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Pluto was recently re-classified, losing its official status as the ninth planet, because a bunch of other dwarf planets were found in its orbital region that were too numerous and insignificant to count as "real" planets, and it quickly became apparent that Pluto wasn't really any different from them.

And yet, one unambiguous fact remains: Pluto was discovered in 1930, and the rest of the Kuiper belt dwarf planets didn't even begin to be discovered until the 1990s. This raises a somewhat obvious question: if Pluto is no more exceptional than other Kuiper belt dwarf planets, why did it take more than half a century of technological advances before we were able to discover the rest of them?

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    $\begingroup$ As you will see if you read about Pluto on Wikipedia, astronomers were searching for something that didn't exist and accidentally found Pluto. $\endgroup$ – called2voyage Sep 3 '15 at 17:52
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    $\begingroup$ Ceres gets no respect. $\endgroup$ – Russell Borogove Sep 3 '15 at 19:10
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    $\begingroup$ As I think @RussellBorogove is trying to point out, Ceres was discovered over 100 years before Pluto. Pluto was not the first dwarf planet discovered. It was the first Kuiper Belt Object discovered. $\endgroup$ – NeutronStar Sep 3 '15 at 22:14
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    $\begingroup$ Maybe it is better for Astronomy.SE? $\endgroup$ – Anixx Sep 5 '15 at 13:00
  • $\begingroup$ Actually, Pluto was the 11th planet, Ceres was the 9th. Or was it the 12th body to be called a planet? Classifications have changed before as knowledge grows. Actually, that's post-Copericus: the sun and the moon were planets before they were reclassified. $\endgroup$ – JDługosz Sep 6 '15 at 6:53
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Pluto was discovered by a manual search of the sky using a blink comparator. This is an extremely laborious process. For Pluto, it made sense to go to all this trouble, because there were indications that a ninth planet had to be out there: Neptune's orbit was perturbed by the gravity of another planet, it seemed.
Later, it turned out that that wasn't the case, but by then Pluto had been found.
Now we know that Pluto is the brightest object in the Kuiper Belt. Everything else is a lot dimmer and harder to find. Because of that, we only started finding KBOs in 1992, when the process could be automated.

Before that, Charles Kowal did an extensive search using a blink comparator:

Between December 1976 and February 1985, Kowal searched 6400 square degrees of sky in the ecliptic plane for distant, slow-moving Solar System objects. Only one object was found beyond Jupiter: 2060 Chiron...

Chiron orbits between Saturn and Uranus.

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  • $\begingroup$ You might have meant to link to Centaurs, which include Chiron. $\endgroup$ – Potatoswatter Sep 7 '15 at 9:04
  • $\begingroup$ No, I meant to link to the History section of the Kuiper Belt article. $\endgroup$ – Hobbes Sep 7 '15 at 9:17
  • $\begingroup$ Then your meaning is a little unclear. The Kuiper Belt was hypothesized for various reasons since the early 20th century and named in 1951, according to that article. The discovery of Pluto and of the Centaurs were each additional evidence. But it doesn't follow that "people were looking for KBOs." As you said, the late 20th century is when computerized image processing made searching feasible. As Wikipedia details, In 1987, astronomer David Jewitt…, became… puzzled by "…emptiness of the outer Solar System". He encouraged… Jane Luu… to locate another object… "If we don't, nobody will." $\endgroup$ – Potatoswatter Sep 7 '15 at 9:40
  • $\begingroup$ … An odd sentiment for a scientist to express, especially considering that the enabling technology was still new. However, it seems to show that the research direction was considered fairly obscure at the time and, although the problem hadn't been sitting neglected by computer-savvy astronomers for many years, nobody was in immediate competition. $\endgroup$ – Potatoswatter Sep 7 '15 at 9:47
  • $\begingroup$ Edited to clarify my point. $\endgroup$ – Hobbes Sep 7 '15 at 10:32
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There are a few special things about Pluto, as compared to the other dwarf planets in the Kuiper belt. These include:

  1. It is the largest dwarf planet known, by diameter. (Note, this was only determined after the New Horizon's flyby)
  2. It orbits relatively close to the Sun, at times it is even closer than Neptune!
  3. It has the largest satellite system of any non-gas giant in the solar system.
  4. It has an atmosphere. It seems unlikely that any other dwarf planet has an atmosphere that has been discovered thus far.
  5. It is the only "double planet", where the planetary satellite system's barycenter outside of the system's main planet!

