In March of 2004, we offered a hearty "welcome" to Sedna, then held up as a candidate for the solar system's 10th planet. Although ultimately Sedna was relegated to non-planet status, over the last year we've learned a bit more about that most distant body -- it's not a Kuiper Belt Object, for example, but most likely a member of the Inner Oort Cloud, and the moon it was thought to have (because of its slow rotation) seems mysteriously missing.
We've continued to keep a watch for objects in orbit around the Sun that could possibly be called a "planet" -- and it turns out that the most recent candidate has been hiding in plain sight. 2003 EL61 (it has yet to be given a formal name), although officially announced yesterday, was spotted initially in 2003, and images of it have been found in archives as far back as 1955. A newly-identified member of the solar system may not be immediately worldchanging, but provides a useful lesson in how science works and how we've come to understand our corner of the universe.
2003 EL61 is currently identified as a Kuiper Belt Object (KBO), meaning that it's part of the population of icy, rocky bodies at the outer edge of our planetary system. KBOs come in a variety of sizes, including bodies that rival planets in size. Pluto, in fact, is more properly referred to as a KBO, not a planet; we continue to refer to it as a planet simply out of cultural inertia.
2003 EL61 was first spotted by Jose-Luis Ortiz at the Sierra Nevada Observatory in Spain in March of 2003; the team's website includes some of the initial details, as well as the first photo used to identify the body. If you read down the page, you'll note that they suggest the possibility that 2003 EL61 is bigger than Pluto, because of its reflectivity. It turns out that it's not, and how we know it's not is an interesting story.
Jose-Luis Ortiz and his group were not the only ones to have stumbled across this object. Mike Brown at Cal Tech also found it, in mid-2004 (although he didn't identify it until December, 2004). It turns out that, in astronomy, naming and primacy of discovery accrues to the team that announces first. If Brown and his group had called a press conference for last week, they'd be given credit for the find, even though Ortiz, et al, had spotted it first. The reason that both groups waited as long as they did before telling anyone was that they needed to get confirmation sightings, and wanted to get better information about the object itself. At 2003 EL61's distance, getting sufficient motion to be able to plot an orbit takes some time. In Brown's case, his group had requested imaging of the body by the Spitzer Space Telescope, and the time required to get the pictures was enough to let Ortiz and company stake their claim.
2003 EL61 is fairly bright -- enough so that, if it were in the asteroid belt between Mars and Jupiter, it would be visible to the naked eye on dark nights. Because of this, Ortiz initially said it could be up to twice the size of Pluto, but the images that Brown's team took spotted a detail that the somewhat less powerful telescope that Ortiz used missed: 2003 EL61 has a moon. Plotting the motion of the KBO and its moon, they could calculate its mass: 32% of Pluto, meaning a diameter of about 70% that of Pluto. 2003 EL61 is about the same size as Sedna.
The most intriguing thing about 2003 EL61 is its orbit: like Pluto, it has an erratic, off-kilter elliptical orbit taking it close to the orbit of the 8th planet, Neptune; in fact, there are points to 2003 EL61's orbit that lie closer to the Sun than the more distant parts of Pluto's orbit. Going by size and orbit, it's not outrageous to suggest that 2003 EL61 is another planet. NASA has made a handy Java-based orbit plotting program for 2003 EL61, allowing you to see where it is on any given day and year. 2003 EL61 is currently out towards its most distant point, but it does come closer in. In fact, back in the early 20th century, its orbit took it close to both Neptune and Pluto; it's sheer luck that Pluto was spotted and identified as the 9th planet instead of 2003 EL61.
I said earlier that Sedna, initially identified as a KBO, isn't. It turns out that the Sedna's orbit never gets into the Kuiper Belt. Sedna is more properly identified as an Oort Cloud Object (OCO), a resident of the much more distant ring of material from which many comets spawn. Mike Brown has a great illustration of where Sedna fits with the rest of the solar system, excerpted below:
So why are 2003 EL61 and Sedna not considered planets, even though they're nearly as large as Pluto -- and, in the case of 2003 EL61, on a similar orbital path? The current scientific argument is based on "populations." If an object that's big enough to form a sphere (instead of a jagged rock) is in an orbit that comes nowhere close to any other similar objects -- as Earth is, for example, or Mars or Saturn or any of the named planets other than Pluto -- it can be called a planet. If the object is one of many similar objects, of varying sizes, in roughly similar orbits, it's more properly identified as an object of that population -- a KBO, for example. By that definition, Pluto -- along with 2003 EL61 -- is not a planet, but a KBO. Sedna, which is too far out to be part of the Kuiper Belt, is so far alone in its orbit; by the population definition, it could be called a planet... except we're almost certain to discover other inner Oort Cloud objects, and we'd have the Pluto Problem (having to redefine a planet as a non-planet, upsetting schoolchildren everywhere) all over again.
What makes all of this interesting from the perspective of what the future might hold is that it looks like our solar system is surrounded by multiple layers of different populations of objects, some approaching planet sizes. Any effort to explore outside of our solar system -- to sample the interstellar medium, for example, or even to try to reach the nearest non-Sun star, Proxima Centauri -- might be able to draw raw materials (for fuel, even for shipbuilding materials) from distant way stations.
Perhaps, in centuries hence, we won't refer to Pluto, 2003 EL61 and Sedna as Kuiper Belt Objects and Oort Cloud Objects, respectively, but instead as Rest Stop Objects.
To me, the most worldchanging thing about this article is the four-piece picture scaling the oort cloud.
How small we and our worries are, and how [insert adjective representing very very hugely enormous] the solar system is, not to mention galaxy, super galactic cluster, and universe.
And the fact that this isn't just an idea, but the reality spinning and tumbling and crashing all around us, it's pretty cool...
I need to be constantly reminded of this, becuase my reality and world-view seems to shrink (making i less full of awe, and less -real-)I'm not careful
This expanded worldview is really worldchanging!
To me, it seems sensible to keep things simple regarding planets and non-planets. These are not small objects by human standards, even if they are on the cosmic scale. I've always been uneasy with saying "it's not a planet if it's part of a large population". Where do you draw the line of what is large?
My ideal definition of a planet is anything orbiting a star, regardless of origin, shape or composition, which is at least 1000km across at its widest point and not undergoing fusion. Anything less than 1km wide is a meteoroid (taking in micrometeoroids and other space junk) and anything in between is a planetoid (taking in comets, centaurs, asteroids and small KBOs). The reason I like this fairly arbitrary definition is that the numbers are round, as are the planets, and we get to keep Pluto, Sedna, Quaoar, EL61 and UB213 and we don't have to include Ceres.
Which makes me a happy boy.
All the best.