Remind Me Again, Why Isn't Pluto a Planet?

Remind me again, why isn't Pluto a planet? Understand the reason Pluto isn't considered a planet anymore and learn what the criteria for planethood is.

Last week, news broke that a team of astronomers using the Hubble Space Telescope had discovered a new moon in orbit around Pluto.

They were actually looking for signs of a ring system when they stumbled across another tiny object - now imaginatively called "P4" - bringing Pluto's moon count to four.

This is quite a nice little news story in its own right, and covered by my Discovery News colleague Irene Klotz (see the July 20 article: "Hubble Discovers New Pluto Moon"). However, what I hadn't anticipated was a deluge of messages asking me if this means Pluto is now a planet again.

Well, the answer is a resounding no. It is still, and always will be, a dwarf planet.

BIG PIC: A Complex and Changing World

But what rules have we applied to Pluto to forever condemn it to the back seats of the solar system rankings?

Before we look at the well established rules of planetary terminology, it's worth remembering why we even need such a definition.

Like many of you, I was taught as a school kid that there were nine planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto along with thousands and thousands of tiny objects called asteroids. For a long time, Pluto was believed to larger than Mercury but after the discovery of Charon (Pluto's largest moon) in 1978, the foundations of its planetary status started to wobble.

By studying Charon, astronomers could accurately determine the mass of Pluto and surprisingly found it to be much smaller than Mercury and even our own moon.

During the late 20th century, more objects started to be discovered at comparable distances to Pluto's orbit and beyond; one of which, Eris, was even thought to be larger than the ninth planet in our solar system. Eris was discovered by Mike Brown and his Palomar Observatory-based team in 2005.

INTERVIEW: Where Are You Hiding Planet X, Dr. Brown?

These discoveries led the International Astronomical Union (IAU) to setup a committee in 2005 to consider an official definition of a planet. There were a number of different definitions that were considered, but in 2006 a final, all-encompassing set of criteria was identified that, once and for all, knocked Pluto off its planetary pedestal.

For a celestial body to be considered a planet, it must;

1) be in orbit around the sun.

Clearly, Pluto is in orbit around the sun, but so do thousands of asteroids. As far as this criterion is concerned, Pluto is still hanging on by the skin of its teeth.

2) have sufficient mass to assume hydrostatic equilibrium.

This is just a posh way to say it is pretty much spherical in shape. Note the careful use of the phrase 'pretty much'; no planet is a perfect sphere. Due to their rotation, often they are a little squashed along the polar axis.

This criterion is just saying they that they must have sufficient gravity to have overcome other forces and mould a more-or-less sphere-shaped body. Pluto maintains hydrostatic equilibrium, whereas many of the asteroids and other minor planets are quite oddly shaped.

3) have cleared the neighborhood around its orbit.

BOOM! That's the nail in the coffin for Pluto.

This final criterion requires that it must have cleared its orbit of all other objects of comparable size, other than its own satellites. This means "a planet" must be gravitationally dominant in its orbit and this is where Pluto fails. Pluto not only shares its orbit with a number of other Kuiper Belt objects, but it also flies inside the orbit of ice giant Neptune!

The small print here says that if it fails on this point alone, then it must be classified as a dwarf planet. And so the case against Pluto finally got laid to rest in 2006 when the IAU voted to "demote" Pluto to the status of dwarf planet.

The recent discovery of another moon in orbit around the tiny distant world doesn't change anything, sadly.

I think in the hearts of thousands of people, Pluto will still always be the ninth planet in our solar system; we just won't talk about it (much).

Artist impression of Pluto and largest moon Charon from the surface of either Nix or Hydra, two smaller moons. "P4" is the new addition.

