2024-07-18 00:03:29
Extreme orbit: Astronomers have discovered an exoplanet with the most eccentric orbit ever seen. The gas giant TIC 241249530b, 1,100 light-years away, orbits its star in a cucumber orbit the wrong way around: it orbits in the opposite direction to the direction of the star’s rotation. This unusual orbit could reveal how cold gas giants get too close to “hot Jupiters” orbit their star – and confirm theoretical models, as the team explains in Nature.
The “Hot Jupiters” are extreme among the planets – and a mystery to planetary research. These exoplanet gas giants orbit their star so closely that they only need a few days to complete one orbit. They often come so close to their star that their envelope of hot gas expands or stretches out into a tail. Some of these “hot Jupiters” have succumbed to their star’s massive tidal forces and spiraled towards their fiery end on a death spiral.
Intermediate degree sought
The real problem, however, is that these giant exoplanets cannot form on the spot. According to current theory, such gas giants only form further out in the protoplanetary disk of a star. These planets must have entered at some point then. According to the models, such hot Jupiter precursors go through a phase in which their orbit oscillates between being close to the star and their original outer orbit.
“Astronomers have been looking for exoplanets that could be exoplanets on a hot Jupiter that represent an intermediate stage of the migration process for more than two decades,” says lead author Arvind Gupta of the National Optical Astronomy Research Laboratory -Infrared (NOIRLab) in Tucson.
Newly discovered gas giant in sight
Now astronomers have found such a precursor. This is the exoplanet TIC 241249530b, which is about 1,100 light-years away and was “captured” by NASA’s TESS space telescope during a transit in front of its star in 2020. To find out more about this Jupiter-sized planet, Gupta focused and his team using several spectrometers from the Kitt Peak National Observatory (KPNO).
The aim was to research in more detail the mass of the distant gas giant and its orbit. Both can be done by measuring the radial velocity – the slight movement of the central star due to the gravity of the planet orbiting it. These subtle fluctuations appear as changes in the spectrum of starlight.
This is what the orbit of TIC 241249530b would look like if placed in the solar system. 
No one has ever been this different
The analyzes revealed something surprising: the planet TIC 241249530b moves in a highly eccentric orbit around its star – its path looks more like a cucumber than a circle. Specifically, the astronomers determined an eccentricity of 0.94 – with these values, zero is considered perfectly circular and 1 is considered maximally eccentric. For example, Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25, while Earth’s slightly elliptical orbit corresponds to a value of 0.02.
“TIC 241249530b has the most eccentric orbit of all the known transiting exoplanets,” says co-author Jason Wright of Pennsylvania State University. If the unusual orbit of this planet, which weighs about five Jupiter masses, were transferred to the solar system, it would come close to the sun within a tenth of the distance of Mercury, and then move out beyond the orbit of the Earth and back again. .
The long-sought connection
“Such highly eccentric planets are extremely rare,” emphasizes Wright’s colleague Suvrath Madevan. So far, astronomers know of only one extrasolar gas giant, HD 80606b, which comes close to the one found now with an imbalance of 0.93. What’s more: TIC 241249530b also orbits its star retrograde – opposite to its star’s direction of rotation. This is also very rare for the planets and usually indicates catastrophic events or major disruptions in their development.
According to the astronomers, the large orbit of this gas giant suggests that they have finally found the transition phase to a hot Jupiter. “We cannot go back in time to see planetary migration in real time. But this exoplanet gives us insight into this migration process,” explains Gupta. Accordingly, TIC 241249530b is already approaching a close orbit of the star.
TIC 241249530b: A gas giant transitioning to hot Jupiter.© NOIRLab Astro
From icy to hot Jupiter
The astronomers used a simulation to recreate how this exoplanet will change from a distant frozen gas giant to a Hot Jupiter. “When this planet formed, it was a frosty world,” says co-author Sarah Millholland of the Massachusetts Institute of Technology (MIT), describing the origins of the gas giant. It was probably thrown out of orbit by the second smaller star in this binary star system as a result of turbulent effects.
As a result of this event, TIC 241249530b is now oscillating back and forth between the extreme star vicinity and the region outside the planetary system – yet. Because every time it passes its star, its gravitational pull slows it down a little. Over time, the gas giant’s orbit will continue to shorten and become more circular. “In about a billion years Jupiter will get hot there,” says Millholland. Then the exoplanet will orbit its star so closely that it will only need a few days to complete one orbit. (Nature, 2024; two: 10.1038/s41586-024-07688-3)
Quelle: National Optical-Infrared Astronomy Research Laboratory (NOIRLab), Massachusetts Institute of Technology, Penn State
July 18, 2024 – Nadja Podbregar
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