An international scientific team, including the Institute of Astrophysics of the Canary Islands (IAC), has discovered the highly eccentric orbit of a gas giant planet outside the Solar System, revealing how hot Jupiters are formed.
This world, called TIC 241249530 b, not only follows one of the most elongated orbits of any known transiting exoplanet, but it also does so in the opposite direction to its host star’s rotation, shedding light on the mystery of how these high-mass gas giant planets evolve into hot Jupiters, with very close, circular paths. The study is published in the journal Nature.
Among the known exoplanets are those that belong to a class known as hot Jupiters: large Jupiter-like exoplanets that orbit close to their star, some even closer than Mercury to our Sun. It’s a mystery how hot Jupiters end up on such close paths, since they couldn’t form there, but astronomers assume that they start out in orbits far from their star and then migrate inward over time.
The early stages of this process have rarely been observed. However, a new analysis of the exoplanet TIC 241249530 b, discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2020, has revealed that its orbit is highly eccentric, describing an elliptical shape with one axis much larger than the other. This odd trajectory suggests that the planet is in a pre-hot Jupiter migration phase. The data also confirm that the exoplanet is about five times more massive than Jupiter.
TIC 241249530 b is the second exoplanet to be discovered that is in the stage of its evolution before it migrates to more closed orbits. The discovery of this type of exoplanet significantly supports the idea that higher-mass gas giants evolve into hot Jupiters as they transition from highly eccentric orbits to closer, more circular ones. This process is due to the fact that as the planet gets closer to its host star, tidal forces on the planet subtract energy from its orbit and cause it to shrink and become increasingly round.
“Although we can’t precisely rewind and see the migration of planets in real time, this exoplanet is a kind of snapshot of the migration process,” explains Arvind Gupta, a postdoctoral researcher at NOIRLab and lead author of the paper published in Nature. “Planetary systems like this are extremely rare and difficult to find, and we hope they will help us unravel the history of the formation of hot Jupiters.”
Find the most unusual planet
The eccentricity of a planet’s orbit is measured on a scale of 0 to 1, with 0 representing a perfectly circular orbit and 1 representing a highly elliptical orbit. This exoplanet has an eccentricity of 0.94, making it the most eccentric planet ever found using the transit method. For comparison, Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25; Earth’s is 0.02.
If this planet were part of our solar system, its orbit would stretch from its closest approach, which is ten times closer to the sun than Mercury, to its farthest point, which is the distance from Earth. This strange orbit would cause temperatures on the planet to vary, ranging from sweltering heat on a summer day to temperatures so hot they could melt titanium at the moment of closest approach to its star.
Additionally, the team discovered that TIC 241249530 b is rotating in the opposite direction than usual, in the opposite direction to its host star’s rotation. This is not something astronomers observe in most exoplanets, nor in our own solar system, and it helps the team explain the strange history of exoplanet formation.
In the research, the team used the 3.5-meter WIYN telescope at the National Science Foundation (NSF) Kitt Peak National Observatory, a program of the NSF NOIRLab. Soon, the team plans to better understand TIC 241249530’s atmosphere through observations with more powerful telescopes such as the James Webb Space Telescope (JWST).
“We are particularly interested in what we can learn about the atmospheric dynamics of this planet as it approaches its star,” says Enric Balli, a researcher at IAC and co-author of the paper. “Telescopes like the James Webb Space Telescope are sensitive enough to detect changes in the atmosphere of a newly discovered exoplanet as it rapidly warms up, so the team still has a lot to learn about the exoplanet.”
“Astronomers have been searching for exoplanets that could be precursors to hot Jupiters or intermediate products of the migration process for more than two decades, so we were very surprised — and excited — to find one. This is exactly what I expected to find,” Gupta concludes.
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