ASTRONOMERS DISCOVER LARGE CIRCUMBINARY PLANET IN HABITABLE ZONE

Armagh Observatory, 14th June 2016: An international team of astronomers led by NASA's Goddard Space Flight Center and San Diego State University (SDSU), and including former Armagh PhD student and research associate Dr Tobias Hinse (Korea Astronomy and Space Science Institute), have used the Kepler Space Telescope to identify the largest planet yet known orbiting two Sun-like stars.

The discovery was announced this week at a meeting of the American Astronomical Society in San Diego, California. It is noteworthy not just for the large size of the newly discovered planet, similar to that of Jupiter in our Solar System, but because it orbits two parent suns in a long-term stable orbit where the temperature is predicted to be just right for the development of life as we know it: the so-called habitable zone. The research has been accepted for publication in the Astrophysical Journal in a paper led by NASA postdoctoral fellow Veselin Kostov.

Co-author Tobias Hinse remarked that the planet is unusual in being right in the centre of the habitable zone of the binary star system. "While life as we know it, at least on the planet itself, is rather unlikely, there is a possibility that life may have developed on a moon orbiting around the planet, or on a body located at one of the Langrangian points of the system, providing a locus for habitable Trojan planets."

The newly discovered Jupiter-mass planet, which has the name Kepler-1647b, is approximately 3,700 light years from Earth in the direction of the summer constellation Cygnus (the Swan). Its two parent stars are believed to be around 4.4 billion years old, comparable in age to Earth and our own Solar System. Each of the two stars is rather similar to our Sun, one being slightly larger and one slightly smaller, so by analogy with our own Solar System it is possible that the new planet could be associated with a form of extra-terrestrial life. This would not necessarily be life as we know it (a gas giant planet like Jupiter has no solid surface), but if the new planet were to have a system of several large rocky or icy moons, like Jupiter and Saturn in our Solar System, then one might speculate that over billions of years life might have evolved on one or more of its moons.

NASA launched the Kepler Space Telescope, now called K2, in 2009 with the primary mission to discover Earth-like planets orbiting Sun-like stars by looking for a small dip in brightness of a star caused by a planet passing directly between the visible star and the telescope's vantage point above Earth. Since then, although the mission formally ended in 2012, its data have become a magnet for astronomers investigating all kinds of time-variable stellar astronomical phenomena. In particular, astronomers can use the shape and size of a slight dip in the brightness of a star observed by Kepler to infer whether or not the dip was caused by a planet passing directly in front of the star, and then to determine the orbital parameters and physical properties of that newly discovered planet. The phenomenon is called a transit, as in the case of the Solar System when either Venus or Mercury passes directly in front of the Sun as seen from Earth.

In the case of so-called exoplanets it can sometimes take years of data to confirm the orbital and physical characteristics of the planet, which of course is too faint to be seen directly. It can be a gruelling process. Laurance Doyle, a co-author on the paper, and astronomer at the SETI Institute, noticed a transit back in 2011, but more data and several years of analysis were needed to confirm that a large, circumbinary planet had caused the transit. A network of amateur astronomers in the "KELT Follow-Up Network", which is helping to observe Kepler-discovered eclipsing binaries and circumbinary planetary systems, provided additional observations that helped the researchers to estimate the planet's mass.

The result is that we now know that the new planet Kepler-1674b has a similar mass and size to Jupiter, and revolves around two Sun-like stars in a nearly circular orbit with a period of revolution slightly more than three years. The two parent stars revolve one around the other on a much smaller orbit with a diameter approximately ten times larger than that of the stars themselves and an orbital period of just 11.26 days. The presence of two central stars immensely complicates the analysis of the system's changing stellar brightness and the search for a circumbinary planet.

"Finding circumbinary planets is much harder than finding planets around single stars", said SDSO astronomer William Welsh, one of the paper's co-authors. "The transits are not regularly spaced in time and they can vary in duration and even depth." The planet also has the longest period of any well-characterised exoplanet found so far, breaking with the tendency for circumbinary planets to have close-in orbits. "Habitability aside", continued Welsh, "Kepler-1647b is important because it may be the tip of the iceberg of a theoretically predicted population of large, long-period circumbinary planets."

A preprint of this work can be found at http://arxiv.org/pdf/1512.00189v2.pdf. Funding and facilities to support this work were provided in part by NASA, the National Science Foundation, KASI and, through the Armagh Observatory, the SFI/HEA Irish Centre for High-End Computing (ICHEC). It is interesting to note that two former Armagh PhD students, Tom Barclay and Geert Barentsen, are currently working on the Kepler/K2 Project at the NASA Ames Research Center, California, USA.

FOR FURTHER INFORMATION PLEASE CONTACT: Tobias C. Hinse (KASI) at tchinse@gmail.com, or Mark Bailey or Apostolos Christou at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; E-mail: meb@arm.ac.uk; aac@arm.ac.uk; URL: http://star.arm.ac.uk/ . Professor Welsh can be reached by e-mail at wwelsh@mail.sdsu.edu

Image Caption: Top: Artist's impression of the rare simultaneous stellar eclipse and planetary transit events on Kepler-1647. Bottom: Comparison of the relative sizes of several Kepler circumbinary planets, from the smallest (Kepler-47b) to the largest (Kepler-1647b). Figure credit: Permission is granted by the artist, Ms Lynette R. Cook, to media/news organizations to use the artwork with appropriate credit in association with coverage of the Kepler-1647b discovery. Others who may wish to use these images are advised to contact the artist directly (E-mail: lynette@spaceart.org; http://lynettecook.com and http://extrasolar.spaceart.org)

Poster

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Poster

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Last Revised: 2016 June 16th