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January 12, 2017

U of C Astronomers Discover "Twin Star," Develop Insight Into Planetary Movement

University of Chicago researchers have discovered a planetary system based around “solar twin” HIP68468, a star that shares many similarities with the Sun.  

Unlike the Sun, this star contains abnormal amounts of lithium, levels that would be more common in a much younger star, as lithium gets consumed by a star during its lifetime. 

“Lithium is like the ‘smoking gun’ evidence in this case that there's something unusual about this star,” said Megan Bedell, a UChicago graduate student and leader of the planet search portion of this project. “[It’s] kind of an unusual element in that it actually gets consumed by the star during the course of its lifetime, so as [the star] grows older, the lithium gets eaten up in the same process that fuses the hydrogen to create energy in the star.”  

High levels of lithium in HIP68468, which usually indicates a young star, contradicts its actual age—6 billion years compared to the Sun’s 4.5 billion years.  

“Planets have lithium, so we inferred that the extra lithium [is due to] planetary pollution on the outside of the star,” Bedell said. “We also see that extra rocky material—iron and things that make up the Earth—tend up to be fairly prevalent in this star compared to other solar twin stars and the Sun, especially.”  

Therefore, Bedell’s group, under the mentorship of her thesis adviser Jacob Bean, assistant professor of astronomy and astrophysics at the University of Chicago, and collaborating with a team of international scientists, including Brazilian scientist and lead author Jorge Melendez, came to the conclusion that star HIP68468 actually devoured nearby planets, which would explain the inconsistencies in the composition of the star.  

Bedell’s discovery of two nearby planets, a “super Earth” and a “super Neptune,” also supports this hypothesis, as they seem to have migrated closer to the star over time.  

“We can guess that the ‘super Earth’ formed somewhere close to where the Earth did [in our solar system] and the ‘super Neptune’ probably formed much further away,” Bedell said. “In both cases today, we see them in a place where they're so close to the star and so hot that it just wouldn't have been possible for the material to condense in that place in the early solar system.” 

Bedell observed these planets with a Chile-based spectrograph called High Accuracy Radial Velocity Planet Searcher (HARPS). This instrument breaks down light from stars into the component colors, and researchers then use the spectrum it creates to find planets and discover the composition of stars.  

“We have to keep using HARPS to observe the star many times over a period of years because you want to see the small changes that happen in the star during the time it takes for the planet to do an orbit around the star,” Bedell said. “I've been going to Chile about three times a year or so for the past few years and using the same instrument to keep track of how the stars are changing through time. Using that, I can then find out whether or not they have planets and what the planets are like.”  

By looking at “solar twins” like HIP68468, researchers gain insight into the future of our own solar system. 

“It's kind of cool to see what the planetary systems of these stars are like, especially if there's a lot of debate about how planet migration might happen over time—maybe the system that's born around a young star doesn't stay stable like that, and it'll get rearranged over the star's lifetime,” Bedell said. “So if you see a really old solar twin, maybe that's what the Sun will end up looking like.” 

In addition, looking at systems like the one surrounding star HIP68468 provides a “stepping stone” to questions about other Earth-like planets, Bedell said.  

“[It’s] got a lot of implications for the solar system and especially how common an Earth-like system is, whether other Earths out there are habitable or whether a lot of stars have this migration event that renders everything too hot to be inhabited,” Bedell said. “It’s nice evidence that we're on the right path for what the current theory is to explain these systems.”  

The group plans on continuing their research on the star system to verify their findings. 

“We would like to keep taking measurements of the star's radial velocity to confirm the planet candidates that we identified and to search for additional planets,” Bean said. “We also want to compare the star's unusual abundance pattern to that of other similar stars to determine how common this sort of phenomenon is.” 

However, even though solar twin HIP68468 might be engulfing nearby planets, Bedell dismisses fears about Earth being similarly destroyed by the Sun any time soon. 

“I mean, if we're going to stick around for 5 billion years, we'll have to worry about the Sun devouring the Earth because eventually the Sun will swell up into a red giant, but I think this points to us being safe until the next phase of the Sun's life starts,” Bedell said. “We're probably not going to go spiraling into the Sun at any moment.”

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