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Looking for Our Sun’s ‘Super-Earth’

An obscure instrument called a blink comparator became world famous following Clyde Tombaugh’s discovery of Pluto in 1930. It was by rapidly switching between astronomical photographs that the young Tombaugh was able to compare objects in the field of view where ‘Planet X’ was presumed to hide. Pluto turned out to be a good deal smaller than Percival Lowell had imagined, leading to thoughts of still more distant planets, but for a time the new planet was best known as a faint dot on a series of plates, moving against a fixed field of stars.


Image: Clyde Tombaugh at the Blink Comparator five years after the Pluto discovery. Credit: Lowell Observatory Archives.

All of this is wonderfully told in Michael Byers’ 2010 novel Percival’s Planet (Henry Holt and Co.), which draws on Tombaugh’s story and depicts the entire Lowell Observatory scene in his time there (see A Tour de Force of Planetary Discovery for my review of the book). Or if you want the inside view, Tombaugh’s own Out of the Darkness: The Planet Pluto takes on his Pluto work and the entire field of planet hunting.

These days we can mimic what a blink comparator did with a computer, but in an era when charge-coupled devices (CCDs) have replaced photographic plates, we’re more prone to use computer software to tease out the motion of distant objects against the background stars, so that the nearby, moving object appears fixed amidst what appear to be the tracks of stars.

We’ll see what technique is to be employed in a new crowdsourced effort to find Planet X, still out there (or so many believe) and referred to by many astronomers as Planet Nine. Brad Tucker (ANU), who leads the project, explains that the new planet is predicted to be a super-Earth, perhaps 10 times the mass of Earth and as much as four times its size.

An object like this roughly 800 AU out would help to explain the unusual orbital characteristics of Sedna and five other ‘extreme trans-Neptunian objects’ (ETNOs) whose orbits are sharply tilted when compared to the plane in which the planets move. Chadwick Trujillo and Scott Sheppard (Carnegie Institution for Science, Washington) made a strong case for this scenario in a 2015 paper, followed up by work from Mike Brown and Konstantin Batygin (Caltech). You can check the Search for Planet Nine site for background on Brown and Batygin’s thoughts.

At ANU, the data in question will come from the Australian National University’s robotic 1.35-meter SkyMapper telescope at Siding Spring, the only telescope that maps the entire southern sky. The instrument will serve up hundreds of thousands of images for public assessment. As with the citizen science effort to study old astronomical prints we discussed on Friday, the ANU search will involve volunteers scanning the SkyMapper images online, in this case to look for any signs of a planet. The incidental discovery of comets or dwarf planets seems likely.


Image: The SkyMapper telescope at sunset. Credit: Steve Chapman.

The project is to be launched by Brian Cox during the BBC’s Stargazing Live broadcast from Siding Spring Observatory, joining other crowdsourced efforts on the Zooniverse site when it becomes available. Bear in mind that we already have Backyard Worlds, a project (likewise on Zooniverse) that uses data from WISE (Wide-field Infrared Survey Explorer) to pursue a similar search, though not targeted as tightly on a single planet. The latter’s animated ‘flipbooks’ are the modern day equivalent of what Tombaugh used for Pluto.

I don’t know what kind of interface the SkyMapper effort will come up with, but it’s pleasing to think of a modern-day analog of the famous Tombaugh search, now spread out over a global network of users. The Stargazing Live series begins on the BBC on March 28 at 2000 UTC. An Australian version of the show will also be broadcast from Siding Spring Observatory next week on ABC TV. Stargazing Live has its own blog page from which to learn more, and ANU Twitter feed @scienceANU is also tracking the rollout.


Comments on this entry are closed.

  • DJ Kaplan March 27, 2017, 12:02

    Wouldn’t it give a false impression to call Planet X a “Super Earth”? I mean, it may be a rocky planet, but that’s likely the only resemblance.

