Current observations of ultra-hot planets by NASA’s Hubble and Spitzer space telescopes mystifies theorists. The planetary spectra suggest unusual compositions.

A team of researchers from the Arizona State University explained in their recently published study that the gas-rich planets have basically normal compositions when we are relying on the known planet formation. The difference is that the atmospheres on their daysides look more like the atmosphere of a star than a planet. Michael Line, an Astrophysicist at the university talked about these exoplanets in the following words:

“Interpreting the spectra of the hottest of these Jupiter-like planets has posed a thorny puzzle for researchers for years.”

The fact which created all the mystery was that why water vapor is apparently absent from these planets when it is found abundantly on similar marginally cooler ones. The researchers involved in the study found that even though oxygen and hydrogen atoms are abundant on ultra-hot planets, their strong dayside radiations increase the temperatures to a degree that it pulls water molecules apart. In case you are wondering what causes all that, orbiting exceptionally close to and with one side permanently facing their star keeps the other side perpetually dark. Consequently, dayside temperatures range from 2,000 to 3,000 degrees Celsius while the night side is almost 1,000 degrees Celsius cooler. Vivien Parmentier, an Astrophysicist at the Aix Marseille University in France and the Lead Author of the study referred to that by saying,

“The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere. In this way, ultra-hot Jupiters stretch out what we think planets should look like.”

Ultra-hot planets have been under surveillance for more than a decade now as exoplanets have a catalog of their own. Limited data has been collected on the night sides of these planets due to the difficulties of the current instruments. Three studies were recently published, co-authored by Parmentier, Line, and others. Based on those studies, the new paper proposes a model based largely on observations and analysis from three ultra-hot planets, WASP-103b, WASP-18b, and HAT-P-7b. It explains the phenomena happening on both sides of these planets.

Findings suggest that fierce winds driven by heat, blow the torn-apart water molecules into the planets’ cooler night side hemisphere. The atoms recombine into molecules there and condense into clouds. They rip apart again as they re-enter the dayside hemisphere. Hot Jupiters, the cooler cousins of ultra-hot planets, with dayside temperatures below 2000 degrees Celsius were the first abundantly found exoplanets in the mid-1990s. It has been proved that these cooler cousins have water in their atmosphere. As a result, astronomers expected to find water on these ultra-hot planets when they were discovered. Unfortunately, it was not there which urged scientists to look for other compositions.

A hypothesis for the apparent absence of water on ultra-hot planets stated that these planets must have formed with extreme levels of Carbon, not Oxygen. However, the infrequent traces of water found at the day-night boundary of these planets confounded this idea. Researchers thus looked at previously established physical models of stellar atmospheres and brown dwarfs, whose properties correspond with hot and ultra-hot Jupiters. Line talked about that and said,

“Unsatisfied with extreme compositions, we thought harder about the problem. Then we realized that many earlier interpretations were missing some key physics and chemistry that happens at these ultra-hot temperatures.”

The researching team adapted the ‘Brown Dwarf Model’ of Mark Marley, one of the paper’s co-authors and a research scientist at NASA’s Ames Research Center in California. They treated the atmospheres of the ultra-hot planets like those of blazing stars instead of considering them colder planets. This allowed them to make some logical sense of the Spitzer and Hubble observations. Line described their work beautifully and concluded that by saying,

“Our role in this research has been to take the observed spectra of these planets and model their physics carefully. This showed us how to produce the observed spectra using gases that are more likely to be present under the extreme conditions. These planets don’t need exotic compositions or unusual pathways to make them.”