Nine years ago, the International Astronomical Union established new criteria for determining what classifies as a "planet," a controversial move that officially (but not in spirit) stripped Pluto's status as the solar system's ninth planet. Left undecided was what to do about all the so-called planets being discovered outside the solar system, a number that now approaches 5,000.
PHOTOS: New Pluto Pics Show Beautiful, Complex World
On Tuesday, astronomer Jean-Luc Margot, with the University of California Los Angeles, unveiled what he describes as a simple "planet test."
The IAU says a "planet" is defined as a celestial body that is a) in orbit around the sun; b) has sufficient mass of self-gravity to overcome rigid body forces so that is assumes a hydrostatic equilibrium shape (in English, that means it's nearly round); and c) has cleared the neighborhood around its orbit.
It's this last criteria that kicked Pluto out of the planet club and into a new category of "dwarf planets," a classification Pluto currently shares with Ceres, located in the Main Asteroid Belt, and Pluto's Kuiper Belt cousins Eris, Haumea and Makemake.
The ability of a planet to dominate its region of space also is the criteria that Margot focused on in devising a classification system for planets beyond the solar system.
PHOTO: New Horizons Returns Photos of Hazy ‘Arctic' Pluto
His test can "determine whether a body can clear a specific region around its orbit within a specific time scale, such as the lifetime of its host star. The test is easy to implement and allows immediate classification of 99 percent of all known exoplanets," UCLA said in a press release.
The proposed criterion requires only estimates of the star mass, planet mass, and orbital period, all of which can be easily obtained with Earth-based or space-based telescopes, UCLA noted.
When applied to the solar system, Pluto still flunks the planet test.
"The disparity between planets and non-planets is striking," Margot said in a statement. "The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature."
GALLERY: Exquisite Exoplanetary Art
Planetary scientist Alan Stern, who heads the New Horizons science team that is studying Pluto, says that's a very narrow way of looking at things.
"I think that there's pretty clear appreciation among planetary scientists that Pluto is much more similar to the planets of our solar system than anything else," Stern said. "Astronomers don't seem to understand that attributes matter."
Back-lit, blue-skied Pluto is still a planet in the heart, though not in official registers.
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
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.