Dust gets everywhere, particularly around young stars. But when it comes to the presence of dust in their interplanetary environments, many mature sun-like stars appear to be very house-proud, providing astronomers with better conditions to seek out and directly image extrasolar planets, or exoplanets.
"Dust is a double-edged sword when it comes to imaging distant planets," said Bertrand Mennesson of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The presence of dust is a signpost for planets, but too much dust can block our view."
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Using the Keck Interferometer (a former NASA project), which combined the power of the W.M. Keck Observatory's twin 10-meter optical/infrared telescopes at the summit of Mauna Kea, Hawai'i, from 2008 to 2011, 50 sun-like stars were studied and around half were found to possess low levels of warm dust than the rest. The results of this survey, and the implications for exoplanetary studies, will be published in online edition of the Astrophysical Journal on Dec. 8.
To date, many exoplanets have been directly imaged by ground- and space-based observatories, but these exoplanets occupy cool orbits far from their host stars. Using a sophisticated combination of advanced instrumentation and maturing imaging techniques, the glare of the target star (that would otherwise obscure any orbiting exoplanet) can be blocked and these (large gas giant) worlds can be resolved.
However, to image smaller, rocky worlds within a given star's habitable zone, there needs to be another technological breakthrough so planets within these compact orbits can be seen. Once this is achieved, astronomers stand a chance of directly imaging small, ‘habitable' exoplanets, potentially fishing out ‘exo-Earths' from their host stars' glare.
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"If you don't turn off the star, you are blinded and can't see dust or planets," said co-author Rafael Millan-Gabet of NASA's Exoplanet Science Institute at the California Institute of Technology in Pasadena.
The habitable zone surrounding a star is the region at which liquid water could persist (if it is present) on the surface of a small rocky world - the temperature of this world would be ‘not too hot' and ‘not too cold' for water to be in a liquid state, which is why the habitable zone is often given the moniker "Goldilocks Zone." Earth is located smack-bang in the middle of our star's Goldilocks Zone and Earth, as we are acutely aware, is habitable and the only world in the Universe known (so far) to possess life. Liquid water is therefore considered critical for the evolution of life as we know it.
But in an effort to better understand the habitable zones surrounding other stars - where astronomers could, some day, directly image a bona fide exo-Earth - the warm dust surrounding sun-like stars needs to be better understood. While its detection may signal the building blocks for rocky worlds are present, dust also blocks reflected light from these exoplanets, only increasing the difficulty for direct imaging.
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Using Keck Interferometer data, the researchers found that mature sun-like stars that are known to have cold belts of dust in their outermost regions also possess warm belts of dust within their habitable zones. Conversely, sun-like stars without these cool bands of dust do not possess warm habitable zone belts of dust. This is the first time the relationship between cool and warm dust has been realized.
Now exoplanet hunters have a tool for identifying mature sun-like stars that have the most potential for astronomical detection of small habitable worlds that can be directly imaged by telescopes - stars without cool dust will likely have less warm dust and will therefore be the better direct imaging candidates.
"We want to avoid planets that are buried in dust," said Mennesson. "The dust glows in the infrared and reflects starlight in the visible, both of which can outshine the planet's light."
This new-found link also provides an answer as to why mature star systems possess warm dust belts at all. Dusty interplanetary environments surrounding young stars are common - planet-forming dust is often a chaotic mess and planet, comet and asteroid collisions are frequent, kicking up more dust. But in mature star systems, like our solar system, this warm dust should have settled to form planets in stable orbits. Therefore, the researchers have found a dynamic link between the outer (cool) and inner (warm) dust belts around mature stars.
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"The outer belt is somehow feeding material into the inner, warm belt," said Geoff Bryden of JPL and co-author of the study. "This transport of material could be accomplished as dust smoothly flows inward, or there could be larger comets thrown directly into the inner system."
Although the first direct image of an exo-Earth is likely a long way off, studies like this provide valuable clues as to the environment surrounding sun-like stars. Next, surveys by the sensitive NASA Large Binocular Telescope Interferometer on Mount Graham, Ariz., will focus on this warm stellar dust, further refining its origin and impact on future exoplanetary studies.