Astronomers Use Stellar Composition To Reveal Solar System Formation
John P. Millis, Ph.D. for redOrbit.com – Your Universe Online
How do solar systems form? Specifically, why do some systems form smaller rocky worlds, while others are dominated by gas giants? A recent study led by Trey Mack, a graduate student in astronomy at Vanderbilt University, may have found the answer.
Stars are dominated by hydrogen and helium, possessing only trace amounts of other elements – what astronomers generically call “metals”. Mack proposed looking at ratios of specific metals, such as aluminum, silicon, calcium and iron – elements that all have melting temperatures above 1,200 degrees Fahrenheit. These elements are crucial in forming rocky, Earth-like planets.
However, if during the formation of the star, the abundance of these elements is mostly absorbed by the star, then there may not be significant enough material left to form such worlds. Instead, any planets that form would more closely resemble Jupiter or Neptune.
To investigate, Mack and his team looked to the binary star system HD 20781 and HD 20782, two objects similar to our Sun. Since these stars would have condensed from the same gas cloud, a similar composition would be expected. However, one star is closely orbited by two Neptune-sized planets, while the other sports a Jupiter-sized planet in a highly eccentric orbit.
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By examining the chemical differences between the stars, the team was able to find how the accumulation of matter influenced the types of planets that formed around them. “Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets,” said Vanderbilt Astronomy Professor Keivan Stassun, who supervised the study. “After obtaining a high-resolution spectrum for a given star, we can actually detect that signature in detail, element by element.”
In this particular study, the results indicated that there were significantly higher levels of the metal elements the team analyzed, relative to the Sun. Additionally, the higher the melting temperature of the element, the greater the abundance – an indication that the stars perhaps swallowed any Earth-like planets that may have been forming in orbit around them.
“Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter,” said Mack. “The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets.
“With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect.”
Based on their initial findings, the team suggests that there are unlikely to be any terrestrial planets orbiting either star. Of course, additional study is needed in other solar systems to further validate the theory. “When we find stars with similar chemical signatures, we will be able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets,” said Mack. “And when we find stars that lack these signatures, then they are good candidates for hosting planetary systems similar to our own.”
“This work reveals that the question of whether and how stars form planets is actually the wrong thing to ask,” added Stassun. “The real question seems to be how many of the planets that a star makes avoid the fate of being eaten by their parent star?”
The results of the study were published online May 7 in the Astrophysical Journal.
Image 2 (below): What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? A spectrum taken of the Sun is shown above. The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF