The Sciences

Three Little Exoplanets, All in A Row

Astronomers have found a solar system that looks surprisingly like ours.

By Veronique GreenwoodJul 26, 2012 5:00 AM
Graphic by Cristina Sanchis Ojeda


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In a universe that exists in at least three dimensions and perhaps many more, our solar system remains an oddly 2-D place. At the center sits the Sun, with the eight planets spinning around it in a tidy plane, parallel to the solar equator. This is a function of the way the planets swirled into existence from the same cloud of dust and gas that gave rise to the sun itself—and is one of the things that got poor Pluto booted from the planet club altogether back in 2006. The ex-ninth planet travels in a steeply inclined orbit, rising above and diving below the solar plane—a clear indication that it's merely an escapee from the vast belt of comet-like objects that circle the solar system.

When astronomers began discovering exoplanets—worlds orbiting other stars—they expected those solar systems to follow the local model. But that's not how things turned out. Planet after planet that was spotted in the earlier days of the search, when all we could detect were very large planets circling very bright stars, were not lined up the way they were supposed to be. Instead, these Jupiter-like gas giants hang at a cock-eyed angle, and sometimes rotate backwards or orbit in the opposite direction of the star's own spin.

Finally, astronomers studying a star known as Kepler-30 have found something that looks reassuringly familiar: one, two, three exoplanets, orbiting in a plane, just like we do. And as reported in the current issue of Nature, it's not only the discovery itself that's cool, it's the way the researchers went about making it.

Exoplanets are typically discovered in one of two ways—and neither involving simply pointing a telescope in the right direction and looking. At solar distances, the planets are just too tiny and often too washed-out by the light of their suns to be spotted visually. Instead, astronomers look for wobbles in the star itself, indicating that something nearby is tugging on it gravitationally. More recently—especially since the 2009 launch of the Kepler Space Telescope—they have relied on the slight dimming in luminosity that occurs as a planet passes in front of its star, blocking a bit of its light. Even that wasn't terribly easy with Kepler-30, a relatively faint star 10,000 light-years distant. Says Joshua Winn, a co-author of the Nature paper and a professor of astrophysics at MIT: "We don't have a [good] picture of this star."

But even so weak a signal could yield some clues. The dimming that takes place when a planet moves in front of a star is not quite so dramatic when it passes over a portion of the surface marred by a sunspot. These magnetic storms give rise to areas of splotchiness that shed a little less light to begin with, meaning there's a little less to block. But sunspots rotate around the star and a planet has to be lined up with it precisely to cross over it at all. Still, if those tumblers fall into place, they can can produce a telltale flicker pattern that, over long stretches of time, can reveal details about a planet's inclination.

Roberto Sanchis-Ojeda, a graduate student in astrophysics at MIT and the lead author of the paper, thus went looking for sunspots on Kepler-30 that were situated parallel to the equator, and when he found one, he waited until the spot was turned toward Earth and closely scrutinized how the star's light output changed. As he hoped, he detected both the dimming of a planet passing in front of the star and the slight reduction of that dimming as it sailed over the sunspot. And then, shortly after, he saw it again. And then again. The unmistakable conclusion: three planets, tidily aligned.

The elegance of both Sanchis-Ojeda's methods and his results leave little room to challenge his findings. But why there are so many systems out there that are misaligned and whether a neat system like ours and Kepler-30 is the exception or the rule is very much an open question. The team plans to use the same technique on other planetary systems in the hope of teasing out some answers, but Winn and another of the paper's co-authors, astrophysicist Daniel Fabrycky of the University of California, Santa Cruz, already have a theory. They believe that all planets start off aligned and then some of them are knocked crooked by the gravitational pull of other planets or nearby stars.

Maybe, but maybe not. "They have not really proven that," says Douglas Lin, an astrophysicist at UC Santa Cruz, who was not involved in the work. "It remains a possibility." Other theories involve planetary collisions that knock worlds off course or simply a large mass of materiallike an asteroid that jumps aboard and, by its mass and impact, tilts the once-flat orbit.

With news from the exoplanet search coming thick and fast, a lot of energy is currently devoted simply to collecting and storing new worlds, readying them for long-term analysis that will likely play out over the course of decades. Finding too many planets to study at once is a whole lot better than finding too few, of course. But it does mean we'll all have to develop a tolerance for a great many new theories—without, for now, a great many proofs.

This story also appears at

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