Solar System Suburbs
Location is everything
Mike Brown found a world that wasn’t supposed to exist. It was an icy planetoid, nearly the size of Pluto, and it was in the wrong place, in what had been presumed to be a relatively empty region of space with just some gas and dust—certainly no objects perhaps a thousand miles across.
It’s not that the planetoid itself was a big surprise. Over a decade ago astronomers predicted that ice worlds smaller than planets would be found in a region named the Kuiper belt, roughly between the orbit of Neptune and a region of space a pretty good hike beyond Pluto. In 1992 astronomers in Hawaii found the first of what is now a list of nearly a thousand icy bodies in that region. Some are pretty chunky— hundreds of miles in diameter.
Brown, an astronomer at the California Institute of Téchnology, has found many of the largest planetoids himself. But the one he found in November 2003 —called Sedna, in honor of an Inuit sea goddess of the Arctic —not only is bigger than any known Kuiper belt object other than Pluto but also is many billions of miles farther from the sun than the Kuiper belt bodies. Sedna at its closest approach to the sun is still two and a half times as far away as Pluto. Its orbit will eventually take it more than 12 times that far away, into a dim exurb of the solar system where the sun looks more like an ordinary star.
Brown wondered if he had made the first discovery of a body in the Oort cloud, a hypothesized reservoir of comets forming a kind of shell around the solar system trillions of miles from the sun. Dutch astronomer Jan Oort theorized in the 1950s that certain comets come from this distant pool. But Sedna is much closer to the sun than those hypothetical comets.
Astronomers have been making new models of our solar system since Earth was thought to reside at its center. Brown thinks it’s time for another. Perhaps the Kuiper belt and Oort cloud overlap. Icy objects may exist from here to there and everywhere in between. “Sedna just blew us away. We had no idea that this intermediate population was out there,” Brown says.
He suspects that there are larger worlds in our solar system. One, by his calculation, might be close to the size of Mars, around 4,000 miles in diameter, which would definitely bump it up to planet status. He estimates that another 60 bodies the size of Sedna will eventually come into view.
No one will be eager to relocate to an ice ball way beyond Pluto, but this kind of research helps us understand how planetary systems form and evolve. In the meantime, anyone who builds a new model of our solar system should make sure it has room for expansion.
National Geographic Magazine
December 2004. (Astronomy Section)
In Terms of Planets
Some things are the same in every solar system. Planets revolve around stars, and planets shine by reflecting the light of the star they orbit. And all planets are created from an aggregation of planetesimals, rocky or icy bodies born during the formation of a solar system. Beyond that, there’s room for variation. In our solar system there are major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), two of which we call inferior because they orbit closer to the sun than Earth (Mercury and Venus) and most of which are superior (all the rest, starting with Mars). That leaves minor planets (which are the same as asteroids). In an ongoing debate over whether Pluto is a planet or an asteroid, some astronomers, such as Mike Brown of Caltech, choose to use the term planetoid, reserved for big minor planets—like some of Pluto’s cousins out in the Kuiper belt and beyond.
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