Unless you’re living under a rock, you’ve probably already heard of this. The International Astronomical Union, aka the Committee on What to Call Astronomical Stuff, has today adopted an official definition of what is — and isn’t — a planet, in the process stripping Pluto of the title it has held since its discovery in 1930. Pluto has now been reclassified as a dwarf planet, meaning our solar system has 8 proper planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Our new solar system .

Text books and encyclopedias will need to be revised accordingly, of course. Not surprisingly, Wikipedia has already locked down its solar system and planet pages, among others, and is in the process of updating their contents.

Personally, I think the IAU’s resolution was spot on. Pluto has for decades seemed the oddball of the solar system. With the recent discoveries of further trans-Neptunian bodies (namely 2003 UB₃₁₃, popularly known as Xena), and the high likelihood of discovering thousands more similarly-sized Kuiper belt bodies in the future, an official definition of the term “planet” was overdue.

While dropping Pluto’s planet status has been described as a triumph of science over sentiment, the Bad Astronomer rightly points out that defining what constitutes a planet is not a scientific decision but rather an arbitrary cultural classification. Nonetheless, the IAU’s new guidelines read lucidly enough:

• A planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighborhood around its orbit.

• A dwarf planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has not cleared the neighborhood around its orbit, and is not a satellite.

Pluto got the boot specifically due to the provision that a planet must clear its neighborhood; Pluto fails this requirement since its eccentric and oblong orbit overlaps with Neptune’s — in fact at perihelion of Pluto’s 248-year orbital period bringing it closer to the Sun than Neptune for a couple of decades at a time (most recently, from 1979 to 1999). Of course, its case wasn’t helped by the fact that it really is quite tiny (heck, it’s smaller than the Moon!)

IAU’s new resolution certainly seems a bit smarter than their harebrained first proposal, announced last week, which would have promoted Xena, Ceres (the largest asteroid) and Charon (Pluto’s moon) to planet status — giving us a 12-planet solar system, with the number promising to keep ever increasing as more Pluto-like icy bodies are discovered in the Kuiper belt. Now, Xena, Ceres and Charon are all considered “dwarf planets”.

The new planet definitions only apply to our own solar system. So far, extrasolar planets, of which over 200 have been discovered, have no classification scheme, though similar criteria will no doubt be applied. More than likely there are enough surprises awaiting in other solar systems to keep the committee arguing semantics for the next couple of centuries.

Putting the Solar System into Perspective

Incidentally, check out these magnificient 3D-rendered pictures of the main solar system bodies. Showing the planets and the Sun in their correct relative scale, the pictures give some idea of just how tiny Earth is in comparison to the gas giants and the Sun.

The inner planets: Earth, Venus, Mars, Mercury (guest starring our Moon).

The outer planets: Jupiter, Saturn (without the ring), Uranus, Neptune.

Looking at the above picture, it may not be altogether inconceivable that three Earths could actually fit inside the Great Red Spot, the gargantuan storm that has raged on Jupiter for at least the past three centuries.

The Sun looming large over the inner and outer planets.

While this picture demonstrates the majestic scale of the Sun compared to even our largest gas giants, the Sun is actually not very large as far as stars go. For instance, the nearby (in astronomic terms) red supergiant Betelgeuse is more than 40 million times more voluminous than the Sun — if it were placed in the location of the Sun, Betelgeuse’s photosphere would extend beyond the orbit of Mars. To picture this, imagine that if Betelgeuse were represented by the Sun in the above picture, then the Sun itself would be the size of the tiny, almost invisible Earth.

Most of the Universe is Missing — No, Really

In other science news this week, NASA has found the first direct evidence that dark matter really exists. Until now, dark matter was a mere theoretical construct necessary to explain (among other things) how galaxy clusters hold together, and there was always the possibility of dark matter being an illusion rising from some flaw in our understanding of gravity. However, the new observations of the collision of two galaxy clusters, some 3 billion light-years away, strongly suggest that dark matter is very real, though still very much of a mystery.