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Cometary Planets

We’re used to thinking about small asteroid-like bodies emitting gaseous tails: They’re called comets.  But could there be such things as cometary planets? Two studies regarding the escaping atmospheres of hot exoplanets have been published this month in the journal Science (1,2). The first is a transiting warm Neptune-mass exoplanet located 20 times nearer its host star than the Earth is to the Sun. The tail of helium being blasted away from this planet by radiation from its orange dwarf host star extends some five planetary radii out. The planet, known as HAT-P-11b, is blown up like a helium balloon, according to the researchers who have been studying it (3).

However, HAT-P-11 is not a young star still blasting away at the primordial atmosphere of a new Neptune-sized world, as you might expect. Instead, HAT-P-11 is 6.5 billion years old; almost 2 billion years old than our own Sun. So, why is it still managing to have such a devastating effect upon the Neptune-sized exoplanet in its midst? Common sense would dictate that you can’t have such an effect going on for 6.5 billion years, as the planet would have been eradicated long ago. Loosely bound helium held in this gaseous ‘envelope’ would surely leak out into space in considerable quantities over time? Like with comets repeatedly transiting around their stars at perihelion, you would think that at some point the volatile gases would all get blown away. Perhaps HAT-P-11b was once a much greater hot Jupiter world which has shrunk to Neptune proportions over time. Or, perhaps this is a case of inwards migration of this world from further out in the star system.

A similar tail of atmospheric helium is being blasted back from another giant exoplanet, this time known as WASP-69b (5). This world is about a quarter of the mass of Jupiter, making it a sub-‘hot Jupiter’ object. The discovery of this helium tail was also made using the Carmenes instrument, installed on the 3.5-meter telescope of the Calar Alto Observatory in Spain. In this case, the host star, is 2 billion years old and just a little smaller than our own Sun (6), making it another orange dwarf. So, again, this effect isn’t due to the young nature of the system – there is a sustained effect taking place over a long period of time if the system has been static for that entire time. How long can such helium leakage be sustained from this ‘evaporating exoplanet’? Previous discoveries of such leaking exoplanets have included a ‘hot Neptune’ planet whizzing around a ~9 billion year old red dwarf, Gliese 436 (7).  Read More…

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