First Known Transiting Exoplanet Didn't Form Where We Currently See It

Artist impression of HD 209458b. Image Credit: University of Warwick/Mark Garlick

An international team of researchers has performed incredibly detailed observations of the atmosphere of exoplanet HD 209458 b. These observations have provided evidence that this planet, which is located very close to its star, did not form where it is currently located but probably over 100 times further out. 

HD 209458 b, located 159 light-years from Earth, is a planet of firsts. It was the first planet discovered with the transit method (where a blip in the light from a star indicates a planet passing in front of it, momentarily blocking the light) in 1999. It was also the first extrasolar planet known to have its atmosphere detected, as well as many of its physical properties measured. Now, astronomers have added something else. Detailing their findings in the journal Nature, this is the first time an exoplanet has revealed six chemical elements detected in its atmosphere.

The team detected hydrogen cyanide, methane, ammonia, acetylene, carbon monoxide, and water vapor. This "chemical fingerprint" was crucial to the research, suggesting a planet that was birthed much further away from its star than HD 209458 b is. The planet has too much carbon for one just 7 million kilometers (4,350 million miles) from its star. This composition would make sense if it had formed further out. In our Solar System, this exoplanet would have formed around the orbit of Jupiter or beyond (750 million kilometers/470 million miles).

“The key chemicals are carbon-bearing and nitrogen-bearing species. If these species are at the level we've detected them, this is indicative of an atmosphere that is enriched in carbon compared to oxygen. We've used these six chemical species for the first time to narrow down where in its protoplanetary disc it would have originally formed,” Dr Siddharth Gandhi of the University of Warwick Department of Physics explained in a statement.

“There is no way that a planet would form with an atmosphere so rich in carbon if it is within the condensation line of water vapor. At the very hot temperature of this planet (1,500K), if the atmosphere contains all the elements in the same proportion as in the parent star, oxygen should be twice more abundant than carbon and mostly bonded with hydrogen to form water or to carbon to form carbon monoxide. Our very different finding agrees with the current understanding that hot Jupiters like HD 209458b formed far away from their current location.”

HD 209458b was also the first exoplanet where water vapor was discovered. In fact, the signal is so clear and strong in infrared light that makes it difficult to see other chemicals, so the team had to work hard to refine their techniques, using the Telescopio Nazionale Galileo in the Canary Islands.

"If this discovery were a novel it would begin with 'In the beginning, there was only water...' because the vast majority of the inference on exoplanet atmospheres from near-infrared observations was based on the presence (or absence) of water vapor, which dominates this region of the spectrum,” lead author Dr Paolo Giacobbe, from the Turin Astrophysical Observatory, explained.

“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."


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