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Earlier this morning, a spacecraft arrived at a dwarf planet that sits in a big asteroid belt between Mars and Jupiter. After a seven-year journey, NASA's Dawn spacecraft is now in its orbit and has begun sending back signals. As NPR's Geoff Brumfiel reports, researchers have known about this tiny world for a long time, but they haven't always known what to make of it.
GEOFF BRUMFIEL, BYLINE: It's called Ceres. It's round like a planet, but really small. Its total surface area would cover just a third of the United States. Carol Raymond is with the Jet Propulsion Laboratory in California. She says astronomers have known about Ceres for hundreds of years.
CAROL RAYMOND: Ceres was discovered in 1801 by Giuseppe Piazzi in Sicily. And at the time, many astronomers were looking for a missing planet between Mars and Jupiter.
BRUMFIEL: For a while at least, Ceres was that planet. But then, astronomers noticed chunks of ice and rock orbiting in the same region. Ceres, it turns out, was actually in the middle of what we now call the asteroid belt.
RAYMOND: And so then Ceres was referred to as an asteroid.
BRUMFIEL: It stayed that way until 2006, when the International Astronomical Union redesignated Ceres - and more controversially Pluto - as dwarf planets. Ceres got the title because it's bigger and rounder than any of the other asteroids.
RAYMOND: You know, in the end, it doesn't really matter whether it's called a dwarf planet, or a planet or an asteroid. We want to understand it.
BRUMFIEL: That's because astronomers think Ceres is a remnant from the earliest days of the solar system. In fact, Ceres may have well been on its way to becoming a real planet until Jupiter got in the way. Ceres was gradually becoming larger as its gravity pulled in dust and rock. But Jupiter's gravity was so much bigger, it stopped Ceres from growing past its present size.
RAYMOND: It came in and just started to throw its weight around, and things started scattering.
BRUMFIEL: Raymond is deputy principal investigator on NASA's Dawn mission, which has spent over seven years getting to Ceres. Part of the trip was spent visiting another body in the asteroid belt called Vesta, which is rocky and oblong. Ceres looks very different. It probably has a rocky core and an ice shell. That shell could have even been a liquid ocean early on in its existence.
RAYMOND: This makes Ceres a really interesting object, a potential habitable environment in the past.
BRUMFIEL: Raymond says the long wait to get to Ceres means there will be some celebration today.
RAYMOND: I think, you know, champagne might be nice.
BRUMFIEL: But not too much because there's a lot of work to be done in order to learn how Ceres fits into our solar system. Geoff Brumfiel, NPR News.
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