Mystery as first known transiting exoplanet didn’t form where we now see it

A giant planet, with a mass some 220 times that of the Earth, has been found to have spiralled into within 4 million miles of its parent star.

HD 209458 b, also known as Osiris, orbits a star around 159 light-years from us.

The giant planet represents a number of significant milestones in exoplanet research – it’s the first planet confirmed through more than one observational method and the first known exoplanet with an atmosphere.

And now that atmosphere has been analysed – leading to a number of intriguing conclusions.

Six distinct gases have been identified in Orisis’s atmosphere; hydrogen cyanide, methane, ammonia, acetylene and carbon monoxide as well as a small amount of water vapour.

That rich chemical mix, especially the carbon, suggests that Osiris began its life far out in its parent star’s planetary disk, only migrating into its current super-close orbit after it formed.

Osiris is so close to its star that its year – the time it takes to complete an orbit around its massive parent star – is just over three Earth days.

The planet's surface temperature is estimated to be in the region of 1,000 °C (about 1,800 °F), meaning that life as we know it is extremely unlikely, despite that complex mix of chemical elements.

Atmospheric analysis of this distant giant was published this month in the peer-reviewed journal Nature.

Lead author Paolo Giacobbe, from the Italian National Institute for Astrophysics, said: "If this discovery were a novel it would begin with 'In the beginning, there was only water…'

"That’s because the vast majority of the inference on exoplanet atmospheres from near-infrared observations was based on the presence (or absence) of water vapour, which dominates this region of the spectrum."

He added: "We asked ourselves: is it really possible that all the other species expected from theory do not leave any measurable trace?

"Discovering that it is possible to detect them, thanks to our efforts in improving analysis techniques, opens new horizons to be explored."

Team member Matteo Brogi of the University of Warwick added: "Detecting as many molecules as possible is useful when we move on to testing this technique on planets with conditions that are amenable for hosting life because we will need to have a full portfolio of chemical species we can detect.”

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