Spotting Comet Dust Around Alien Suns

If you wake extra early on these chilly mornings (and you’re north of the equator), you might spot the “false dawn,” a diffuse, white glow above the eastern horizon. Now astronomers have observed a similar light around nine nearby stars.

Perseids and the zodiacal light

Perseid meteors flash by a backdrop of the zodiacal light, as seen from Jiangxi, China. (Click for larger image.)
starwisper, S&T Online Photo Gallery

The night sky is full of hidden gems such as nebulae, clusters and galaxies. But my personal favorite is the zodiacal light. Seen in the night sky after twilight or before dawn (depending on the time of year and your location) this diffuse cone of white light towers above the horizon. Its glow comes from sunlight scattered off the grains of comet and asteroid dust in the plane of our solar system.

Now astronomers have spotted exozodiacal light — the same phenomenon, albeit much brighter — in nine other planetary systems.

“We are slowly reaching the point where a few other extrasolar planetary systems will be well documented … from the giant planets down to the smallest grains,” says Jean-Charles Augereau (University of Grenoble, France). “We’re not yet there, but we’re getting closer and closer, and the presence of exozodiacal dust is part of the story.”

The Sun’s heat vaporizes comet ices, so every time a comet passes through the inner solar system, it contributes billions of dust particles. Asteroids occasionally collide and contribute dust too. All of these grains settle into a vast cloud within the plane of our solar system.

So it’s no surprise that the same phenomenon occurs in other planetary systems. But observing such rarefied light is challenging to say the least.

“The problem is that this dust is always very close to a much brighter star which outshines it,” says lead author Steve Ertel (University of Grenoble, France, and the European Southern Observatory).

Auxiliary Telescopes of Very Large Telescope Interferometer

The domes of the four auxiliary telescopes used as part of the Very Large Telescope Interferometer.
F. Millour / OCA

The team combined near-infrared light beams collected via four 1.8-meter Very Large Telescope Interferometer telescopes, for the first time resolving exozodiacal light near the habitable zone around nine stars. The stars themselves, typically 100 times brighter than the dust around them, remain points of light in these images, making their light easier to subtract.

Because the team observed near-infrared emission, they’re seeing the hottest dust in the system, and it’s not clear whether they’re seeing starlight scattered off the dust, or heat emission from the dust itself. All the same, near-infrared interferometry is the only technique capable of discovering and studying these systems.

Protoplanetary disks are often observed around young stars, dusty cocoons that eventually coalesce into planets, comets, and asteroids. But these disks dissipate over time, the star’s radiation pushing the disk’s grains and gas out of the system. The dust Ertel’s team observed surrounds older stars, long after protoplanetary dust has disappeared, so it must have been created recently.

The Origin of Exozodiacal Light

The question is, where was the dust created? The team thinks the likely source is a massive reservoir of comet-like bodies may be located far from the star. Whenever one ventures inward, it releases dust, though the team is still working on what mechanism can scatter the bodies toward the star at high enough rates.

The most promising mechanism is that a chain of planets scatters comet-like bodies inwards. More exotic ideas, like the slow evaporation of planets the size of Mercury, have also been proposed.

A diagram (not to scale) to illustrate how some particles may be scattered into the inner planetary system.  Bonsor et al. 2014

A diagram (not to scale) to illustrate how some particles may be scattered into the inner planetary system.
Bonsor & others, 2014

But other astronomers remain skeptical that the observations reveal exozodiacal dust in the first place. The origin of the near-infrared glow is still an open question, perhaps coming from some stellar phenomenon, suggests Rafael Millan-Gabet (Caltech). But he adds, “It is not negative criticism — all those things are common in state of the art science.”

If this bright exozodiacal dust does exist, it could pose a challenge to future missions aiming to image Earthlike planets, Ertel says. NASA and ESA will have to take this into consideration.

Reference:

Steve Ertel et al. “A Near-infrared Interferometric Survey of Debris-disc Stars. IV. An Unbiased Sample of 92 Southern Stars Observed in H-band with VLTI/PIONIER.Astronomy and Astrophysics, Accepted


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