Tale of the Shepherd Planets

Until recently, the search for planets beyond our solar system was a matter of calculating the odds and laying out theories of solar system formation. Circumstantial evidence began to trickle in, a color shift in a stars light as a planet tugged on it, or a dipping in its light as a planet passed in front.

When would our technology allow us to see through the bright light of stars to see these alien worlds directly? Scientists using the Hubble Space Telescope began examining a star visible in the southern hemisphere, just 25 light years away.

Called Fomalhaut, it's much hotter than our sun and 15 times as bright. In fact, it's one of the brightest stars in our night sky. What makes it so curious is the large ring of gas that surrounds it. The ring is slightly off center from the location of the sun. That suggests there's a gravitational presence, a planet, that's distorting its shape. With a coronagraph in place to block the star's light, Hubble zeroed in on the ring. Right there in the data, it turns out, was a bump, perhaps a planet.

Hubble photographed this planet a second time, two years later when it had progressed in its orbit. Based on the change in position, astronomers calculated that it takes about 872 Earth-years to complete an orbit.

Astronomers thought it to be a huge planet, many times larger than Jupiter. The reason they could see is that it may have a very large and reflective ring system. From the ring's narrow width, the planet seems to be sculpting it, by pushing on and sharpening its inner edge.

Hubble had delivered a rare direct image of an extrasolar planet, the first one ever in visible light. Astronomers could not confirm its presence by looking in the infrared portion of the light spectrum. That seemed to be the end of it.

Enter ALMA, the not quite completed Atacama Large Millimeter-Submillimeter Array in the high desert of Chile. It produced this picture of the disk, which showed that its inner and outer edges are both relatively sharp.

Computer simulations refined the parameters of planets. There is not one, but two, on both edges of the ring. They had to be relatively small to avoid destroying the ring: larger than Mars, but only a few times larger than Earth. There's little chance that these frigid worlds harbor life. What they do is extend our vision, and our ideas on what it takes to form a solar system.

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