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  Geotimes - October 2007 - Reaching for the stars in planet formation

Planetary Science
Reaching for the stars in planet formation

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Most known exoplanets orbit stars that appear to be rich in metals. New research suggests these stars may be polluted with metal from planetary debris — or even a planet — that collided with the star.

Since the detection of the first planet outside our solar system in 1992, astronomers have found more than 200 additional exoplanets. In cataloging all of these discoveries, one thing in particular has stood out: Most of the stars around which these planets orbit appear to be rich in metals. (To astronomers, “metal” means any element other than hydrogen and helium.) This finding has left some researchers wondering if having a metal-rich star is a requisite for planet formation or if these stars just appear metal-rich due to planetary debris polluting the star’s surface. It’s the classic chicken versus egg debate, researchers say, and solving this problem may help astronomers better understand how planets form. Now, an international team of researchers says it might have an answer, suggesting planetary pollution is really to blame for these stars appearing to be metal-rich.

One reason why solving this problem has been difficult is because studying the composition of stars is no easy task. Researchers can only detect the makeup of the surface layers of a star, and even that is only from analyzing telescopic observations of stellar spectra. Most exoplanets that have been found orbit stars similar in mass and age to the Sun. These stars have relatively thin outer layers. This means it’s nearly impossible for a researcher to know if one of these stars appears metal-rich because it truly has metal throughout its entire body, or if it just has some metal in its outer layers due to planetary debris, or even a planet, colliding into its surface. Luca Pasquini, an astrophysicist at the European Southern Observatory in Garching bei München, Germany, and Artie Hatzes, an astrophysicist at Thüringer Landessternwarte in Tautenburg, Germany, and colleagues thought looking at a different kind of star, a red giant, might be a way to get around this problem.

Giant stars, Hatzes says, are stars “in their retirement age.” A main sequence star, such as the Sun, fuses hydrogen in its core. As the star ages, it begins to exhaust its supply of hydrogen, which causes the star to expand. Eventually, when the star runs out of hydrogen, it transforms into a puffed-up, red giant. “In giants,” Pasquini says, “almost the whole star is fully mixed, so the gas at the surface has the same composition as the interior,” which means researchers can look at the outer makeup of a giant, and, unlike a main sequence star, know its interior composition.

This fact led Pasquini and his team to study the metal content of 14 red giants known to have orbiting planets. They found that these stars were not particularly metal-rich, and had no more metal than red giants without planets. The most likely explanation for the lack of metal in these giants, they concluded in a paper in an upcoming issue of Astronomy & Astrophysics, is that metal-rich main sequence stars are just polluted. When planetary debris crashes into the surface of a main sequence star, it hangs out in the star’s outer layers. But as the main sequence star begins to expand and evolve into a red giant, the metal in their outer layers gets diluted and fully mixed into the rest of the star, explaining why this metal is not detectable in the red giants. Pasquini likens this to sprinkling cocoa powder on cappuccino. At first, the cocoa powder is visibly clumped together on the coffee’s surface, but disappears as it gets stirred together and incorporated into the rest of the cappuccino.

“This is a major new finding with potentially important implications for how planetary systems form and evolve,” says Alan Boss, an astrophysicist at the Carnegie Institution of Washington in Washington, D.C. The results, he says, call into question whether the widely accepted core accretion model of planet formation can really explain how all exoplanets form.

All of the known exoplanets are planets very similar to Jupiter — massive planets consisting mainly of gas with small rocky cores. Traditionally, astronomers have used the core accretion model to explain their formation: After a star forms, it leaves behind a disk of gas and dust. Over millions of years, rocky material in this disk collides, merging together to slowly build up a rocky core. Once the core reaches a certain mass threshold, it begins to accrete gas leftover from the star’s formation. The trick, however, is to build up a rocky core fast enough before all of the gas in the disk floats away. The more metallic materials available in the disk, the easier and faster this build up process will be. A metal-rich star may be an indicator that its disk would have been abundant in metals, thus explaining why planets seem to preferentially form around metal-rich stars, says Greg Laughlin, an astrophysicist at the University of California Observatories in Santa Cruz.

If some stars with planets aren’t actually metal-rich as the new evidence suggests, Boss says, then alternative hypotheses on planet formation, such as his “disk instability” model, may be a more viable alternative because they suggest planets like Jupiter can form very quickly — in thousands, not millions, of years — and therefore, they don’t need as much metal.

However, Laughlin says there’s an alternative explanation of the new results that’s perfectly in line with the core accretion model. Laughlin points out that the red giants in this study have masses that on average are much greater than the mass of the main sequence stars with known orbiting planets, and thus Pasquini, Hatzes and his colleagues did not compare apples to apples. This isn’t because all red giants are more massive than main sequence stars as their name might suggest. Instead, when the red giants in this study were younger (when they were main sequence stars themselves), they would have been several times more massive than our sun, and therefore, would naturally have more metal than less-massive stars, Laughlin says. That means, all else being equal, more-massive stars should also have more metal available for planet formation. High-mass stars with planets, therefore — such as the red giants in this study —won’t appear particularly metal-rich when compared to high-mass stars without planets. So, he suggests, a planet can form via the core accretion model either around high-mass stars or around smaller stars that are especially rich in metal.

Pasquini and Hatzes recognize that the mass of the stars is something that needs to be taken into consideration. The next step, Hatzes says, is to look for planets around giants with a mass more similar to that of our Sun to see if they are metal-rich, to test Laughlin’s scenario. Pasquini agrees that there is still much to learn before the question of planetary formation is really solved. “Exoplanet science is very young and we are really just scratching the tip of the iceberg,” he says. “I am sure that we will see many new results in the [upcoming] years, so stay tuned.”

Erin Wayman

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