Local grad discovers planets in star cluster

VERGENNES –– Two Jupiter-sized planets were recently discovered by a research group led by Waltham native Sam Quinn and his adviser, Russel White.
Quinn, 26, graduated from Harvard University in 2005 with a degree in astronomy and astrophysics and is now a PhD candidate at Georgia State University.
After college Quinn worked at the Harvard-Smithsonian Center for Astrophysics. He worked on NASA’s Kepler Mission, which identifies possible planets orbiting stars. The experience influenced the 2003 Vergennes Union High School graduate’s decision to continue that line of work.
“That really invigorated me to make that field into my career,” he said.
In his work Quinn began researching an unresolved dilemma concerning star clusters that ultimately sparked the discovery.
“We started looking in clusters because we know planets are common,” he said. “Most stars form in clusters and most stars have planets, so that was motivation for looking in a cluster. Up to this point no one had found planets in clusters.”
If most stars have planets, he explained, it would make sense that there would be planets in large clusters of stars.
Historically, researchers have looked at individual stars rather than clusters. Despite the lack of exploration of star clusters, it can actually be easier to work with them, Quinn said.
“For stars in clusters we’re able to sort of cheat a little bit because we know they all formed out of same molecular cloud of hydrogen,” he said. “The stars in that cluster should have the same composition. They should have the same age. Knowing composition and age lets us figure out a star’s mass and radius. If you know the properties of a star very well then you’re able to figure out properties of the planet.”
The field of searching for planets is quite young, said Quinn. The first planets discovered outside our solar system broke the mold of what astronomers expected to find.
“People expected to see what we have in our solar system,” Quinn said. “The planets they found were all large like Jupiter, but they were close to their stars. They are 20 to 30 times closer to their star than Earth is to the sun. They are called hot Jupiters because they receive a lot of radiation.”
With the planets’ proximity to their stars, said Quinn, a new question arose.
“They can’t form next to stars because it’s too hot to accumulate ice, rock and gas that close,” he said. “So that became a big question, this formation and subsequent migration inward.”
Quinn and his fellow researchers looked in the Praesepe cluster to find planets that might answer this question and were successful in their efforts.
“We surveyed 53 stars and discovered two hot Jupiters, one of them at an orbital period of a little more than two days and one with a period of four and a half days. That would place them probably about 30 times closer to their star than Earth is to the sun,” Quinn said. “The exciting thing is now that we’ve found these planets, we know that they do migrate inward in open clusters. We haven’t yet determined the mechanism, but it’s certainly a start because there are a few proposed ways that planets can move inward.”
Importantly, they resolved the issue that sparked their research.
“The most significant discovery is just resolving this apparent discrepancy between the fact that planets are common and stars form in clusters, but we hadn’t found hot Jupiters in clusters,” Quinn said. “Resolving that then allows us to examine the mechanisms for formation and migration of these planets.”
To find the new planets the group used the wobble method, explained Quinn.
“What that means is as a planet orbits its star the planet has a mass too, it’s sort of like a child holding a parent’s hand while running round in a circle,” he said. “The parent will move a bit, but not as much as the child. So based on laws of physics we can see if a star is wobbling around in a circle. We can measure how fast it’s moving in its circle, based on its movement we can see what the mass is of the planet it’s moving with.”
The star and its newly discovered planets are relatively close to Earth, said Quinn.
“It’s about 570 light-years away, which makes it not one of the closest stars to Earth, but in the grand scheme of things it’s pretty close,” he said. “The Milky Way is 100,000 light-years across and this star is only 570 away.”
The group’s paper will most likely be published in the Astrophysical Journal Letters after a reviewing process. The exposure will probably encourage more research in the field.
“We’re on our way now to distinguishing formation and migration mechanisms and starting to learn how these giant planets come to be,” Quinn said. “I’m hoping this will energize people to start looking in clusters because there’s a lot to be learned.”
Quinn is happy with the outcome of the group’s research and the impact it will have on the field.
“I think I’d like to continue this particular line of research looking for planet clusters,” he said. “It’s been successful so far, we’ve learned a lot, and I’d love to continue that.”

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