One lesson astronomers have learned from the avalanche of new-planet discoveries over the past decade and a half is that all solar systems are by no means created equal. One has five Neptune-size planets jammed right up against their star; others feature single, superhot, Jupiter-size worlds where an entire year lasts just a day or two. Very few places — so far, at least — resemble our old, familiar solar system much at all.
But that may be changing. Astronomers are reporting at this week’s American Astronomical Society meeting in Long Beach, Calif., that they’ve spotted what appears to be evidence of an ongoing demolition derby of material around the nearby bright star Vega. And that points, if only indirectly, to a family of planets that could resemble our own.
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The actual evidence, says astronomer Kate Su, of the University of Arizona, who led the research, is a belt of dust whose infrared glow was picked up by NASA’s Spitzer Space Telescope and also by the Herschel Space Observatory, run by the European Space Agency. Dust by itself wouldn’t prove much of anything; after all, our solar system was made of nothing but dust and gas at first. If alien astronomers had taken a picture at the time, they wouldn’t have had any reason to assume there were any planets lurking there. But most of our primordial dust was gone by the time the sun was 100 million years old or so; it had congealed into planets, comets and asteroids. Vega is already a half-billion years old. “It [the dust] has to have been generated recently,” Su says. “It has to be constantly replenished.”
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The best explanation: the dust is being produced as a result of crashes among larger bodies, most likely asteroids. Indeed, our solar system has a similar haze of dust pervading the asteroid belt, generated precisely the same way. The only major difference is that Vega’s belt is about four times farther from the star than ours.
That’s the first step in Su’s chain of reasoning. The second is that astronomers already know from earlier observations that Vega has a belt of colder dust orbiting much farther out — almost certainly the debris from crashes among icy bodies like those that inhabit the solar system’s Kuiper belt.
But there’s a large, dust-free gap between the inner belt and the outer, a void that almost certainly has to be created by orbiting planets sweeping up asteroids or comets that wander too close. In that region, at least, there wouldn’t be many collisions, so there wouldn’t be much dust.
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The gap is large enough, says Su, that it must be policed by either a single, gigantic planet or a number of smaller but still reasonably large ones. The gigantic one would be relatively easy to spot, but it hasn’t been yet, so Su favors the smaller-planets option. Though we don’t yet have the technology to spot those worlds, the James Webb Space Telescope, currently slated for launch in 2018, should be up to the job.
Su also offers hope that Vega may host small, rocky planets like Earth, Venus, Mercury and Mars as well. She notes that some of the first exoplanets ever found were so-called Hot Jupiters, huge worlds that presumably formed in the outer parts of their solar systems, then spiraled in to hug their stars. “If you have a large planet moving inward,” says Su, “you will destroy the asteroid belt and any inner planets that might be there.” The fact that Vega’s belt appears to be intact, she says, means there’s at least a chance that the star harbors rocky planets as well.
Even more encouraging is that this isn’t the first time these sorts of double dust bands, one hot and one cold, have been seen: they also exist around Fomalhaut, Epsilon Eridani and a third star with the drab name HR 8799. And in the last case, astronomers don’t have to infer the existence of planets in the gap between dust belts: HR 8799 has four large worlds keeping the dust in check right where you’d expect them to be. “We have four,” says Su, “and there are more coming.”
Our solar system may still not be typical in the Milky Way. But it’s not as far from the mainstream as we might have feared.
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