While Pluto had always been an oddball, the 2005 discovery of a body even bigger than Pluto in the same general vicinity forced astronomers to rethink what it takes to be a planet.
In 2006 the International Astronomical Union declared that Pluto would be reclassified as a dwarf planet, joined by the then newfound, larger body—dubbed Eris—and the previously known Ceres, which had been considered an asteroid.
Late last month astronomers using telescopes in Chile watched Eris cross in front of an unnamed star. This so-called occultation event caused the star to appear to blink out for a few seconds.
Due to the angle of observation, scientists knew the length of the blink corresponded to the size of Eris—which in this case showed that Eris can be no more than 1,454 miles (2,340 kilometers) across.
That's just a hair smaller than Pluto's estimated 1,456-mile (2,344-kilometer) width.
Still, both dwarf planets' measurements are a bit fuzzy, considering that astronomers have been calculating sizes for the distant objects with either ground-based or Earth-orbiting telescopes. Pluto is 39 times farther from the sun than Earth, and Eris's elongated orbit takes the object more than twice as far away as Pluto.
"The reports that Pluto is bigger than Eris all take the largest possible size of Pluto and the smallest size of Eris," said Mike Brown, the Caltech scientist who discovered Eris, but who wasn't involved in the new research.
According to Brown, "we really don't know" which is bigger.
Looking for a Vanishing Star
Capturing an occultation event isn't easy, because they're often observable from only a few points on the globe, and predicting where those locations will be isn't an exact science.
Finding a tiny object's shadow from such distances is like trying to eyeball a quarter from 62 miles (100 kilometers) away, said the Paris Observatory's Bruno Sicardy, who coordinated the occultation-watchers.
What's more, during an occultation, researchers need at least two separate observances to calculate the target object's width.
"The trajectory [of Eris's occultation] was anywhere between Alaska and Atacama," a South American desert, said Alain Maury of the San Pedro de Atacama Celestial Explorations Observatory, one of the researchers who captured images of the event.
"As [Eris] was getting closer [to the occultation point], Bruno Sicardy called me the day before and said, 'Maybe you can try to observe this.'"
Maury's group and another team at La Silla Observatory, 460 miles (740 kilometers) to the south, were both able to capture the star's disappearance, giving the necessary data to calculate size. (See an Atacama map.)
When NASA's New Horizons spacecraft reaches Pluto in 2015, the probe may turn up new information on Pluto's atmosphere, which would lead to a more accurate estimate of Pluto's width. But to pin down Eris's size, astronomers would need to witness another occultation with more powerful telescopes.
Eris Smaller but Denser Than Pluto?
Figuring out Eris's size would in turn help astronomers figure out the dwarf planet's likely composition.
Previous studies of Eris confirmed that it's about 25 percent more massive than Pluto. That means if Eris is about the same size or slightly smaller than Pluto, Eris must be denser, and so made of more rock and less ice.
This has some researchers baffled, since both Pluto and Eris are members of the Kuiper belt, a region beyond the orbit of Neptune that's filled with hundreds of thousands of icy bodies larger than 62 miles (100 kilometers) and an estimated trillion or more comets.
(Related: "Pluto Gets 14 New Neighbors.")
"To go from something that's more or less half rock and half ice to almost all rock—you can't do that in the standard picture of the solar system," Caltech's Brown said. "Something funny certainly has been going on."
Other scientists aren't so sure: "You might expect a bunch of variety" in Kuiper belt objects, said planetary scientist Alan Stern of the Southwest Research Institute in Boulder, who was also unaffiliated with the new research.
"They're not all clones of each other, formed identically. ... We also see that among the other classes of planets."
For example, though Mercury, Earth, and Mars are all rocky planets, their inner cores vary widely in size. Mercury's core is much bigger relative to the planet's overall size than Earth's, while Earth has a much bigger core than Mars.
Size Aside, Pluto's Still a Dwarf Planet
Caltech's Brown added that, even if Pluto is proven to be the biggest dwarf planet, it's not likely to be reinstated on that evidence alone. (Related: "Pluto a Planet Again—On Friday the 13th, in Illinois.")
After all, size isn't the only thing that matters in the new definition of a planet. Having a neighborhood full of similar bodies, for example, keeps Pluto in dwarf status.
"It's not like two weeks ago we thought Eris was a planet [simply] because it was bigger" than Pluto, Brown said.