Defining things in space can be problematic. What makes a planet a planet? A star a star? Astronomers look for many shared characteristics and this sometimes leads to the discovery of objects that are star/planet hybrids, like brown dwarfs. Now a new class of objects might be half star and half planet – the ultrahot Jupiters.
These objects, which have been known about for roughly a decade, are massive gas planets whose close proximity to their stars means they are tidally locked. One side is constantly facing the star, the other is in perennial darkness. By studying the planetary atmospheres of these ultrahot Jupiters, researchers have discovered that the two sides of the exoplanets behave in different ways. This dual nature is reported in Astronomy & Astrophysics.
"The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere," lead author Dr Vivien Parmentier, from Aix Marseille University in France, said in a statement. "In this way, ultrahot Jupiters stretch out what we think planets should look like."
The team focuses on one particular ultrahot Jupiter, WASP-121b. The team looked for the presence of water molecules. The dayside of these giants reaches temperatures between 2,000 and 3,000 degrees Celsius (3,600 and 5,400 degrees Fahrenheit). That's so hot that oxygen and hydrogen break apart, meaning that there are no water molecules. But on the dark side, water is present. The atoms recombine in the twilight zones of the planet and break apart at dawn.
The observations and this particular model add to our understanding of exoplanets, but they don't provide an all-encompassing explanation. The model explains the observations of several notable ultrahot Jupiters and definitely separates them from their slightly cooler cousins, the hot Jupiters. The latter group sports water features on both their daysides and nightsides.
The model is limited because it is unable to explain all ultrahot Jupiters. Explaining the atmosphere of these ultrahot planets clearly benefits from a stellar and planetary approach like the one used in this study, but these objects might have behaviors that are unique to them.
"We now know that ultrahot Jupiters exhibit chemical behavior that is different and more complex than their cooler cousins, the hot Jupiters," said Parmentier. "The studies of exoplanet atmospheres is still really in its infancy and we have so much to learn."
Future observatories like the James Webb Space Telescope, expected to launch in 2021, will hopefully provide more in-depth observations and help us understand just how in-between stars and planets these objects really are.
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