the orbits of the 23 named TNOs (or Kuiper Belt objects). Shown as a bright mauve line is the orbit of Ultima Thule, the present target of New Horizons.



The crowded outer solar system: this map shows the orbits of the major planets (with the Sun at the centre), dwarf planet Pluto, and the 23 named Centaurs plus the 23 named Kuiper Belt objects (though for clarity only a few of the names are shown here). The New Horizons spacecraft is currently completing a flyby of (486958) 2014 MU69, otherwise known as Ultima Thule. Permission is hereby granted for the use of this image by the media and others, on the condition that the Centre for Space Science Technology logo/credit is not removed; a high-resolution PDF version is available here.



The image above is obviously very crowded, despite the fact that only a small percentage of the known Kuiper Belt and Centaur objects have been plotted; and those we have discovered so far also represent a minuscule fraction of the total number of bodies out there. In the following map, for clarity I have plotted the paths of only the major planets, Pluto, and Ultima Thule.






The orbital path of Ultima Thule compared to those of the outer planets and Pluto. Permission is hereby granted for the use of this image by the media and others, on the condition that the Centre for Space Science Technology logo/credit is not removed; a high-resolution PDF version is available here.







Personally, I am looking forward to seeing the images of Ultima Thule as they become available. The New Horizons satellite is so far away from Earth that the transmission data rate is slow, and it is expected that it will take around 20 months for all the collected and stored information to be sent back. Think on that next time you feel like complaining about your internet speed!

One of the reasons I have an interest in the understandings we will gain from studying this object in the outer solar system is that for some decades I have argued that these pose a hazard to life on Earth that has been underestimated by most researchers in the field. With colleagues I have published numerous papers in this regard, our most recent summary of the situation was we see it having appeared in Astronomy & Geophysics, a journal published by the Royal Astronomical Society, in late 2015. A more extensive discussion of my own view on the subject was published by the Geological Society of America in 2014.

There’s a lot happening right now in terms of deep space exploration, and also many new understandings being obtained as we gather more and more information about our own planet by staring down from satellites in orbit around the Earth. One could say that we are gaining an awful lot out of space.

Update January 3rd (NZ time): First detailed images of Ultima Thule are becoming available here and here.


0 Responses to “It’s crowded at the edge of the solar system  ”

  • I still love the scale of diagrams like those. Earth? Oh, that’s included in the tiny little circle in the middle 🙂
    Plus the joy at primary school of arguing with the teacher that Neptune is the planet furthest from the sun because said teacher left out a critical qualifier like “average”. I vaguely recall that that has changed now insofar as it’s relevant at all with Pluto being called a dwarf (the term is “little planet”, surely?)