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Astronomers have discovered a third planet surrounding a pair of stars in the Kepler 47 system.
The illustration of an artist shows the three planets of the Kepler 47 system. The large middle planet represents the recently discovered Kepler 47d.
NASA / JPL-Caltech / T. Pyle
Kepler 47 has grown. Jerome Orosz (San Diego State University) and his colleagues announced in the May issue of the Astronomical Journal that the "Tatooine" system, with two previously known planets orbiting two stars, has a third planet. The system is 3,340 light-years away from the constellation Cygnus and so far it is the only known system for hosting several planets. The binary pair consists of two stars of the main sequence, one similar to our Sun and the other, a red dwarf. The new planet, Kepler 47d, orbits between the two planets discovered earlier. In fact, all three orbit the stars so tightly that they could fit into Earth's orbit around the Sun.
Astronomers once thought that circumbinary planets – worlds gravitating around a pair of stars – could not exist because the dynamic interaction of stellar double gravities would eliminate stable planetary orbits. The discovery of Kepler 16b about 8 years ago dispelled this idea and reshaped some models of planetary behavior. Now, confirmation of a multi-planet system could extend these models even further.
Illustration of Kepler planetary orbits-47
NASA / JPL Caltech / T. Pyle
The existence of the Kepler 47 system suggests that gravitational interactions limit the size of circumbinary planets, which could explain why rocky planets are much less likely to be found around binaries. When planets form in the disk of dust and gas surrounding the young stars, their interactions with the disk cause their internal migration. Co-author Nader Haghighipour (University of Hawai'i) explains that the competing gravitational forces of the two stars eventually drive out most of the smaller planets. The giant planets, meanwhile, continue to migrate inward and eventually collapse into the stars. Only the mid-sized planets remain.
This diagram shows the orbits of the planets in Kepler 47. The dark green region represents a conservative estimate of the habitable zone of the stars; light green indicates a more optimistic estimate. The red circle represents the distance of the stars where the planetary orbits remain stable despite the changing gravity field around the two stars.
Orosz et al. / Astronomical Journal 2019
Kepler-47 also demonstrates that circumbinary systems can be filled with planets in stable orbits. "I think the big breakthrough in this area," says David Martin (University of Chicago), "is what we call a complex system." Martin, who did not participate in the Kepler-47 research, explains: Do not place any other planet in between these three and let the system survive. This is important to understand how planets are formed. "
The transits are delicate
The three planets around Kepler 47 were discovered with NASA's Kepler Space Telescope. Launched in 2009, Kepler searched for exoplanets via the transit method – detecting slight changes in the brightness of starlight as planets passed their suns.
Finding exoplanets by their transits can be difficult. They must be large enough to measurably attenuate the light of the stars as they pass. Then, the inclination of their orbital plane must be sufficiently small compared to the Earth to allow observers to see periodic transits of planets through the face of their stars. The orbital planes of the circumbinary planets are further complicated by the wobbling; they vary slightly in time. "For each [circumbinary] the planet you see, you are missing about eight or nine, "says co-author of the study, Bill Welsh (San Diego State University).
Another type of research
Data on the Kepler 47 system was extended only in 2013, but NASA's Transiting Exoplanet Survey Satellite (TSS) was launched in 2018 to collect observations. TESS will be able to review Kepler 47 this summer and researchers are excited to see new data on the system after an interval of five years. TESS, however, is designed to image larger expanses of sky than Kepler, searching for planets around the nearest 200,000 stars. His research will offer a new way to study circumbinary systems.
"TESS will look at hundreds of thousands of binary stars," says Welsh. "Kepler has about 3,000, but TESS will add a few hundred thousand. With TESS, the goal is to start making statistics, looking at patterns and trends between these systems.
One of the goals is to understand how planets can form in the gravitational, unstable environment around binary stars. Ongoing research gives astronomers a wealth of data to work with. "We do not only find planets around binaries, we find them at a rate comparable to that of stars," notes Martin. "It seems like nature likes to live on the edge."
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