Astronomers have discovered two Earth-sized planets that survived getting caught in the red-giant expansion of their host star.
Steve Kawaler, an Iowa State University professor of physics and astronomy and a leader of the Kepler Asteroseismic Investigation, helped the research team study data from the Kepler space telescope to confirm that tiny variations of light from a star were actually caused by two planets of that star.
The findings are published in the Letters section of the Dec. 22 edition of the journal Nature. Stéphane Charpinet of the Institut de Recherche en Astrophysique et Planétologie in Toulouse, France, is the lead author and leader of the research team.
"This is a snapshot of what our solar system might look like after several billion more years of evolution," Kawaler said. "This can help us learn about the future of planetary systems and of our own sun."
Kawaler said the researchers have studied pulsations of the planets' host star (KIC 05807616, an old star just past its red-giant state and with an exposed core) for about two years. While analyzing the data, Charpinet noticed two tiny variations repeated in 5.76 and 8.23 hour intervals.
He asked other astronomers -- including Kawaler -- to analyze the original Kepler data and a subsequent set of data to see if they could also see the variations.
"We saw them in the same place and the same periodicity," Kawaler said. "So we knew they were real."
That led to the next question: "So what are they?"
Kawaler, working 26 years ago with the late Carl Hansen of the University of Colorado, had studied the fastest, and slowest, rates that stars could pulsate. Using that result, the team could conclude the variations seen by Kepler were too slow to be caused by the star itself. And so the astronomers started testing the idea that the variations were from two planets orbiting the star.
Astronomers believe the variations from the two planets -- KOI 55.01 and KOI 55.02 -- are caused by reflection of the star's light on the planets and by differences in thermal emissions from the hot day-sides and cooler night-sides of the planets.
The astronomers also report the two planets are 76 percent and 87 percent the size of Earth. That makes them among the smallest planets detected around a star other than our sun.
They further report the planets are very close to their host star, only .6 percent and .76 percent the distance between the sun and Earth. That means conditions on the planets are very harsh with temperatures up to 16,000 degrees Fahrenheit.
That's so close the host star's expansion as a red giant would have engulfed the planets, possibly stripping gaseous giants similar to Jupiter down to their dense cores. The planets also could have contributed to the host star's unusual loss of mass.
The research team said the discovery of the two planets raises many questions about their ability to survive such harsh conditions. It also raises questions about how planets can affect the evolution of their host stars.
Kawaler said NASA's Kepler Mission, launched in March 2009, is a tremendous tool for studying stars and planets. So much so, astronomers are working to extend the Kepler Mission another four years, from 2012 into 2016.
Kepler's primary job is to detect tiny variations in the brightness of the stars within its view to find Earth-like planets that might be able to support life.
The Kepler Asteroseismic Investigation is also using Kepler data to study different kinds of stars. The investigation is led by a four-member steering committee: Kawaler, Chair Ron Gilliland of the Space Telescope Science Institute based in Baltimore, Jorgen Christensen-Dalsgaard and Hans Kjeldsen, both of Aarhus University in Denmark.
- S. Charpinet, G. Fontaine, P. Brassard, E. M. Green, V. Van Grootel, S. K. Randall, R. Silvotti, A. S. Baran, R. H. Østensen, S. D. Kawaler, J. H. Telting. A compact system of small planets around a former red-giant star. Nature, 2011; 480 (7378): 496 DOI: 10.1038/nature10631
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