The gas giants in our solar system—Jupiter and Saturn, along with their ice giant cousins Uranus and Neptune—orbit far from the sun. However, that’s not the case for some of the giant planets we’ve discovered beyond the solar system. Many of them are extremely close to their stars, closer than Mercury is to our sun. These close-in giants are often referred to as “hot Jupiters,” and a new paper published July 17 in the journal Nature promises to shed some light on the process of their formation.
[Related: How lightning on exoplanets could make it harder to find alien life.]
The study describes a newly-discovered exoplanet called TIC 241249530. The research team spotted its presence in data from the Transiting Exoplanet Survey Satellite (TESS), and studied its properties via several telescopes and other instruments. They concluded that the planet is around five times as massive as Jupiter, and that it is orbiting a star “slightly hotter, larger and more massive than the Sun”.It’s the way in which TIC 241249530 is orbiting that star, however, that makes it especially noteworthy. The planet’s orbit is extremely eccentric, or highly elliptical. In fact, its orbit is the single most eccentric ever observed in an exoplanet; according to the research team, 241249530’s trajectory is “closer to a cucumber than a circle.” The planet’s orbit is made even stranger by the fact that it is “backwards”: TIC 241249530 travels in the opposite direction to its star’s rotation.
As well as being remarkable for its extreme nature, TIC 241249530’s orbit also provides evidence for a theory about how hot Jupiters form, and how they end up so close to their parent stars. That theory suggests that the planets coalesce at relatively large distances from their stars, but migrate inward over time, entering eccentric orbits that take them extremely close to their parent stars. As each pass reaches its periastron—the point at which the planet comes closest to the star—tidal forces leach energy from the planet’s orbit, eventually causing that orbit to stabilize in a very tight circle close to the star.
It appears that this is exactly what is happening with TIC 241249530, providing astronomers with an opportunity to watch the process as it unfolds. “This exoplanet serves as a sort of snapshot of the migration process. Planets like this are hard to find and we hope it can help us unravel the hot Jupiter formation story,” Arvind Gupta, study co-author and a postdoctoral researcher at NOIRLab, said in a statement.
[Related: A nearby exoplanet reeks of rotten eggs.]
While such planets are indeed rarely observed, we have seen at least one exoplanet similar to TIC 241249530: HD 80606b, first described in a 2001 paper. HD 80606b is another immense planet, just over four times the mass of Jupiter, and was the record holder for orbital eccentricity in an exoplanet until the discovery of TIC 241249530. Notably, both it and TIC 241249530 seem to be following the sort of trajectory predicted by the inward-migration theory. As such, this new discovery presents further evidence in support of that theory.
However, TIC 241249530 also promises to provide a wealth of new information about hot Jupiters. Scientists can derive a great deal of data about exoplanets by observing them as they transit their parent stars, i.e. pass in front of the star from our point of view. TIC 241249530 reaches its periastron—the point at which it is closest to its parent star—just six hours before its transit. As the paper notes, this presents “a unique opportunity to observe how [the planet’s] atmosphere responds to … rapid, extreme heating”—an opportunity that will hopefully allow us to make direct observations of the exotic processes that leave these huge planets orbiting in extreme proximity to their stars.