On November 5, several teams of astronomers in the Chilean Andes observed Eris pass in front of a 17th magnitude star in the constellation Cetus. Many additional astronomers observed the efforts of one of these teams remotely, via the Internet. The best observation was made by Jose Luis Ortiz at the Institute of Astrophysics of Andalusia, Spain using a 16-inch telescope. Their occultation video can be found here: http://www.youtube.com/watch?v=HxQ0b8p4c
By viewing Eris pass over this star, astronomers can determine Eris’ diameter. What they discovered is that Eris has a diameter of 2340 kilometers or 1454 miles or less, making it a bit smaller than Pluto, which has a diameter of 2344 kilometers or 1456.5 miles, plus or minus 10 kilometers.
Because Eris’ discovery re-opened the already ongoing debate over the definition of planet, Eris is often portrayed as Pluto’s rival, the cause of its demotion. This viewpoint misses the mark completely because the two planets were never in competition. On the contrary, Eris’ existence confirms that Pluto is not a loner, that there is an entire class of small planets beyond Neptune, which share features with one another as well as with Neptune’s largest moon, Triton.
Small objects at such distances are difficult to measure under the best of circumstances. Eris is three times further from the Sun than Pluto. Previous Hubble images showed it to have a slightly larger diameter than Pluto, but even the best such measurements have a degree of uncertainty.
According to Kelly Beatty in Sky and Telescope magazine, Eris may have previously appeared larger than it really is because its axis is pointing toward the Sun, warming up the hemisphere facing the Sun and thereby leading to inflated infrared measurements. The article can be found here: http://www.skyandtelescope.com/news/1068
Eris’ mass is currently believed to be 2.5 grams per cubic centimeter, making it more massive than Pluto even if it is smaller. Pluto’s mass is estimated at 1.8 to 2.1 grams per cubic centimeter. Higher density likely means it is composed of more rock and less ice—another argument against classifying either Eris or Pluto as comets or “dirty snowballs,” given that the majority of their composition is rock. And Eris has a reflectivity of about 90 percent, significantly higher than that of Pluto, meaning it is highly reflective of sunlight falling upon it. This too could make the planet appear bigger than it really is.
The diameter numbers for Pluto and Eris are so close as to be almost identical. No one should be surprised if the “contest” over which is bigger goes back and forth many times as new data becomes available.
The fact that Eris and Pluto are so similar works against the mindset that demoted Pluto in the first place. The argument was we have four terrestrials, four gas giants, and one misfit, Pluto. Eris’ discovery illustrates that Pluto is not and never was, a misfit. It was the second (as Ceres was the first) of a third class of planets to be found in our solar system, small objects large enough to be planets because unlike asteroids, they are large enough for their own gravity to pull them into a round shape. Dwarf planets are simply small planets not large enough to gravitationally dominate their orbits. There is no rational explanation to label them as not being planets at all other than the artificial, convenience-based argument that our solar system cannot have too many planets, as children will never be able to memorize them.
Pluto and Eris are not rivals; they are two of a kind. Makemake and Haumea are likely similar in composition, mass, and density. Interestingly, all these objects also bear striking similarities to Neptune’s largest moon, Triton. Triton orbits Neptune backwards, meaning it revolves in the opposite direction as Neptune does around the Sun. It is in an unstable orbit, meaning it likely was once a planet with its own orbit around the Sun and was subsequently captured by Neptune.
With a diameter of 2700 kilometers, Triton has a composition strikingly similar to that of Pluto. Both have surfaces covered with frost of carbon dioxide, carbon monoxide, nitrogen, and methane, and both have a pinkish-red color. These similarities are enough to raise questions about whether Triton, Pluto, and Eris, as well as possibly Makemake and Haumea, have similar origins.
Anyone interested in more detail about Pluto’s troposphere (lower atmosphere) can find it at http://www.astro.ex.ac.uk/exoclimes2010/p
Even among the four terrestrials and four gas giants in our solar system, no two planets are identical in size, composition, geology, number of moons, etc. The similarities and differences that we pick and choose to categorize these objects are largely subjective. Saturn has the lowest density of all the planets; it would float if we could find a large enough ocean in which to place it. Jupiter and Saturn have more in common with the Sun than with the Earth when it comes to composition, as the former are composed mostly of hydrogen and helium, and neither has a solid surface.
Astronomers determined Jupiter and Saturn are not stars, however, because they never conducted hydrogen fusion and therefore never produced their own light—neither is anywhere near massive enough to do this. Yet at the same time, they chose to put Jupiter and Earth in the same overall category of planet even though the two bodies have little in common. Earth actually has more in common with Pluto than with Jupiter. Both Earth and Pluto are rocky with solid surfaces; both have nitrogen in their atmospheres, and both have large moons believed to have formed via giant impacts with the original planet.
The next time you hear people use the argument that Pluto is “not like the other planets,” the old “one of these things is not like the others,” remember that inherent in this statement is a subjective choice of characteristics used to determine similarity and difference. A different choice of characteristics will yield a different categorization system. If we use hydrostatic equilibrium to classify an object as a planet, spherical moons of planets like Triton are essentially planets themselves and deserve their own category of secondary or satellite planets.
The best way to learn the specifics about these bodies is to go there. An Eris Express would take about 30 years, but if we can find a way to speed up that travel time and to fund such a mission, the returns would be priceless. The same is true for further exploration of Uranus and Neptune and their satellites, possibly through orbiters like Galileo did for Jupiter and Cassini is doing for Saturn.
Until we actually see these places up close, all conclusions should be understood to have a tentative quality about them. People want quick, easy answers, but that does not work for small objects at such distances. For the foreseeable future, we just have to learn to accept that there is no “final answer.”