Okay, so why else was it discovered early? Here's a few reasons:

  1. Telescopes didn't grow much in size for some time around the discovery of Pluto.
  2. Discovering new objects so far out takes a lot of time. Someone really has to be looking for such objects, or else have a dedicated computer program to do so. In fact, often objects are discovered to be in pictures from long ago, such as Orcus, discovered in 2004, but pictures were taken of it in 1951.
  3. Computers didn't come to be popular for such purposes until the 1990s, which is why the number of such objects discovered increased dramatically at that time.
  4. Pluto is quite a bit brighter than the other dwarf planets, Ceres excluded.
  5. Astronomers were looking for a new planet in the outer Solar System when Pluto was discovered. Pluto was not the planet they were looking for, still, it was there, largely by coincidence.
  6. The 1990s saw a dramatic improvement in telescopes, namely in that the mirror could be moved in a way to correct for atmospheric issues, called adaptive optics.
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    $\begingroup$ "These are not the planets you are looking for..." $\endgroup$ – Bigbio2002 Sep 3 '15 at 19:16
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    $\begingroup$ @MackTuesday Eris is more massive but occupies less volume than Pluto. $\endgroup$ – called2voyage Sep 3 '15 at 19:42
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    $\begingroup$ @MackTuesday, recent measurements from the New Horizons spacecraft show that Pluto is slightly larger than Eris. Eris is still more massive though. $\endgroup$ – NeutronStar Sep 3 '15 at 22:11
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    $\begingroup$ So, very short version, a fair bit of luck plus, "nobody thinks that brightness should be a criterion of planethood"? $\endgroup$ – Steve Jessop Sep 4 '15 at 4:08
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    $\begingroup$ @JDługosz: The best estimates of Eris's radius come from a stellar occultation in 2010 — essentially, Eris eclipsed a (very faint) star, and astronomers were able to figure out the size of the shadow from observations at different points on the Earth. Eris's radius is now known to within 6 km, Pluto's within 2 km, and Pluto's radius is (best estimate) about 23 km larger. The same technique has also been used to measure Makemake and Quaoar. $\endgroup$ – Michael Seifert Sep 9 '15 at 14:49
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One reason is its brightness.

Pluto passes near and far and the atmosphere freezes and thaws. So while most bodies are pretty dingy, Pluto is covered with fresh snow.

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One reason not mentioned is an accident of timing. But for an orbital coincidence, what we now call Eris might have been discovered before, or at about the same time as, Pluto.

Eris has an absolute magnitude of about -1.17, versus Pluto whose absolute magnitude is -0.7. The greater absolute brightness of Eris is one reason Eris was originally thought to be larger than Pluto in volume.

Since Eris is intrinsically brighter, it would have been seen first or, at least, at about the same time, if only they had been at comparable distances. But Eris follows a highly eccentric orbit and happened to be close to aphelion when astronomers were looking for a planet beyond Neptune. Had Eris been in a different part of its orbit, our narrative of Kuiper Belt discovery might have been different.

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  • $\begingroup$ How far away was Eris from Earth in 1930 when Pluto was discovered and what was the distance when Eris was discovered in 2005? $\endgroup$ – Uwe Mar 22 at 12:08
  • $\begingroup$ My understanding is that Eris was discovered near aphelion (97 AU from the Sun) in 2005, and it would not have moved very far around in 75 years out of a 557-year orbit. $\endgroup$ – Oscar Lanzi Mar 22 at 13:10
  • $\begingroup$ "Precovery images of Eris have been identified back to September 3, 1954." So there was a chance to discover it 5 decades earlier and 24 years after Pluto. But aphelion was around 1977, about 23 years after precovery and 28 years before discovery. When Eris was discoverd, it was the most distant known object in the Solar System. $\endgroup$ – Uwe Mar 22 at 16:25
  • $\begingroup$ Since the orbit runs 557 years and is highly eccentric, the above comments imply Eris would indeed have been near aphelion during the whole 20th and early 21st centuries. $\endgroup$ – Oscar Lanzi Mar 22 at 23:02

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