On September 9, 1999, a team of astronomers from the High Altitude Observatory made a groundbreaking discovery. Using the radial velocity method, they detected a massive, Jupiter-sized planet orbiting a Sun-like star HD 209458. This was the first exoplanet to be discovered using this method, and since that fateful day, this system has become a champion of firsts.
The planet is HD 209458 b, and nicknamed “Osiris” after the Egyptian god of the underworld. Osiris is a fitting nickname for this planet, as it is a very hot and hostile world. HD 209458 b orbits its star very closely, and its surface temperature is estimated to be around 1,000 degrees Celsius. This makes it too hot for liquid water to exist on its surface, and it is unlikely to be habitable.
Even so, this planet is remarkable in many ways. It occupies first place in a long list of discovery milestones. Here are the areas Osiris occupies first place.
It is one of the first exoplanets to be discovered using the transit method, which means that astronomers were able to detect it by observing how its star’s light dimmed as the planet passed in front of it. This was a major breakthrough, as it showed that it was possible to directly detect exoplanets.
In fact, seeing its transit makes it the first exoplanet to be detected by more than one method: both transit and radial velocity. Further, it was the first planet to have its orbital speed measured, allowing astronomers to determine its mass directly
Osiris is also the first exoplanet to have its atmosphere directly detected. In 2001, astronomers using the Hubble Space Telescope were able to measure the absorption of light from HD 209458 b’s atmosphere. This allowed them to identify the presence of sodium and other elements.
And in 2007, astronomers from Lowell Observatory found evidence of water vapor in the upper atmosphere. Another first.
These observations were a major breakthrough in exoplanetary research. It showed that it was possible to learn more about the composition of exoplanet atmospheres, and to search for biosignatures, which are molecules that could be indicative of life.
Osiris is also the first exoplanet to be observed losing its atmosphere. Astronomers have detected a large plume of hydrogen gas escaping from the planet. This is likely due to the intense heat and stellar winds from the star, stripping it away, leaving a stream of material behind it.
The discovery of the evaporating atmosphere showed that some exoplanets could actually lose their atmospheres, and that this could have a significant impact on their habitability.
This exoplanet, some 157 light years away, has single handedly had a profound impact on our understanding of exoplanets and the search for life beyond Earth. It has advanced our understanding of other worlds outside our solar system perhaps more than any other exoplanet we’ve seen. It has shown that it is possible to directly detect exoplanets and their atmospheres, find out what they are made of and that some exoplanets are losing their atmospheres. This information is essential for our understanding of the diversity of exoplanets and the processes that can affect their habitability.
The discovery of HD 209458 b was a major inspiration for the search for life beyond Earth. Astronomers now realized what was possible when studying other worlds in the distant galaxy. Although Osiris is not a good candidate for life, its discovery showed that there may be many other exoplanets out there that are more habitable. The detection of water vapor in HD 209458 b’s atmosphere is a promising sign for the search for life beyond Earth, as water is essential for life as we know it.