Video: Artist’s impression of the unusually elliptical orbit of the planet TIC 241249530 around its host star. © Abigail Minnich/Penn State
It whizzes past its star “precariously” close, but then only returns to it after a long journey through the system: researchers have discovered a giant planet that moves in an extremely elliptical orbit. According to model calculations, it will orbit its star less and less eccentrically and more widely in the future, which will ultimately make it a so-called hot Jupiter. The discovery sheds light on the development histories of these special exoplanets, say the researchers.
Among the more than 5,600 worlds that astronomers have now discovered around distant stars, there is one category that cannot be found in our solar system: the so-called hot Jupiters are exoplanets that resemble our gas giants Jupiter and Saturn, but occupy a very different position in their systems: they orbit closer to their stars than Mercury orbits the sun, which causes them to heat up considerably. It is assumed that these celestial bodies did not originally form in these tight orbits: models suggest that they first formed in the outer regions of their systems and only later settled into their tight orbits.
“Astronomers have been searching for more than two decades for exoplanets that could be precursors of hot Jupiters or intermediate products of the migration process,” says first author Arvind Gupta of Pennsylvania State University in University Park. So far, there has only been one case of evidence of a planet that could be in this development process. Now Gupta and his colleagues are presenting another candidate. It is the planet called TIC 241249530, which orbits a star about 1100 light years from Earth.
Amazingly extravagant on the road
It was originally discovered using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) using the transit method, which detects the shadowing created by the planet passing in front of its star. Instruments at the Kitt Peak National Observatory in Arizona then provided additional data that provided clues based on the star’s radial velocity. Spectral analysis of the starlight was able to reveal the “wobble” of the star, which is due to the gravitational pull of the planet as it orbits. The information collected was then used to deduce the characteristics of TIC 241249530 and its orbit.
As the team reports, the data analysis showed: The planet is five times more massive than Jupiter and moves extremely eccentrically during its orbit, which lasts around six months: It comes very close to its star at times, but then moves far away from it in a highly elliptical orbit. If the planet were part of our solar system, it would come closer to the sun than Mercury, but then move out into the area of the Earth’s orbit, the researchers explain. TIC 241249530 now has the highest value of eccentricity recorded to date for a planet observed in transit, say the researchers. It thus replaces the previous record holder HD 80606 b, which was already considered a candidate for an evolving hot Jupiter.
“This new planet is experiencing really dramatic changes in irradiation during its orbit: its atmosphere must heat up extremely every time it approaches the star,” says co-author Sarah Millholland from the Massachusetts Institute of Technology in Cambridge. The researchers also discovered another peculiarity of TIC 241249530 that may have something to do with its special history: the planet orbits in a direction that is opposite to the rotation of its parent star. Such a counter-rotation does not occur in our solar system and has only rarely been observed in exoplanets.
On the way to hot Jupiter
To shed light on how the unusual orbit came about and how it might evolve, the researchers conducted model simulations of the orbital dynamics and possible influencing factors. These show that TIC 241249530 is indeed a planet that originally formed on a circular orbit in the more outer region. It then apparently drifted to its eccentric orbit because the system is under the influence of a neighboring star, the researchers explain. “Although we can’t really hit a kind of rewind button and observe the process of planetary migration in real time, the results do represent something like a snapshot of the migration process,” says Gupta.
As for the future evolution of the orbit, the model simulations showed that TIC 241249530 will gradually lose momentum until it eventually assumes an increasingly circular orbit very close to its host star. Specifically, the researchers predict that in about a billion years the planet will orbit its star in just a few days. It will then have fully evolved into a hot Jupiter. “This new planet therefore supports the theory that some hot Jupiters have undergone a migration history with high eccentricity,” says Millholland.
As the team concludes, in addition to the new clues about the evolutionary histories of hot Jupiters, the planet offers further potential for astronomical research: “We are particularly interested in what we can learn about the dynamics of this planet’s atmosphere after it has completed one of its scorchingly close passes to its star,” says co-author Jason Wright of Pennsylvania State University. “Telescopes like NASA’s James Webb Space Telescope now have the sensitivity needed to study the changes in the atmosphere of this newly discovered exoplanet,” says the astronomer.
Source: Massachusetts Institute of Technology, Pennsylvania State University, specialist article: Nature, doi: 10.1038/s41586-024-07688-3