    • J. Jason Wentworth March 28, 2017, 20:55

      That seemed odd to me, too. Maybe–just as astronomers refer to “metals” in stars as being elements heavier than helium (not all of which are metals in the normally-accepted sense)–the term “Super Earth” is sufficiently ‘loose’ to include solid planets that are made largely of solidified gases? As far from the Sun as Planet X is thought to be (as “nearer” Pluto and Triton showed, even though they contain some rocky materials and have very thin atmospheres), a world consisting chiefly of a methane/nitrogen/ammonia “atmosphere” could have its atmosphere frozen solid, particularly if it isn’t massive enough–as Uranus and Neptune are–to have significant internal heat to keep the atmosphere gaseous and (deeper down) liquefied.

    • xcalibur March 29, 2017, 23:05

      It may be a super-earth or a mini-neptune, depending on composition. It remains to be seen.

  • Harry R Ray March 27, 2017, 14:33

    If Planet Nine is indeed discovered as a result of this effort, out of the thousands and thousands of Clyde Tombaugh wannabe’s, will the “one lucky winner”s name be placed beside the principle investigator’s in the discovery paper? Back in 1930, things were done QUITE DIFFERENTLY. Vesto Slipher was the “principal investigator” in Percival Lowell’s “Planet X” search. If the discovery were made today instead of 1930, HE would have been credited with the discovery, NOT Clyde Tombaugh! Case in point: Didier Queloz actually DID the observations and data reduction that led to the discovery of 51 Pegasi b, but Michel Mayor got the credit for the discovery as the principal investigator, and was the LEAD AUTHOR of the discovery paper.

    • Cameron Jack March 28, 2017, 5:06

      The person who makes the discovery is not necessarily the person who writes the academic paper, or who leads a major academic project. It’s very rare these days for individuals to receive much credit for major research work as much of it is done by large, international teams. In extreme cases the lead author is sometimes simply the person with the first name in alphabetical order!

    • ljk April 4, 2017, 8:34

      Just as Jocelyn Bell did the work to find the first pulsar in 1968 but her boss Anthony Hewish got both the credit and the Nobel Prize:


  • Mark T. March 27, 2017, 14:39

    “(Trujillo and Sheppard) made a strong case for this scenario in a 2015 paper, followed up by work from (Brown and Batygin).”

    They made a strong case for the orbits being perturbed, but arguably did not make a strong case for the perturber being a currently existing 5-15 Earth mass object orbiting 200-1,200 AU away. That fits Brown’s simulation, but about half the time Brown originally said he would need to find Planet X has elapsed and we are still waiting.

  • Geoffrey Hillend March 27, 2017, 16:47

    Planet X is supposed to be a Neptune sized world according to Caltech’s theoretical calculations or computer simulations and these are of course only hypothesis, so maybe the name Super-Earth might not be best. Super- Earth’s in the outer solar system became gas giants due to the lower temperature which results in the greater ability to retain an atmosphere like the moon Titan. Consequently, due to this law of planetology, a a planet with the mass of a Super-Earth in the outer solar system would have to migrate there from our solar system or another star system which seems very unlikely. I don’t expect astronomers to find a Super Earth or even another Neptune sized planet in the outer solar system since it is only theoretical and the evidence is not crucial but maybe only another dwarf planet or small rocky planet.

    • Mauldred March 28, 2017, 6:03

      If it formed in its current place, we can also imagine an over-sized Pluto. It could be the same size as Neptune but with a solid icy surface and only a 1- to 3-bar helium-rich atmosphere. The surface would be made mostly of nitrogen and methane ice, with possibly lakes and seas of liquid hydrogen, and we could walk on it due to the bearable ~1.3 g surface gravity and tolerable atmospheric pressure, just with a very efficient protection against the extreme cold and a reserve of air to breathe.

      But of course Nature is often more imaginative than ourselves, so it is very hard to guess with models what Planet 9 might look like, because the models are themselves limited by our human imagination and the fact that the Oort Cloud is still largely unknown.

      I would not be so surprised that Nature surprises us one more time, although I will certainly be surprised, as the rest of us, by the characteristics of Planet 9, when we’ll find it. :)

  • Tom Mazanec March 27, 2017, 17:44

    How long till it becomes obvious there is no Planet Nine if we do not find anything?

