NASA’s New Horizons team says the debate is over: Pluto is the largest of the worlds that patrol the fringe of the solar system.
Just a day away from the mission’s closest approach past the dwarf planet, the team on Monday reported refined estimates for its size: 2370 kilometers in diameter, plus or minus 20 kilometers. That makes Pluto larger than Eris, another distant world with a diameter of 2326 kilometers across, plus or minus 12 kilometers. Estimates for Pluto’s size have generally grown over the past decade, whereas Eris’s have remained similar. So Pluto can now claim to be king of the Kuiper belt, the region of thousands of icy worlds that orbit the sun beyond Neptune.
The new measurement “settles the debate about the largest object in the Kuiper belt,” says Alan Stern, the principal investigator for the mission, who presented the result at a press conference at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, which is managing the New Horizons mission. Coming in at the higher end of estimates that have ranged between 2300 and 2400 kilometers, Pluto’s bigger size means that estimates for density go down slightly. That in turn means that Pluto’s proportion of less dense ice to rock is a bit higher.
The size measurement, made by fitting spacecraft images to circular disks, still has some uncertainty, due to pixel fuzziness. Stern says the precision in the measurement will ultimately be narrowed to just a few kilometers. Typically, astronomers measure the size of Kuiper belt objects (KBOs) by waiting for stars to pass behind the objects during occultation events; by measuring the timing of these tiny eclipses, they can calculate sizes—but errors accrue when, as in the case of Pluto, the objects have atmospheres that refract the light of the background star.
Though it may be smaller in size, Eris remains about 30% more massive than Pluto, says Mike Brown, a planetary astronomer at the California Institute of Technology in Pasadena who led the team that discovered Eris in 2005. Brown has been called a “Pluto killer” because the discovery of Eris played a part in Pluto’s demotion to dwarf planet in 2006 by the International Astronomical Union. “Most scientists care much more about mass than size,” Brown notes slyly. “I’m glad that Pluto gets a little bit of a claim to fame by being larger.”
Brown says the refinements in estimates for Pluto’s density shouldn’t matter too much. At about 2 grams per cubic centimeter, it is still quite high compared with smaller KBOs. He says the relatively high density fits with the idea that a collision led to the formation of Pluto and its largest moon Charon; the collision probably stripped away some of the less dense ice, leaving behind a body that is more heavily composed of rock.
The New Horizons spacecraft is closing the remaining gap between it and Pluto. Just before 8 a.m. Eastern Standard Time on 14 July, the spacecraft will pass make its closest approach. It will pass within 12,500 kilometers of the surface and take pictures with resolutions better than 100 meters per pixel. Stern says the data are already “mouthwatering,” and that the energy level of the mission team is “electric."
*See Science’s full coverage of Pluto, including regular updates on the New Horizons flyby.