Choked in Dust

Spitzer Space Telescope, the fourth and final element in NASA’s family of Great Observatories.
Credit: Ball Aerospace & Technologies Corp., 2003

Our solar system is awash in a haze of dust. Thrown off by collisions in the asteroid belt, or streaming off comets as they are heated by the sun, this dust can sometimes be seen as a faint twilight glow, a "zodiacal light" caused by sunlight bouncing off the miniscule dust grains.

The Spitzer Space Telescope has detected a similar dusty film around the star HD 69830. Spitzer detects infrared light – or heat – so what actually was detected is the heat of the dust as it is warmed by the star. The heat ranges from several thousand degrees Fahrenheit where the dust is closer to the star, to room temperature further out. The average temperature of the dust is about 450 degrees F.

Slightly less luminous than our sun, the star HD 69830 is also a little younger, about 2 to 4 billion years old. The dust layer around this star is extremely thick – a thousand times thicker than our own. Why would a star so similar to the sun have so much more dust than our own solar system?

Charles Beichman, from the California Institute of Technology and lead author on a paper to be published in the Astrophysical Journal, speculates that the dust could be coming from collisions within a large asteroid belt orbiting the star.

If the asteroid belt around HD 69830 is confirmed, it would be the first asteroid belt found to orbit a star so similar to our sun. The two other known asteroid belts orbit younger, more massive stars.

Artist rendition of zodiacal light, HD 69830.
Credit: NASA

"We’re interested in asteroid belts in these systems because they may mark either the construction sites that accompany the formation of rocky planets, the junkyards that remain after the formation of such planets, or simply mark places where, for one reason or another, material just couldn’t assemble to form planets at all," says Beichman.

Beichman says the asteroid belt orbiting HD 69830 could lie anywhere between .5 to 1 AU from its star. (1 AU is the distance between the Earth and the sun). To generate as much dust as they are seeing, this asteroid belt would need to have 25 times as much material as our own asteroid belt. Beichman says that collisions within this asteroid belt would occur relatively frequently, about every thousand years.

In our own solar system, the asteroid belt currently lies about 2 to 4 AU from the sun, or between the orbits of Mars and Jupiter. Jonathan Lunine, Professor of Planetary Sciences and Physics at the University of Arizona in Tucson, says that our asteroid belt was more massive during the early years of planetary formation. The rocky planets Mercury, Venus, Earth and Mars incorporated many of these asteroids as they were forming.

Lunine notes that the gas giant planet Jupiter aided in this asteroid clean up. The immense gravity of Jupiter pushed the asteroid debris into eccentric orbits around the sun, rather than the more circular orbits of the still-forming plants. This meant the asteroids crossed the planets’ paths more frequently, allowing the planets to sweep up asteroids more quickly than they would have otherwise.

Another possible source of the dust surrounding HD 69830 is a large comet the size of the planet Pluto. The spectral measurements of the dust are strikingly similar to the mineral composition of the comet Hale-Bopp.
Credit: NASA

"The fact that, in the HD 69830 system, there is still a massive asteroid belt located in the terrestrial planet formation zone makes that system different from our solar system," says Lunine. "Somehow that massive belt survived. That might be a signature that there is no giant planet in that system available to sweep the debris up efficiently. So this might be a system that is in a somewhat younger or more primitive stage of evolution. Perhaps we can even speculate that there are no rocky planets present, that the evolution of the debris did not proceed to the point where rocky planets formed."

Radial velocity measurements of star HD 69830 have found no orbiting gas giant planets. Methods currently used to hunt down extrasolar planets can’t detect planets less massive than Saturn, however, so smaller planets can’t yet be ruled out. But if there were planets located in the inner solar system of HD 69830, they should have already swept up much of the dust, as well as many of the asteroids.

Due to the lower luminosity of the star, the habitable zone – the zone where water could be liquid on the surface of a terrestrial planet- would be closer than it is in our own solar system. Lunine speculates that for HD 69830, the habitable zone would be about .8 or .9 AU from the star (versus .7 to 1.5 AU in our solar system.). If a rocky planet somehow was located in that habitable zone, life would have a difficult time surviving because the planet would get pummeled by asteroids.

Large impacts occur on Earth approximately every 100 million years, and at least one extinction event on Earth may have been caused by a meteor impact 65 million years ago, ending the reign of the dinosaurs. For habitable terrestrial planets orbiting HD 69830, such impacts would occur every million years.

Beichman and his colleagues used Spitzer’s infrared spectrograph to observe 85 sun-like stars. HD 69830 is about 40 light years away, and is located in the constellation Puppis. It is too faint to be seen with the naked eye.

Another possible source of the dust surrounding HD 69830 is a large comet the size of the planet Pluto. The spectral measurements of the dust are strikingly similar to the mineral composition of the comet Hale-Bopp. Both the dust and Hale-Bopp have a strong signal of forsterite, a magnesium-rich silicate that forms in high temperature processes. Forsterite can be found in the green sand beach of Hawaii, which is volcanic in origin.

But Beichman thinks the comet hypothesis is unlikely, because it would mean they were looking at the system at the exact same moment in time that a comet happened to be passing through it.

Future observations of the star, using Spitzer and ground-based telescopes, should help determine whether asteroids or comets are the source of the dust. Upcoming NASA missions, such as the Space Interferometry Mission (SIM), the James Webb Space Telescope, and the Terrestrial Planet Finder (TPF), may be able to determine if there are any terrestrial planets orbiting this star.

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James Webb Space Telescope

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