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Massive exoplanet has extreme orbit around red giant

Jenny Winder, News Writer
Feb 28, 2015, 19:36 UTC

Sen—Two groups of astronomers from Germany have, independently of each other, discovered an unusually dense and massive planet which orbits a red giant star. 

The teams, one led by Mauricio Ortiz of the Centre for Astronomy of Heidelberg University (ZAH), and the other by Simona Ciceri of the Max Planck Institute for Astronomy (MPIA) in the same city, report that Kepler-432b has six times the mass of Jupiter, but is about the same size, making it one of the most dense and massive exoplanets, a planet outside our Solar System, known so far. 

Kepler-432b was first noticed when NASA’s Kepler space telescope recorded tiny dips in the brightness of the planet’s host star, as the planet passed directly in front of the star, a “planetary transit”. The two Heidelberg teams were able to confirm the planet using the CAFE spectrograph at the 2.2-metre telescope at Calar Alto Observatory and the Nordic Optical Telescope on La Palma in the Canary Islands, to detect the planet’s traces in the spectrum of the star, the “radial velocity method”.

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The 2.2 metre telescope at Calar Alto observatory that the groups used to confirm the existence of the planet Kepler-432b. Image credit: MPIA

Only five planets, including Kepler-432b, have been observed which are unusually close to their red giant hosts. Of these, only two, namely Kepler-432b and Kepler-91b have been observed sufficiently closely to determine both their mass and their size

Dr Davide Gandolfi, from the state observatory Königstuhl, told Sen: "The highly eccentric orbit brings Kepler-432b to 'only' 24 million km (15 million miles) from its host star, before taking it to about three times as far away. This creates enormous temperature differences over the course of the planet's year, which is equivalent to 52 Earth days.

“The majority of known planets moving around giant stars have large and circular orbits. With its small and highly elongated orbit, Kepler-432b is a real ‘maverick’ among planets of this type,” he added. 

The orbit of Kepler-432b is highly elongated. As a consequence, the distance between the planet and the star, as well as the temperature on the planet, change dramatically over the course of the planet’s year.

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Illustration of the orbit of Kepler-432b (inner, red) in comparison to the orbit of Mercury around the Sun (outer, orange). Image credit: Dr Sabine Reffert

“During the winter season, the temperature on Kepler-432b is roughly 500° Celsius. In the short summer season, it can increase to nearly 1,000° Celsius,” said astronomer Dr Sabine Reffert, also based at Königstuhl.

Of the nearly 1,900 exoplanets known, around 50 orbit red giant stars, which are stars in the later stages of their lives. Kepler-432b's parent star has already exhausted the nuclear fuel in its core and is gradually expanding. Its radius is already four times that of our Sun and it will get even larger in the future. Planets too close to such a star will be swallowed up, and planets orbiting too close to the red giant’s surface are likely to be drawn in and swallowed within tens or a few hundreds of million years.

Astronomers do not expect to observe many examples of such a fairly short-lived phenomenon. Ciceri said. “At this point, there are two possibilities: Either we have been unusually lucky to observe two rare, close planetary orbits such as those of Kepler-432b and Kepler-91b. Or else, planets like these survive for much longer than was previously assumed.”

Ortiz added: “The days of Kepler-432b are numbered. In less than 200 million years, Kepler-432b will be swallowed by its continually expanding host star. This might be the reason why we do not find other planets like Kepler-432b—astronomically speaking, their lives are extremely short”.