University of Texas Astronomer Explores Planet Formation Around Our Galaxy's Smallest, Most Abundant Stars
AUSTIN, Texas — A study published in this week’s edition of Astrophysical Journal Letters, led by University of Texas at Austin graduate student Jacob Bean with research scientists Michael Endl and Fritz Benedict, brings new insight into how planets form around the most populous stars in our Milky Way galaxy. Bean’s work shows that the chemical make-up of these “red dwarf” stars with orbiting planets is different from most of Sun-like stars that harbor planets — and indicates that astronomers must take chemical composition into account in their planet searches around these stars.
Red dwarfs have lower mass than any other type of star, ranging from just 8 per cent of the Sun’s mass to as much as 60 per cent. They also give off correspondingly less light, making them more difficult to study. Despite their stature, though, red dwarfs are the most numerous stars in the galaxy. Of the hundreds of billions of stars in our Milky Way, at least 70 per cent are red dwarfs. “This factor alone makes them a crucial sample for determining the fraction of stars that are orbited by planets,” Bean says.
Roughly 200 planets have been found around Sun-like stars. Most of these planets are several times the size of Jupiter, the largest planet in our solar system. In contrast, only three red dwarf stars were known to have planets or planet-candidates at the time of Bean’s study: Gliese 876, Gliese 436, and Gliese 581 (a possible fourth was recently announced). Gliese 876 harbors two Jupiter mass planets, with a third lower mass planet suspected.
One interesting trend that has emerged from studies of the Sun-like hosts to Jupiter-mass planets is the larger amount of “metals” — that is, elements heavier than hydrogen and helium — in their atmospheres compared to the Sun’s atmosphere. This property is known as “metallicity.”
The amount of heavy elements in a star’s atmosphere is thought to be a clue to the composition of the cloud of gas and dust from which it — and its planets — formed.
Benedict recounts a bit of project history: “Our original motivation was to determine the metallicity of red dwarfs in binary stars to help disentangle a completely different problem. Jacob early-on recognized the value of applying his techniques to planet-bearing red dwarfs.”
Planets probably grow faster in a proto-stellar cloud with higher metallicity . “Just as rain drops need a speck of dust in the air around which to form, the formation of planets is thought to be assisted by a similar successful first step,” Benedict says. “More dust in the protoplanetary disk might increase the chances for planet formation.”
According to Bean, “the formation of high-mass planets like Jupiter is also a time issue. A rocky core with sufficient mass to gravitationally pull in a lot of gas must form before the star switches on and its strong radiation pressure pushes the remaining gas away.” Apparently Sun-like stars have about a ten percent chance of having a planet. The chance for red dwarfs seems far less.
The purpose of Bean’s study was to find out if the red dwarfs with known planets also have high metallicity values. “It is predicted that high-mass planets should be rarer around red dwarfs because there should have been less material overall to form the star and potential planets in the primordial cloud,” Bean says. “That theory is supported by surveys discovering fewer of these types of planets around red dwarf stars. But, the dependence of high-mass planet formation on metallicity complicates what would otherwise be a straightforward result. If the red dwarfs being surveyed for planets have lower metallicities than the Sun-like stars that are being surveyed, that could also cause the discovery of fewer high-mass planets. The purpose of this work was to try and disentangle the two effects.”
Bean used the 2.7-meter Harlan J. Smith and 9.2-meter Hobby-Eberly telescopes at The University of Texas at Austin McDonald Observatory in West Texas to study the compositions of the three red dwarfs.
Analyzing the light from these tiny, dim stars is difficult, he says, because of the low temperatures in their atmospheres — the region from which the light to be analyzed is coming. “Molecules form in the star’s atmosphere,” Bean says, which “produce spectra that are very complex — a forest of lines all blended together.”
Bean’s work involves not only telescope observations, but computer modeling as well. He studied and improved computer-generated “low-temperature model atmospheres” for red dwarf stars.
When he analyzed his telescope data with these improved models, he found that these red dwarfs with planets contain significantly fewer metals than Sun-like stars that harbor planets.
Current theory holds that red dwarfs have fewer high-mass planets because the formation rate of high-mass planets depends on the mass of the host star. The low-frequency of planet detections around red dwarfs seems to support this theory. Now, however, Bean’s result shows that the effects of metallicity cannot be ignored when testing planet formation theories around red dwarfs. If planet searches are biased toward lower metallicity red dwarfs, then that could account for the low numbers of high-mass planets found around these stars.
Bean’s result is a preliminary finding from an ongoing project to determine the metallicities of all the red dwarfs included in the McDonald Observatory Planet Search. Because three stars is not a sufficient number upon which to establish a significant trend, Bean will determine many more red dwarf metallicities.
His co-author, Michael Endl, has searched for planets around about 100 red dwarfs to date. “Red dwarf stars represent very interesting targets for planet hunters,” Endl says. “For the red dwarfs with the lowest masses, like Proxima Centauri, we are sensitive to planets down to two Earth masses using the standard radial velocity technique.
“There appears to be a paucity of giant planets detected around red dwarfs, as compared to more Sun-like stars. This could mean that gas giant planet formation is less efficient around low mass stars. But it is possible that the majority of red dwarf giant planets orbit their star at larger separation and still await their discovery.
“Jabob’s result is an important step toward a better understanding of the planet formation history around the most common stars in the galaxy,” Endl says.
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