How to Make a Hot Jupiter

Image Courtesy of Uriel Teague.

If you’re reading this, you likely live in a solar system. I’d venture to say that you live on Earth, too, making you a resident of its solar system. Given these assumptions, you’re likely familiar with its structure: four rocky planets on the inside followed by four gaseous planets on the outside, all orbiting the Sun in nearly identical equatorial planes. This structure feels standard, safe. But every tellurian astronomer was forced to confront this biased assumption upon the discovery of the first extrasolar planet.

In 1995, astronomers Didier Queloz and Michel Mayor discovered planet 51 Pegasi b, the first known planet found outside of the solar system. This planet is large, hot, and extremely close to the sun-like star that it orbits—much closer than Mercury is to the Sun. It is what is now called a “hot Jupiter,” a gas giant planet with an orbit extremely close to its star. This single planet scrambled astronomers’ understanding of solar system formation since they had previously assumed that gas giants orbited far from their stars, much like our beloved Jupiter. Now that over 5,000 exoplanets have been discovered, we can create a “normal distribution” of solar systems, and ours is far from the center of the bell curve.

Though they are familiar to us, our system’s orbits, which transit the equator of the Sun, are just as extreme as orbits wherein a system’s planets orbit from pole to pole. So while large, gaseous planets completing orbits around their stars in just a few days may seem foreign to us, it is frequently observed in other planetary systems. The only problem is that we don’t understand how they do it.

Yale astronomy researchers Malena Rice and Greg Laughlin are tackling this issue head-on. Fifth-year Ph.D. student Malena Rice was studying how a warm Jupiter, a gas giant with an orbital period of over ten days, fit into existing planet formation theories when she noticed an interesting correlation. “I made about a hundred plots looking at how this planet fits in with the bigger picture of all the [previous] measurements… and I realized that, no matter how you look at them, the eccentric planets tend to be more misaligned,” Rice said. This misalignment of eccentric hot Jupiters, or those with more elliptical rather than circular orbits, could be the key to understanding how they form.

Heat Your Jupiter to 2000 °F

But how did hot Jupiters get their characteristic elliptical orbits in the first place? This issue has separated astronomers into a few different camps. Perhaps hot Jupiters formed in their original places in a process called in situ formation. However, this is difficult to do because there isn’t a lot of planet-forming material near the stars they orbit. Others argue that they might have formed farther out and migrated inward, shepherded by other planets in the solar system through gravitational interactions. In this process, the hot Jupiters would spiral slowly and gradually toward the inner orbits of the system. There is strong support for this latter theory in astronomical communities.

The third camp argues in favor of high-eccentricity migration, a framework in which hot Jupiters are born farther from their host stars or the stars that they orbit. Through scattering and nudging from other sources, the hot Jupiters are knocked onto highly elongated orbits and spiral inwards to reach much closer orbits over time. “One way to distinguish which of these is actually correct is by looking at whether or not hot Jupiters are aligned with the plane of their host star’s equator,” Rice said. 

In general, we expect host stars to spin in the same direction as their surrounding disks of dust, gas, and other debris. These disks form around newly-made stars to eventually form planetary bodies through collisions of the orbiting particles. In the beginning, everything should form in the same plane. So, planets that are misaligned or orbiting in directions different from their host stars could indicate that the system underwent a dynamically dramatic process. These misalignments require a kick hard enough to tilt entire systems, such as one planet tossing another into a different orbit, a star flying by and tilting the disk, or one planet being thrown into the host star and being engulfed. The fact that systems with hot Jupiters are rarely observed with other planets indicates that the Jupiters’ would-have-been neighbors could have been thrown out early on or engulfed by the chaos caused by the dramatic inclination shift.

Observe Your Jupiter Transiting Its Star 

Hot Jupiters are incredibly well-studied by astronomers because they are the easiest to observe. To detect these planets, astronomers use the transit method, a detection technique where astronomers measure how much light a planet blocks as it passes or transits its host star. “You get the size of the planet by looking at how much of the starlight has been blocked, you get the period by seeing how often it happens, and you get information about the orbit from the duration of the transit,” said Greg Laughlin, Yale professor of astronomy. 

Over the past decade, a lot of effort has gone into measuring the angles between planetary orbits and the stars’ equators. Enough of these measurements have been collected such that patterns are now starting to emerge. One of the most interesting patterns is that stars that are more massive than the Sun by about twenty or thirty percent tend to show planets that are badly misaligned, whereas the stars that are less massive than the Sun tend to show better alignment. 

Extrapolate Your Jupiter’s Evolutionary History

Researchers Malena Rice and Greg Laughlin found that when the planet’s orbit had a higher eccentricity, there was more misalignment. This finding aligns with the idea that the planets get to their current locations through scattering rather than steady, slow disk migration. “That doesn’t mean that high-eccentricity migration is the only process that could take place, but is probably dominant—if you assume that it’s the only mechanism at play, it is consistent with all of our observations,” Rice said. 

For now, they need more data, but what they’ve collected so far is promising, confirming that disks should be aligned with their hosts. If wide-orbiting warm Jupiter planets had started in misaligned orbits, they would continue to have those peculiar orbits since they orbit too far from their host star to be realigned over time. So, the fact that we mostly see aligned systems, particularly on wider orbits, further indicates that something drastic happened to the closer-orbiting hot Jupiters after they had all formed to become misaligned.

This result isn’t what would be expected, as it implies that these hot Jupiters rely on random events rather than a systematic process. “I had assumed either that the planets are forming in situ or that they’re migrating, and I hadn’t really appreciated the fact that we can explain the distribution simply through chaotic scattering and planet-planet interactions, kinds of one-off events,” Laughlin said.

But what advantages does characterizing these planetary systems bring? Having two well-studied types of planetary systems is much better than just one, especially when those two systems are fringe-type oddballs on each end of the spectrum. From these systems, one can interpolate between the two extremes and extrapolate the evolutionary histories of more average systems. “What’s really exciting about this paper is that it gives us really good reason to believe that this very dramatic set of events, which are unlike anything that happened in the solar system, is actually happening on a regular basis,” Rice said. “It’s providing a completely new perspective on the different ways that planetary systems form; we are starting to piece together the possibilities and move away from being biased by our one exquisitely detailed data point.”

About the author: Brianna Fernandez is a junior in Pierson College studying astronomy and earth and planetary sciences. In addition to writing for YSM, she is one of the magazine’s layout editors. Outside of YSM, she researches exoplanets with Professor Debra Fischer and advocates for incarceration-impacted individuals with the Yale Undergraduate Prison Project.

Acknowledgments: The author would like to thank Malena Rice and Greg Laughlin for their time and enthusiasm in sharing their research. 

Extra reading:

Rice, Malena, et al. “Origins of Hot Jupiters from the Stellar Obliquity Distribution.” The Astrophysical Journal Letters, vol. 926, no. 2, 2022, 

Hamers, Adrian S., and Scott Tremaine. “Hot Jupiters Driven by High-Eccentricity Migration in Globular Clusters.” The Astronomical Journal, vol. 154, no. 6, 2017, p. 272., 

Kraus, Stefan, et al. “Spin–Orbit Alignment of the β Pictoris Planetary System.” The Astrophysical Journal, vol. 897, no. 1, 2020, 

Wallace, Spencer. “Unraveling the Formation History of Hot Jupiters.” Astrobites, 27 June 2019,