Earth-like exoplanets could announce their presence through trailing clumps of dust – and new observations of the Earth's own dust cloud could provide a way to find them. Over the course of five years, the Spitzer Space Telescope drifted through a diffuse but extensive ring of dust particles that orbit the sun in lockstep with the Earth, showing astronomers for the first time what the dusty signature of an exo-Earth might look like.

"For the first time we can measure the structure of that cloud along the Earth's orbit, using this moving space probe that travels through the cloud," said astronomer William T. Reach of the Universities Space Research Association, the author of a paper to appear in the journal Icarus. "We can use that as a key, as a template, to understand the dust around other stars."

The observations showed that a ring of dust from comet tails and broken asteroids follows the Earth in its orbit, something astronomers had already suspected. The dust particles are about 0.02 millimeters in diameter or larger. An extra-thick cloud of these particles about 7 million miles wide trails behind the Earth at about 80 times the distance from the Earth to the moon. Spitzer, which follows the Earth in orbit around the sun, sent images from directly inside this cloud from its launch in 2003 until its coolant ran out in 2009.

Astronomers' first whiff of this trailing dust clump came in 1984, when the IRAS spacecraft showed that the sky is brighter in infrared wavelengths when looking backward along the Earth's orbit than when looking forward. Because dust glows in the infrared, the lightened sky was a clear sign that more dust follows the planet than leads it.

"We couldn’t figure out for the life of us what the hell was going on," said astronomer Mark Sykes, now the director of the Planetary Science Institute in Arizona, who worked on the IRAS project. No good explanations emerged until the early '90s, when astronomer Sumita Jayaraman, also now at the Planetary Science Institute, realized that individual dust particles could get temporarily trapped in a special gravitational relationship called a resonant orbit with Earth.

Most of the dust in the plane of the solar system, called the zodiacal cloud, will eventually spiral into the sun. But particles of the right size, tens of micrometers across, can feel a little gravitational push as they float by the Earth. That push counteracts the sun's pull just enough to hold the dust particles in a loose halo around the sun. The subtle interactions of the Earth and the dust grains' movements lead to the backward-facing clump.

Mathematical models of the dust ring gave astronomers an idea of the clump's extent, but the Spitzer observations were the first chance to test them.

"This work is great because it provides us a novel way of probing the structure of this cloud, which could then feed back into these detailed dynamical models of the dust," Sykes said.

The observations can feed models of what dust rings associated with extrasolar planets might look like. Of the few extrasolar planets to have their pictures taken by direct imaging, at least two hinted at their presence by warping the disk of dust and gas around their star. Earth-like planets that are too small or dim to find through usual methods may have a subtle but detectable influence on their dust disks.

"It's a way that we can recognize planets around other stars that we can't necessarily see," said NASA exoplanet scientist Marc Kuchner. "This result make it much easier to compare solar system dust clouds with ones we see in the disks."

But the dust can be misleading too, Kuchner warns. "They can be bad news if you're trying to directly image a planet, because they can masquerade as planets themselves," he said. "It's both the signal and the noise."

Image: 1) NASA. The S-shaped blue band in this infrared image from the COBE satellite is the zodiacal cloud of dust in the solar system. 2) William T. Reach. A simulation of the Earth's dust ring, with the Spitzer spacecraft's path traced in red.

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