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The Real Planet X

A day after the news about 2003 EL61, a Kuiper Belt object originally thought to be larger than Pluto, we now have another world that appears significantly larger still. 2003 UB313 was discovered with the Samuel Oschin Telescope at the Palomar Observatory by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University). Evidently the lower limit of its size is Pluto, and it may be (and probably is) larger.

View of 2003 UB313

Image: Three views of the new planet. Credit: Mike Brown, California Institute of Technology.

Now some 97 AU from the Sun, the planet is the farthest-known object in the Solar System. A news release from Caltech quotes Brown on 2003 UB313 and its credentials as a planet:

“It’s definitely bigger than Pluto,” says Brown, who is professor of planetary astronomy. Scientists can infer the size of a solar-system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the planet is not yet known–in other words, it’s not yet possible to tell how much light from the sun is reflected away–but the amount of light the planet reflects puts a lower limit on its size.

“Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto,” says Brown. “I’d say it’s probably one and a half times the size of Pluto, but we’re not sure yet of the final size.

“But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system.”

But there is also an upper size limit, one determined by the fact that the Spitzer Space Telescope cannot detect this world. That pegs its diameter at something less than 3000 kilometers (Pluto’s is 2300 kilometers). And yes, we’ll have a better name than 2003 UB313 for this object soon once the name the astronomers have proposed to the International Astronomical Union has been accepted. More on the new planet can be found here; this page is likely to be updated significantly over the weekend.

Centauri Dreams‘ note: If Clyde Tombaugh had known to look 44 degrees off the ecliptic, he might have found 2003 UB313 instead of Pluto. No one would have expected a planet in an orbit with such a steep inclination, raising all kinds of questions about what forces drove the world to its present position. The elliptical orbit takes 560 years to complete and brings 2003 UB313 as close as 3.3 billion miles from the Sun (inside the orbit of Pluto). At 97 AU, it is currently nine billion miles out.

Meanwhile, this further information about 2003 EL61 — remember, this is a different object — also from Caltech in Brown’s Web pages. The evidence strongly suggests 2003 EL61 is actually a good deal smaller than Pluto:

“Many times when objects like this are discovered we don’t actually know how big we are, just how bright they are. They could be bright because they are large or they could be bright because they are highly reflective, like a ball of snow. In the case of 2003 EL61, however, we have gotten lucky, because we have discovered a moon orbiting it. By following the orbit of the moon over the course of 6 months we are able to precisely determine the mass of 2003 EL61 and its moon. The mass is about 32% that of Pluto, implying that it has a diameter of perhaps 70% that of Pluto or around 1500 km. We don’t know the diameter for sure, though, just the mass. It could be made of high density material like rock and be smaller or it could be made of low density material like ice and be larger and still be the same mass. If the size is indeed 1500 km 2003 EL61 is larger than any other known object in the Kuiper belt other than Pluto, with the closest runners-up being Quaoar and Pluto’s moon Charon at about 1250 km. It is also possibly larger than Sedna, a similar object well outside the Kuiper belt.”

Brown, Trujillo and Rabinowitz had been studying 2003 EL61 at the same time the Sierra Nevada team was working on it, and were delaying announcement of the object until further observations could yield more information about its size. The Spanish team thus gets credit for the discovery, while the Palomar team supplies a slew of further detail. Their observations at the Keck Observatory, for example, have shown that 2003 EL61’s moon appears to compose only about one percent of the mass of the system, making it the smallest satellite relative to its primary thus far found in the Kuiper Belt (Charon is roughly ten percent of Pluto’s mass).

Comments on this entry are closed.

  • Rahul July 4, 2006, 7:56

    Just one question
    Have anybody seen the explaination of why earth is tilted 23 degrees on axis and saturn at 90 degrees?

  • Administrator July 4, 2006, 9:51

    Saturn’s axial tilt is 26.7 degrees. I think you have Uranus in mind, where the tilt is indeed extreme at over 97 degrees. And what I’ve always heard as the explanation was a primordial impact that caused this drastic change, though if there are other theories out there, I’d also like to hear about them.

  • James April 5, 2007, 11:26

    Acually the axis of saturn is 26.8 degrees.

  • Caitlin April 9, 2007, 20:29

    i dont think that Pluto is a real planet because it is so cold it wouldn’t have enough strengh to hold it’s own parts because it is so cold! i also think that Sedena is a real planet i don’t know why but i think it really is!

  • Adam April 10, 2007, 4:51

    Hi Caitlin

    Cold doesn’t affect gravity, but it does make holding on to gases, like nitrogen and methane, a lot easier. The warmer gases are, the faster their molecules move, and to hold on to them a planet’s gravity has to be stronger.

    Over all Sedna is smaller than Pluto and less able to hold onto any gases it might have. Both are now called dwarf planets, which has nothing to do with what they are and more to do with how they affect smaller objects around them. A planet, like Earth or Jupiter, tells all the smaller objects around it where to go with its gravity – it is gravitationally dominant. Pluto, Sedna, Eris and Ceres, plus the other dwarf planets, are too small to do that in their orbits and so they’ve been given a new name to make a clear distinction.

    Keep learning, you’re asking good questions!

  • Edg Duveyoung April 10, 2007, 9:31

    I bet that if Mercury was as small as Pluto, it would still be called a planet by the same folks who dumped Pluto into secondary status.

    Maybe 30 years from now, Pluto will be a planet again — the world, the entire world, has been taught “officially” in public schools that Pluto is a planet, and like the attempt to change America and make it use the metric system, anti-pluto-ists might find that a cultural momentum resists the new classification enough that, later, down the line, the popular media will be typically using the word planet just as much as miles, yards, feet and inches are still used today.


  • ljk April 10, 2007, 9:54

    Pluto is a Kuiper Belt Object that was mislabeled almost from the
    moment of its discovery in 1930. We didn’t find any other KBOs
    until 1992, but they were suspected for years.

    Pluto is smaller than Earth’s moon, it has a very eccentric orbit
    compared to the eight main planets, there is at least one other
    KBO larger than it with several more nearly as large, plus it
    swims around Sol in the same region as most of the KBOs. Had
    Pluto been discovered in the last decade or so, it would have
    been called a KBO and not a major planet.

    To still call Pluto a planet with our current knowledge is pure
    nostalgia. Besides, the KBOs are fascinating worlds in themselves
    and worthy of study by space probes, in case anyone is worried
    about that not happening just because Pluto is not a planet.

    Recall that the early space probes were meant to study primarily
    the major planets, but in the cases of the Jovian worlds, their moons
    were found to be just as fascinating and often moreso. Now places
    like Europa, Enceladus, and Titan merit their own missions.

    The day that answers to science questions are chosen by popular
    vote like some kind of American Idol contest is the day we have
    gone back to a theocracy, or worse.

  • Edg Duveyoung April 10, 2007, 10:12


    Just to be sure I’ve communicated. I’m not invested in Pluto being called a planet — I was merely noting that maybe the new classification will have a long hard slog, before the present “psychic momentum” that the masses currently have behind the, now knee jerk, built in, automatic use of the word “planet” when Pluto is mentioned. “Kuiper Belt Object” just doesn’t roll off the tongue, ya know?

    I read and post at several astronomy sites, and, frankly, the memberships are small, and the posts are not all that frequent, even with Hubble and the rovers creating headlines with a nice regularity, not many folks are studying astronomy — the masses are not paying attention! It’s going to be a long while before Pluto’s proper classification becomes common knowledge to such a degree that people begin to self-correct when the word comes to their minds.

    I still reach for a Kleenex, not a tissue, ya know?