    • djlactin March 28, 2017, 8:04


    • Mark T. March 28, 2017, 15:37

      “I’m pretty sure, I think, that by the end of next winter — not this winter, next winter — I think that there’ll be enough people looking for it that … somebody’s actually going to track this down,” Brown said during a news conference Wednesday (Oct. 19) . . . Brown said that eight to 10 groups are currently looking for the planet.”


      On October 19, 2016, Mike Brown thought it would be done by March 20th, 2018.

  • fbj March 28, 2017, 4:16

    Some research groups think Planet Nine is unlikely to exist. They do not see clustering when simulating the outer solar system:

    “Shankman et al. (2017) demonstrated a drawback of the P9
    scenario; they showed that P9 generically drives ETNOs into gi-
    ant planets crossing orbits, which subsequently decouples them
    from any shepherding mechanism of P9”

    “In Lawler et al. (2017), the authors evolved primordial planetesi-
    mal disk for 4 Gyr in the solar system with the present-day plan-
    etary architecture plus an additional planet…They found no
    evidence for orbital clustering in the ETNOs region at the end of
    the simulation neither for a circular nor an eccentric ninth planet.
    These findings bring some controversy into the P9 scenario”


  • kzb March 28, 2017, 8:05

    Geoffrey, I don’t think you should dismiss it being a terrestrial planet. For one thing, the lower mass limit is only a few times Earth. For another, some papers argue that our current solar system architecture could only come about if one or more planets were ejected. Planet 9 could be this “ejected” planet, only it did not quite make it out of the solar system.

    • MGP2001 March 28, 2017, 23:21

      I’m not an orbital mechanic but the ejection theory seems a bit of a stretch. Any sufficiently large body, if ejected and recaptured, would maintain an orbit roughly in the same plane as the rest of its peers given conservation of energy and angular momentum laws. Although, I suppose two large bodies could have ejected each other…

  • Geoffrey Hillend March 28, 2017, 18:28

    I don’t dismiss the idea the there could be a planet X between 1 and 10 Earth masses. It would be nice if there was one there. One shouldn’t forget about the principles of planetology though. Usually, Earth size planets in the outer solar system become Gas giants. Jupiter has a rocky core. Maybe there was not a lot of gas out in the Kuiper belt for a Super-Earth to be forced to become a Gas giant. The reason being is that there is much less radiation from the sun in the outer solar system. Too much radiation can increase the velocity of gas molecules when a planet is close to the Sun but when it is far away it allows a planet to retain a much larger atmosphere. The planet X is supposed to be further out than 100 Au which makes it difficult to find or prove. Neptune was discovered by the irregularities of the orbit of Uranus which was being perturbed by the gravity of Neptune but these are near to each other compared to 100 Au’s. I was wondering if there is a way to prove or disprove it by the radial velocity method since a Neptune sized planet would definitely affect the motion of the Sun?
    It is the evidence of planet x is not strong. Its based on computer modeling and that some of the dwarf planets orbit cluster together which might be explained as coincidence or something else rather than being perturbed by the gravity of a distant Kuiper belt gas giant.

  • kzb March 29, 2017, 13:17

    Geoffrey: the idea is that Planet 9 would have formed much closer in. It could have formed as a terrestrial rocky planet in the inner solar system. There was not enough material to form a large planet at its current (supposed) orbital radius, it must have formed further in.
    As for the orbital inclination (30 degrees), other trans-Neptune objects also have high inclinations. Admittedly they are far smaller objects, but I don’t know how much of a problem it really is for orbital mechanics.

  • xcalibur March 29, 2017, 22:44

    Some have pondered why super-earths and mini-neptunes are common in other systems, but are not found in our own. Planet X may resolve this issue.

    There definitely seems to be evidence for something out there. The fact that we haven’t exhaustively mapped the outer solar system is a reminder that we’re still in the early phases of discovery.

  • ljk April 4, 2017, 8:37

    Narrowing down to four candidates. I refuse to use the real title of the following article as it is very misleading, making one think they have found Planet X already.

    In brief:

    A 3 day search for an undiscovered planet in our solar system has produced 4 possible candidates. The hunt for Planet 9 was part of a Zooniverse citizen science project, and shows what we can achieve when we collaborate on scientific projects.

    Full article here: