Have Any Dwarf Planets Been Discovered Outside Our Solar System?

Question posed by Courtney.


The thing with finding extrasolar* planets is that the bigger ones are easier to find, and the smaller the planet the harder it is to find. Dwarf planets are pretty much planets except they can't lay claim to one (or both) of the following:

  • Heavy enough to have achieved hydrostatic equilibrium**.
  • Big enough to have cleared the local neighbourhood of other stuff.
Both of these are affected mostly by an object's mass- that's how much stuff it's made of. The smaller an object is, the less likely we are to find it when it's located far away in another star system, but the limits of what we're able to detect are gradually creeping lower: the first extrasolar planet discovery was only confirmed in 1992***. Most early exoplanet discoveries were so-called 'hot Jupiers': massive planets that orbited very close to their stars. These hot Jupiters surprised astronomers (they seemed to go against prevailing planet-formation theories), but as planet-detecting has become a more successful endeavour, it appears that such planets are actually in a minority.

As planet-spotting techniques have developed, smaller exoplanets have been found. The smallest so far was discovered in January of this year (2012) and orbits a star called KOI-961 with at least two other planets. It is about the size of Mars. This planet was discovered by NASA's Kepler mission, and Kepler program scientist Doug Hudgins stated that "Finding [an exoplanet] as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us."

As smaller bodies are discovered it may well prove difficult to decide whether they are planets or dwarf planets: we can make inferences as to their shape from estimates of their mass and composition, but, from such a distance, we can't know for sure; neither can we make truly accurate assessments of how clear their local neighbourhoods are.

* 'Extrasolar' just means 'outside the solar system'.
** That basically means that it's generally spheroidal, or ellipsoidal in shape. For more on Dwarf planets check out this post, or ask a question!
*** The first exoplanet to be discovered was actually discovered in 1988, but this one wasn't properly confirmed until 2003!


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