On February 28, 2005, a frigid and snowy night befitting a talk on the outer regions of the Sun’s domain, Caltech researcher Mike Brown spoke at the American Museum of Natural History’s Hayden Planetarium on “Sedna and the Birth of the Solar System”. Brown, whose team discovered Sedna, Quaoar, and many other denizens of the outer solar system, started his talk with the discovery of Pluto by Clyde Tombaugh in 1930. Tombaugh’s mentor, Percival Lowell, had photographed Pluto decades earlier in his search for “Planet X”— a hypothetical world that he mistakenly believed was perturbing Neptune’s orbit—but expecting a larger world had failed to recognize Pluto, which appeared only as a dim, starlike point. Tombaugh realized that the only sure way to detect a new solar-system body was by its motion, and discovered Pluto by blinking between images of the same region of sky taken several days apart, looking for anything that moved.
But did Pluto mark the edge of the solar system? And if not, what else lay out there? Gerard P. Kuiper hypothesized in 1950 that instead of additional high-mass planets beyond Neptune and Pluto, numerous smaller planetoids might orbit there instead; this theory was borne out after his death when the belt of objects that now bears his name was discovered in the 1980s. As of the lecture, there were 996 known Kuiper-belt objects.
No one since Tombaugh’s time had attempted to survey the entire sky for distant, slow-moving bodies when Brown and colleagues Chad Trujillo and David Rabinowitz started their search 2-1/2 years ago, using the 48-inch Samuel Oschin telescope at Mt. Palomar (and the same blinking technique—though now largely automated—as Tombaugh had used). Brown told of the discovery of Quaoar—which at the time was the largest solar-system body found since Pluto. He said that if it had been found before the other Kuiper-belt objects, because of its size it would probably have been considered as a planet in its own right—though he believes that only objects that are considerably larger than the population around them should be considered planets. By his definition, neither Quaoar nor Sedna (intermediate in size between Quaoar and Pluto) qualify as planets—nor, for that matter, does Pluto. Though most astronomers expected the Kuiper belt to extend outward indefinitely, no KBOs have been found much farther out than Pluto’s aphelion point. (Pluto, in its orbit, ranges approximately from the inner to the outer edge of the Kuiper belt, and apart from its size seems a typical Kuiper-belt denizen.)
Brown described the finding of Sedna, the first solar-system object discovered outside the Kuiper Belt, which due to its distance was the slowest-moving object that Brown had ever found. He did not mention the controversy over his team’s announcing the name Sedna before the object was officially named by the IAU; on his website he defends their action, saying that people are emotionally invested in their solar system, and they wouldn’t have gotten nearly as excited over “2003 VB12”, its original designation—as unusual as their discovery is by any name. As Sedna is the coldest object yet discovered in the solar system, Brown’s team named it for the Inuit goddess of the sea, a mermaid-like creature who lives in an ice-cave at the bottom of the Arctic Ocean. (After its discovery, they received a case of wine from a California vineyard that made Sedna wine, replete with an image of the goddess on each label. The vineyard was gratified that the naming of the planetoid had suddenly brought a much higher profile to its brand. On the heels of that success, Brown quipped, his team is thinking of naming the next such object “Dom Perignon”.
Sedna moves in a highly elliptical orbit that takes it between 3 and 18 times Pluto’s distance from the Sun. It’s not a Kuiper-belt object, nor is it a member of the Oort Cloud, a spherical cloud of comets that extends much of the way to the nearest star. (The inner edge of the Oort Cloud is believed to be about 10 times as far from the Sun as Sedna’s farthest point.) It takes Sedna 11,500 years to circle the Sun; Brown said that Sedna would only be close enough to the Sun to be discoverable for about 200 years, or about 1/60 of its orbit (it won’t reach perihelion until 2076) and that there are probably 60-100 similar objects out there, some of them larger than Pluto, perhaps one or two nearly as big as Mars.
As to where Sedna formed, and how it got into such an elliptical orbit, and what it and its presumed kin can tell us of the formation of the solar system, the answers are still out. “This whole population of objects is like a fossil record from the earliest part of the solar system,” Brown said, “and once we can read this record, I think we will have an astounding amount of information about what was really happening in the early solar system.”
It is a mystery that awaits the discovery of more Sedna-like objects to be resolved. Among the theories are that Sedna was perturbed by a Mars- or Earth-sized object that formed at 70-80 AU from the Sun (near Sedna’s perihelion point) and presumably is still out there, or it was deflected by another star passing fairly close (90-160 AU) to the Sun. It’s even possible that it orbited the other star and was captured by the Sun—if we were to find similar planetoids with inclinations of greater than 40 degrees, it would support that hypothesis.