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HATS-14b

November 25, 2015


The planet HATS-14b, a hot gas planet, shows an unusual inclination of its orbit and shakes the previous conceptions about how such planets could have formed [1] [2].


In our own solar system, the orbits of the planets are all in the same plane, perpendicular to the axis of rotation of the sun, or in the same plane as the solar equator. However, massive exoplanets are known which circle their star at a small distance on strangely inclined orbits. Until now, the conditions of planetary formation were thought to explain this: All the planets in the same system have formed from the same accretion disk, so that an additional interference factor, such as interstellar gas, an irregularly shaped dust disk or magnetic fields, would have been responsible for the observed deviations. It also appeared that small, cooler stars often exhibit large gas planets at a small distance, the orbits of which lie in the rotational plane of the star, while larger and hotter stars show more often hot gas planes on inclined orbits. [2]

Joshua Winn from the Massachusetts Institute of Technology suggested that small and cool stars have a relatively dense atmosphere. The gravitational force of the star atmosphere would act on the planet and bring its orbit into the rotational plane with time [2] [3].

HATS-14b, however, about 1672 light years away from us, shakes down this explanation. The planet orbits a small star on a narrow orbit, which should lie in the plane of the star equator. Instead, however, its orbit is inclined by 76 ° [4].

So far, it is not known how such an orbit can come about. There is, however, another weakness in Winn's theory, which he himself, along with Gonjie Li of Harvard University, admits: As soon as the gravitational force of the star pushed the nearest gas planets into the equatorial plane, it simultaneously deprived him of orbital energy, leading to decay of the orbit and eventual crash into the star. Gas planets in narrow orbits in the equatorial plane of the star should therefore only exist in the short term. However, this is in contradiction with the fact that such planets are often found [5].

This question has not yet been resolved.



References:
1. Mancini et al. (2015): HATS-13b und HATS-14b: two transiting hot Jupiters from the HATSouth survey. Astronomy and Astrophysics 580: A63
2. Winn et al. (2010): Hot stars with hot Jupiters have high obliquities. The Astrophysical Journal Letters 718, L145
3. Winn & Fabrycky (2015): The Occurrence and Architecture of Exoplanetary Systems. Annual Reviews of Astronomy and Astrophysics.
4. Zhou et al: (2015): A high obliquity orbit for the hot-Jupiter HATS-14b transiting a 5400 K star.
5. Li & Winn (2015): Are tidal effects responsible for exoplanetary spin-orbit alignments?



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