Friday 1 May 2015

Kepler-432: a Red Giant Star with at least two giant planets.

Stars form from when vast clouds of gas and dust condense under their own gravity and contract into a single body. As this body contracts it eventually becomes so hot and dense that hydrogen atoms begin to fuse to form helium atoms in its core. This produces massive amounts of energy in the form of heat and light, that push against the gravity of the collapsing star, sweeping away any remaining material in the surrounding gas cloud that has yet to accrete onto the star and holding the star itself in a form of stasis; prevented from exploding by its gravity and from collapsing by the energy released by the fusion of hydrogen. Stars can (and do) remain in this state for billions of years, though larger stars, where the gravity is higher and therefore the pressure in the core greater, burn their fuel more quickly. Eventually stars run out of hydrogen and gravity begins to win the battle, and the star begins to collapse again. However this is not the end of the story, since as the hydrogen-depleted star collapses it eventually becomes so dense that it can fuse helium in its core. This releases more energy than the fusion of hydrogen, pushing harder against the gravity and causing the star to swell into a much larger (in terms of volume) body; a Red Giant Star. Such a star will eventually run out of helium as well, and begin to collapse again, though the largest stars can go through successive stages of fusing a whole series of elements before they eventually die.

These giant stars are of interest to planetary scientists because they offer the potential to study planets further out in the stellar system. Most exoplanets are detected by either the gravitational pull they exert upon their parent star, or the light they block when they pass in front of it. However planets further out in a stellar system are unable to produce enough of a gravitational pull to enable detection, and are much less likely to pass in front of their host star when seen from Earth. With giant stars the second of these is less of a problem, since the star itself is a bigger target, so there is more chance of a planet moving in front of it.

In a paper published on the arXiv database at Cornell University Library on 20 April 2015, and accepted for publication in the AstrophysicsJournal, a team of scientists led by Samuel Quinn of the Department of Physicsand Astronomy at Georgia State University describe the discovery of planets around the Red Giant Star Kepler-432.

Kepler-432 was originally designated as an ‘object of interest’ in 2013, and given the designation KOI-1299 (‘Kepler Object of Interest 1299’). It is 870 parsecs (convert to light years) from Earth within the Kepler Field (the field of stars in the sky to which the Kepler Space Telescope permanently pointed), and is calculated to have a mass of 1.32 times that of the Sun but a radius 4.06 times the Sun’s and an effective surface temperature of 4995 K (compared to 5778 K for the Sun). From which it is calculated to be an early-stage Red Giant, about 3.5 billion years old (younger than our Sun, but more massive, giving it a shorter life). This star was observed to be the subject of regular occulations (periods of dimming assumed to be caused by something passing in front of it) occurring every 52.5 days.

As well as the initial observations with the Kepler Space Telescope, Kepler-432 was the subject of follow-up observations with the 3.5 m WIYNtelescope on Kitt Peak in Arizona, the Near InfraRed Camera 2 on the Keck II 10 m telescope at Keck Observatory on Mauna Kea and the TillinghastReflector Echelle Spectrograph on the 1.5-m Tillinghast Reflector at the FredL. Whipple Observatory, also in Arizona.

The first discovery made from these observations was that a close by faint star originally thought to be a background star is in fact a bound companion, calculated to be an M-class Red Dwarf star with a mass 52% of that of the Sun and an effective surface temperature of 3660 K, which orbits the primary star at a distance of about 750 AU (i.e. about 750 times the distance at which the Earth orbits the Sun), completing one orbit in about 15 000 years. Since this is now known to be a two-star system the primary star is designated Kepler-432A and the companion Kepler-432B (when naming bodies in other stellar systems stars are designated with upper case letters and planets lower case letters).

Near InfraRed Camera 2 image of Kepler-432 showing a faint companion star. Image is four arcseconds square (the sky is a sphere comprising 360 degrees, with each degree divided into 60 arcminutes and each arcminute into 60 arcseconds). North is up and east is left (the reverse of what is seen on a ground map, since the viewer is looking up instead of down). Quinn et al. (2015).

The occulating body is calculated to be a planet with a mass equivalent to 5.41 times that of Jupiter and a radius 1.145 times that of Jupiter orbiting Kepler-432A at a distance of 0.301 AU (30.1% of the distance at which the Earth orbits the Sun), with an orbital period of 52.5 days, which is given the designation Kepler-432b. However this planet’s orbit is not completely regular, from which the presence of a second planet is deduced, the gravity of which perturbs the first planet slightly on each orbit. This second planet is calculated to have a mass equivalent to 2.63 times that of Jupiter, and to orbit Kepler-432A at a distance of 1.188 AU, completing one orbit every 411 days. This second planet is given the designation Kepler-432b.

See also…

The Kepler Space Telescope has discovered over 4000 candidate planets, around 40% of which are in systems with multiple planets. Many of the early multiple planet systems discovered contained one or more...
In December 2011 a team lead by Stephane...
Iota Draconis is a K-type orange giant star in the constellation of Draconis, 103 light years from Earth. It has an apparent magnitude of 3.31, making it naked-eye visible. In 2002 a paper in The Astrophysical Journal by a team lead by Sabine Frink of the Center for Astrophysics and Space Sciences at the University of California, San Diego reported the discovery of a...

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