Once again, an astronomical celebrity is in its sights: the exoplanet, which was first identified using the transit method in 1999, has now given detailed insights into its atmosphere. The composition of the gases suggests that the planet has had a “hot” migration history, the astronomers report. According to this, it formed comparatively far away from its star and did not migrate into its extremely narrow orbit until later. This confirms the assumption that exoplanets of the “hot Jupiter” category have typically moved into the centers from the outer regions of their star systems in the course of their history.
Planets also revolve around distant stars – astronomers have now impressively confirmed this. A particularly large number of exoplanets entered the network for the planet hunters using the so-called transit method. This detection method can be used if the orbit of a planet is such that, when viewed from Earth, it is passing exactly in front of its star. It generates periodic reductions in its brightness. This method was first used successfully in the search for exoplanets in 1999, when astronomers identified the planet HD 209458b in this way. It orbits a sun-like star located about 150 light-years from Earth in the constellation Pegasus. After its discovery, HD 209458b remained firmly in the eye of astronomers and they have already been able to uncover some of its features.
A distant atmosphere in sight
So he advanced to the archetype of the exoplanet category of the so-called hot Jupiter, for which numerous examples have meanwhile been discovered. HD 209458b is similar in size to the largest planet in our solar system, but orbits its central star even closer than Mercury to the sun: this giant whizzes around its hot star at a distance of only about seven million kilometers. Astronomers were able to gain initial information about its atmosphere beforehand. Accordingly, due to the close proximity to the star, it literally evaporates and forms a kind of tail like a comet. There were also indications of ingredients in the gas envelope of HD 209458b, but the current study now enables a more detailed insight into the atmospheric composition for the first time.
The results are based on data from the Telescopio Nazionale Galileo in La Palma. The telescope captured the light shining through the gas envelope of HD 209458b as the planet passed in front of its central star. Spectral analyzes of this shimmer made it possible to draw conclusions about the presence of certain substances in the atmosphere. The astronomers have now been able to detect six components: hydrogen cyanide, methane, ammonia, acetylene, carbon monoxide and small amounts of water vapor. As they explain, the abundance of carbon compounds (hydrogen cyanide, methane, acetylene and carbon monoxide) is remarkable. The results show that the bottom line is that there are about as many carbon atoms as oxygen atoms in the atmosphere. According to the scientists, this is twice as much as would be expected against the background of models of the formation of planetary systems.
Further place of birth
“It seems impossible that this planet with such a carbon-rich atmosphere formed where it is now,” says co-author Siddharth Gandhi University of Warwick. “Because at the high temperatures, oxygen should occur twice as often as carbon and usually be bound with hydrogen to form water or with carbon to form carbon monoxide,” explains the scientist. The findings, however, show that the planet accreted a particularly large amount of carbon-rich gas during its formation. According to the researchers, the most plausible explanation for the results is that HD 209458b once orbited much further from its star than it does today.
To explain: A planetary system begins its life as a disk of material that surrounds the star and merges into the solid cores of the planets, which then collect gaseous material to form an atmosphere. In the vicinity of the star, where it is hotter, much of the oxygen in the atmosphere is retained as water vapor. Further out, where it is cooler, this water condenses into ice and is trapped in the planet’s core, leaving an atmosphere that is more composed of molecules based on carbon and nitrogen. Therefore, planets orbiting close to the sun are expected to have atmospheres rich in oxygen rather than carbon. Against this background, the findings for HD 209458b indicate a history of migration.
“Based on the findings, we were able to narrow down where in the protoplanetary disk the planet might originally have formed,” says Gandhi. According to this, HD 209458b was created at similar distances to its star as Jupiter or Saturn have to the sun. From this position it has migrated to the “hot” orbit in the center of the system in the course of its history. “This confirms the assumptions that hot Jupiters like HD 209458b arose far from their current positions,” Gandhi sums up.
Source: University of Warwick, specialist article: Nature, doi: 10.1038 / s41586-021-03381-x