A day after the news about 2003 EL61, a Kuiper Belt object originally thought to be larger than Pluto, we now have another world that appears significantly larger still. 2003 UB313 was discovered with the Samuel Oschin Telescope at the Palomar Observatory by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University). Evidently the lower limit of its size is Pluto, and it may be (and probably is) larger.
Image: Three views of the new planet. Credit: Mike Brown, California Institute of Technology.
Now some 97 AU from the Sun, the planet is the farthest-known object in the Solar System. A news release from Caltech quotes Brown on 2003 UB313 and its credentials as a planet:
“It’s definitely bigger than Pluto,” says Brown, who is professor of planetary astronomy. Scientists can infer the size of a solar-system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the planet is not yet known–in other words, it’s not yet possible to tell how much light from the sun is reflected away–but the amount of light the planet reflects puts a lower limit on its size.
“Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto,” says Brown. “I’d say it’s probably one and a half times the size of Pluto, but we’re not sure yet of the final size.
“But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system.”
But there is also an upper size limit, one determined by the fact that the Spitzer Space Telescope cannot detect this world. That pegs its diameter at something less than 3000 kilometers (Pluto’s is 2300 kilometers). And yes, we’ll have a better name than 2003 UB313 for this object soon once the name the astronomers have proposed to the International Astronomical Union has been accepted. More on the new planet can be found here; this page is likely to be updated significantly over the weekend.
Centauri Dreams‘ note: If Clyde Tombaugh had known to look 44 degrees off the ecliptic, he might have found 2003 UB313 instead of Pluto. No one would have expected a planet in an orbit with such a steep inclination, raising all kinds of questions about what forces drove the world to its present position. The elliptical orbit takes 560 years to complete and brings 2003 UB313 as close as 3.3 billion miles from the Sun (inside the orbit of Pluto). At 97 AU, it is currently nine billion miles out.
Meanwhile, this further information about 2003 EL61 — remember, this is a different object — also from Caltech in Brown’s Web pages. The evidence strongly suggests 2003 EL61 is actually a good deal smaller than Pluto:
“Many times when objects like this are discovered we don’t actually know how big we are, just how bright they are. They could be bright because they are large or they could be bright because they are highly reflective, like a ball of snow. In the case of 2003 EL61, however, we have gotten lucky, because we have discovered a moon orbiting it. By following the orbit of the moon over the course of 6 months we are able to precisely determine the mass of 2003 EL61 and its moon. The mass is about 32% that of Pluto, implying that it has a diameter of perhaps 70% that of Pluto or around 1500 km. We don’t know the diameter for sure, though, just the mass. It could be made of high density material like rock and be smaller or it could be made of low density material like ice and be larger and still be the same mass. If the size is indeed 1500 km 2003 EL61 is larger than any other known object in the Kuiper belt other than Pluto, with the closest runners-up being Quaoar and Pluto’s moon Charon at about 1250 km. It is also possibly larger than Sedna, a similar object well outside the Kuiper belt.”
Brown, Trujillo and Rabinowitz had been studying 2003 EL61 at the same time the Sierra Nevada team was working on it, and were delaying announcement of the object until further observations could yield more information about its size. The Spanish team thus gets credit for the discovery, while the Palomar team supplies a slew of further detail. Their observations at the Keck Observatory, for example, have shown that 2003 EL61’s moon appears to compose only about one percent of the mass of the system, making it the smallest satellite relative to its primary thus far found in the Kuiper Belt (Charon is roughly ten percent of Pluto’s mass).