In 2006 (yes, it was that long ago – yikes) the International Astronomical Union (IAU) officially adopted the definition of dwarf planet – they are large enough for their gravity to pull themselves into a sphere, they orbit the sun and not another larger body, but they don’t gravitationally dominate their orbit. That last criterion is what separates planets (which do dominate their orbit) from dwarf planets. Famously, this causes Pluto to be “downgraded” from a planet to a dwarf planet. Four other objects also met criteria for dwarf planet – Ceres in the asteroid belt, and three Kuiper belt objects, Makemake, Haumea, and Eris.

The new designation of dwarf planet came soon after the discovery of Sedna, a trans-Neptunian object that could meet the old definition of planet. It was, in fact, often reported at the time as the discovery of a 10th planet. But astronomers feared that there were dozens or even hundreds of similar trans-Neptunian objects, and they thought it was messy to have so many planets in our solar system. That is why they came up with the whole idea of dwarf planets. Pluto was just caught in the crossfire – in order to keep Sedna and its ilk from being planets, Pluto had to be demoted as well. As a sort-of consolation, dwarf planets that were also trans-Neptunian objects were named “plutoids”. All dwarf planets are plutoids, except Ceres, which is in the asteroid belt between Mars and Jupiter.

So here we are, two decades later, and I can’t help wondering – where are all the dwarf planets? Where are all the trans-Neptunian objects that astronomers feared would have to be classified as planets that the dwarf planet category was specifically created for? I really thought that by now we would have a dozen or more official dwarf planets. What’s happening? As far as I can tell there are two reasons we are still stuck with only the original five dwarf planets.

One is simply that (even after two decades) candidate dwarf planets have not yet been confirmed with adequate observations. We need to determine their orbit, their shape, and (related to their shape) their size. Sedna is still considered a “candidate” dwarf planet, although most astronomers believe it is an actual dwarf planet and will eventually be confirmed. Until then it is officially considered a trans-Neptunian object. There is also Gonggong, Quaoar, and Orcus which are high probability candidates, and a borderline candidate, Salacia. So there are at least nine, and possibly ten, known likely dwarf planets, but only the original five are confirmed. I guess it is harder to observe these objects than I assumed.

But I have also come across a second reason we have not expanded the official list of dwarf planets. Apparently there is another criterion for plutoids (dwarf planets that are also trans-Neptunian objects) – they have to have an absolute magnitude less than +1 (the smaller the magnitude the brighter the object). Absolute magnitude means how bright an object actually is, not it’s apparent brightness as viewed from the Earth. Absolute magnitude for planets is essentially the result of two factors – size and albedo. For stars, absolute magnitude is the brightness as observed from 10 parsecs away. For solar system bodies, the absolute magnitude is the brightness if the object were one AU from the sun and the observer.

What this means is that astronomers have to determine the absolute magnitude of a trans-Neptunian object before they can officially declare it a dwarf planet. This also means that trans-Neptunian objects that are made of dark material, even if they are large and spherical, may also fail the dwarf planet criteria. Some astronomers are already proposing that this absolute magnitude criterion be replaced by a size criterion – something like 200 km in diameter.

It seems like the dwarf planet designation needs to be revisited. Currently, the James Webb Space Telescope is being used to observe trans-Neptunian objects. Hopefully this means we will have some confirmations soon. Poor Sedna, whose discovery in 2003 set off the whole dwarf planet thing, still has not yet been confirmed.