Hot Jupiters: The OG Exoplanets

Author: Hayley Beltz.
Editors: Alison Claire Ludzki, Callie Corsa, and Sarah Kearns

Take a moment to remember that you and everyone you know live on a small blue orb hurling itself around a hot ball of hydrogen and helium that pays us no mind. Furthermore, we are only one of eight staggeringly diverse planets within our solar system that have been making this trip for billions of years. These planets range from hot rocks too small to even hold onto an atmosphere to cooler, massive gas giants where a day lasts less than 10 hours. Our solar system is only one of many (billions) and is only a small sample of the set of possible planet types and configurations. When astronomers started to look outside our solar system at nearby stars and the planets that orbit them–known as exoplanets–we began to understand just how strange other worlds can be.  Our solar system was unable to prepare us for what we saw first: Hot Jupiters.

Generally, Hot Jupiters are a class of gas giants that revolve around their host star in small, quick circular orbits. But before talking about a day in the life of a typical Hot Jupiter, let’s delve a little bit into why these  planets’ properties make them suitable for discovery and how exoplanets can be detected. One of the methods for detecting exoplanets is called the radial velocity method (sometimes just called “RV”) shown in the gif below:

When a planet orbits around a star, its gravity tugs on the star which results in the star moving as well. The more massive the planet and the closer its orbit, the more the star moves and the faster its (radial) velocity is. The radial velocity of the star is the speed that the star travels directly toward or away from us.  We measure that velocity here on Earth by measuring the light from the star. When the light moves towards us, its wavelength is shifted to look more blue. When it moves away from us, the light looks more red. So if we watch a star long enough and notice its light wiggling back and forth regularly, that’s a pretty good sign that there’s a planet there! Hot Jupiters are very massive and VERY close to their host star, meaning that they are wonderful for RV detection.

Despite the name, Hot Jupiters are more like a distant cousin than a sibling to our Jupiter. While Hot Jupiters and our Jupiter are made of the same stuff (mostly gas) and are much less dense than terrestrial/rocky planets like Earth or Mars, that’s where the main similarities end. In reality, these cousins live in vastly different neighborhoods. The division in these two “types” of Jupiter stems from the fact that Hot Jupiters can be 100 times closer to their host star than Jupiter is to our sun.  Length of orbit scales with distance from the star and as such, Hot Jupiters tend to orbit their host stars in less than a week. For reference, Mercury (the closest planet to the sun in our solar system) orbits the sun in only 88 days so Hot Jupiters are fast. All this heat makes the planets puffy and they often have larger radii than our Jupiter, despite being less massive.

Hot Jupiters are so close to their host star that they’re thought to be in a synchronous orbit (like our moon). That means that the length of a day on the planet is the same as the length of a year. In other words, during the time it takes for the planet to orbit the star it will have completed exactly one revolution. This also means that there would be a permanent day side which is always illuminated and a permanent night side which never gets to see starlight. Hot Jupiters will likely not be a popular vacation destination for future advanced civilizations and are certainly not a feasible location for humans to live.

To understand how these aspects would affect the atmosphere of an exoplanet, astronomers like myself use models. My research involves using a 3D atmospheric model combined with observational data to make predictions about Hot Jupiter atmospheres. When we compare data taken by a telescope with models, we can learn more about the system than from either method alone. For example, my current project involves using models and observational data to prove that a particular Hot Jupiter is in a synchronous orbit–the first time this has ever been done for emission spectroscopy! The emission spectra of a planet is the light the the planet emits. When we look carefully at this light, we can learn about what sort of compounds the light traveled through on its way to us–in this case we learn about the atmosphere of the planet itself! In these models, we program the necessary physics (i.e. heat transfer through atmospheric layers) and initial conditions (planet mass or star radius) and allow the model to calculate the atmospheres parameters. For example, it can calculate the temperature or wind speed at each atmospheric layer for a specified number of orbits.  Depending on the computational complexity of the model, these calculations can take minutes or months.

From these models, we can forecast the weather on Hot Jupiters, though every day is likely the same: hot and windy. To understand just how windy, let’s check in with the familiar cousin. Jupiter in our solar system has an iconic giant storm known as the Great Red Spot.  Faster than a category 5 hurricane on Earth, this storm has been going for several hundred years. How does this compare to winds on Hot Jupiters? Our model shows a very strong eastward jet traveling around the equator of these planets with wind speeds over 3000 m/s–that’s well over 10 times the speed of Jupiter’s Great Red Spot.

Hot Jupiters may not be a habitable category of exoplanets but are a true testament of extreme planet conditions. Of the nearly 4,000 confirmed exoplanets, over 800 fall into the Hot Jupiter regime.  It’s important to remember though that  our planet detection techniques like the RV method mentioned earlier are biased towards big planets. How common Hot Jupiters are is still an open question in astronomy. Although these are one of the most common types of exoplanets confirmed so far, they are by no means thought to be the most abundant type. Hot Jupiters were the first extreme type of exoplanet to be discovered, and we’ve only just begun to discover what other solar systems exist in our galaxy.