Dwarf Planet candidates

[Danzi]

Hello
Looking on wikipedia at the list of possible Dwarf planet candidates, there are quite a few. So i ask, why has the IAU not classified some of them as Dwarf planets, there are loads on the list, but so far there are only 5!

Why has the IAU not classified more of the candidates as Dwarf Planets, such as Sedna, or 2007 OR10?

Thanks

 

[MeteorWayne]

The answer is because they are speculative reports. The size estimates are just that, estimates. When objects are first discovered, the size is based on assumptions about the reflectivity of the surface. They vary by +/- 50% or more.

In fact, for almost all these abjects, they look like points, so there are no real size measurements. Pluto is an exception.

 

[Danzi]

In that case, how come they could work out the size of Eris, Haumea and Makemake, but can of objects closer?

 

[h2ouniverse]

In that case, how come they could work out the size of Eris, Haumea and Makemake, but can of objects closer?

Hi Danzi,

The H=1 limit for AUTOMATIC classification as dwarf by IAU is not innocent. It guarantees a diameter >824km even if albedo is 1 (fully reflective body). Beyond 800km, it is commonly accepted that a body should be quite well hydrostatically relaxed (not an absolute notion, even Titan or Earth are not fully relaxed) whether it is rocky or icy.

Icy bodies (say e.g. with density <2.5) are believed to be relaxed if >400km. The threshold might even be lower ( see http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1050.pdf ). Sedna and 2007OR10 have their minimum guaranteed diameter at 625km and 544km respectively. So IF they are icy, they should qualify as dwarves even with a high albedo.
But until you get a mass assessment (e.g. from the presence of a satellite) it is difficult to constrain the density. Iapetus and Haumea have both average diameters of about 1500km and yet the former has a density close to 1 whereas Haumea is close to rocky bodies (about 3). So can be highly variable.
Another way is to constrain the albedo but this is not easy.

Best regards.