Exquisite Exoplanetary Art

Sept. 19, 2011 --

They're alien worlds orbiting distant stars far out of reach of detailed imaging by even our most advanced telescopes. And yet, day after day, we see vivid imaginings of these extrasolar planets with the help of the most talented space artists. The definition of an extrasolar planet -- or "exoplanet" -- is simply a planetary body orbiting a star beyond our solar system, and nearly 700 of these extrasolar worlds have been discovered so far (plus hundreds more "candidate" worlds). With the help of NASA's Kepler space telescope, the ESO's High Accuracy Radial velocity Planet Searcher (HARPS), French COROT space telescope and various other advanced exoplanet-hunting observatories, we are getting very good at detecting these worlds, but to glean some of the detail, we depend on artist's interpretations of fuzzy astronomical images and spectral analyses. That's the way it will be until we build a vast telescope that can directly image an exoplanet's atmosphere or physically travel to an alien star system. So, with the flurry of recent exoplanet discoveries, Discovery News has collected a few of the dazzling pieces of art born from one of the most profound searches mankind has ever carried out: the search for alien worlds orbiting other stars; a journey that may ultimately turn up a true "Earth-like" world.

The Transit

As an exoplanet passes in front of its star as viewed from Earth, a very slight dip in starlight brightness is detected. Observatories such as NASA's Kepler space telescope use this "transit method" to great effect, constantly detecting new worlds.

Hot Jupiters

Some exoplanets orbit close to their parent stars. Due to their close proximity and generally large size, worlds known as "hot Jupiters" are easier to spot than their smaller, more distant-orbiting cousins.

Habitable Worlds

The primary thrust of exoplanet hunting is to find small, rocky worlds that orbit within their stars' "habitable zones." The habitable zone, also known as the "Goldilocks zone," is the region surrounding a star that is neither too hot nor too cold. At this sweet spot, liquid water may exist on the exoplanet's surface. Where there's water, there's the potential for life.

A Phantom

Usually, exoplanet hunters look for the slight dimming of a star or a star's "wobble" to detect the presence of an exoplanet. However, in the case of Kepler-19c, its presence has been detected by analyzing its gravitational pull on another exoplanet, Kepler-19b. Kepler-19c is therefore the Phantom Menace of the exoplanet world.

Keeping Warm

The habitable zone seems to be the pinnacle of extraterrestrial living. If you're an alien with similar needs to life on Earth, then you'll need liquid water. If your planet exists outside your star's habitable zone, well, you're in trouble. Either your world will be frozen like a block of ice, or boiling like a kettle. But say if your world had the ability to extend your star's habitable zone? There may be some atmospheric factors that might keep water in a comfy liquid state. Even better, if you like deserts, a dry world could even be oddly beneficial.

Crazy Aurorae

Planets with a global magnetic field, like Earth, have some dazzling interactions with the winds emanating from their stars. The high-energy particles bombard the planet's atmosphere after being channeled by the magnetism. A wonderful auroral lightshow ensues. But say if there's an exoplanet, with a magnetosphere, orbiting really close to its star? Well, stand back! The entire world would become engulfed in a dancing show, 100-1000 times brighter than anything we see on Earth.

The "Candidates"

"Candidate" exoplanets are often mentioned, especially when talking about detections by the Kepler space telescope. But what does this mean? As a world passes in front of its star, slightly dimming the starlight, this isn't considered a "confirmed" exoplanet detection. To make sure that signal is real, more orbital passes of the exoplanet need to be logged before a bona fide discovery can be announced. Until then, these preliminary detections are called exoplanet candidates.

Angry Suns, Naked Planets

Exoplanets come in all sizes and all states of chaos. Some might have wonky orbits, others might be getting naked. Other times, they're simply being ripped apart by X-rays blasted from their parent star. Bummer.


Super-Earths get a lot of press. Mainly because "Earth" is mentioned. Sadly, most of these worlds are likely completely different to anything we'd call "Earth." And you can forget calling the vast majority of them "Earth-like." It's simply a size thing -- they're bigger than Earth, yet a lot smaller than Jupiter, hence their name, "super-Earth." Easy.

Let's Go!

For now, we have to make do with artist's renditions of exoplanets for us to visualize how they may look in their alien star systems. However, plans are afoot to send an unmanned probe to an interstellar destination. Although these plans may be several decades off, seeing close-up photographs of these truly alien worlds will be well worth the wait.