There’s more to discover about a distant gas giant planet than you might think.
Astronomers have discovered that the exoplanet, or extrasolar planet, not only has one of the strangest orbits ever observed, but it is also transforming into a “hot Jupiter”-type world. Understanding this transformation could help scientists better understand how worlds in this curious category form.
The exoplanet, designated TIC 241249530 b and located about 998 light-years from Earth, was first detected by NASA’s Transiting Exoplanet Survey Satellite (TESS) in January 2020 when it crossed, or “transited,” the face of its parent star.
The planet orbits its star, TIC 241249530, at a distance of about 12% of the distance between Earth and the Sun. This proximity means that it completes a full orbit in just 15.2 Earth days. But that’s not what makes this planet’s orbit so extreme.
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Most planets do not have perfectly circular orbits. Instead, most planetary orbits are elliptical with some degree of flattening, which astronomers call “eccentricity.” TIC 241249530 has one of the most stretched and flattened orbits astronomers have ever observed. Additionally, this Jupiter-sized planet orbits its star “backwards” relative to the star’s rotation.
However, hot Jupiters are exoplanets that orbit their planet at distances that allow them to travel a full year in just 10 Earth days or less. This means that TIC 241249530 b is not a hot Jupiter — at least not yet. Currently, astronomers don’t really understand how hot Jupiters can get so close to their parent star, with scientists suggesting that these planets form farther away from their star and then migrate inward.
Yet the early stages of this migration process remain frustratingly elusive, even after astronomers have observed and confirmed the existence of at least 5,600 exoplanets.
A team of astronomers used two instruments on the 3.5-meter WIYN telescope at Kitt Peak National Observatory (KPNO) to observe TIC 241249530 b and reveal it as an early hot Jupiter.
“Astronomers have been searching for more than two decades for exoplanets that are likely precursors to hot Jupiters or intermediate products of the migration process, so I was very surprised – and excited – to find one,” Arvind Gupta, team leader and NOIRLab postdoctoral researcher, said in a statement. “It’s exactly what I was hoping to find.”
A hot Jupiter in the making
Scientists first used the NN-EXPLORE Exoplanet and Starspot Imager (NESSI) to remove “flickering” patterns caused by Earth’s atmosphere, as well as to reduce noise from other light sources that could pollute the signal from the star TIC 241249530 as its planet transits its face.
Then they measured the speed of the exoplanet around the star using the NEID spectrograph to determine the shift in the star’s light.
“NESSI gave us a sharper view of the star than would have been possible otherwise, and NEID precisely measured the star’s spectrum to detect shifts in response to the orbiting exoplanet,” Gupta said.
The team’s analysis of this spectrum confirmed that TIC 241249530 has a mass about five times that of Jupiter. The study also revealed the planet’s extremely eccentric orbit. The eccentricity of a planet’s orbit is measured on a scale of 0 to 1, with 0 being a perfectly circular orbit and 1 being a highly elliptical orbit.
The dwarf planet Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25, while Earth’s nearly perfectly circular orbit has an eccentricity of 0.02. The orbit of TIC 241249530 b has an eccentricity of 0.94, which is more eccentric than the orbit of any other exoplanet ever discovered using the transit method of exoplanet detection.
There is another planet with a flatter orbit, HD 20782 b, a gas giant located 1117 light-years away. Its orbit has an eccentricity of 0.956, but this world was not discovered using the transit method.
If TIC 241249530 b were placed in the solar system, its orbit would take it 10 times closer to the Sun than Mercury (which is about 4.8 million kilometers away) and would be at Earth’s maximum distance from the Sun (about 153 million kilometers). This would cause temperatures on TIC 241249530 b to vary from those of a bright summer’s day on Earth to temperatures hot enough to melt lead.
The motion of TIC 241249530 b around its star also has another unusual feature. The planet orbits its star in the opposite direction to the star’s rotation, a so-called “retrograde motion.” This is a phenomenon rarely observed in exoplanets and a characteristic that only two planets in the solar system, Venus and Uranus, demonstrate.
These two aspects of TIC 241249530 b’s orbit alerted the team to its imminent transformation into a hot Jupiter. The team believes that as this highly eccentric orbit brings the planet closer to its star, the orbit will begin to fill out like the 2D shadow of an inflating beach ball. This is expected to happen because tidal forces generated by the star’s gravity are pulling orbital energy away from the exoplanet.
As the planet’s orbit “circulars,” it will also shrink, bringing TIC 241249530 b closer to its star and giving it a year that lasts less than 10 Earth days, signaling that the transformation into a hot Jupiter is complete.
TIC 241249530 b is only the second exoplanet discovered that appears to be in the pre-hot Jupiter migration phase. Both TIC 241249530 b and the previous example of such a hot Jupiter precursor appear to support the transformation of higher mass gas giants into hot Jupiters via their migration from highly eccentric orbits to tighter, more circular orbits.
“While we can’t really go back and observe the process of planetary migration in real time, this exoplanet serves as a kind of snapshot of the migration process,” Gupta concluded. “Planets like this are incredibly rare and hard to find, and we hope it can help us unravel the history of hot Jupiter formation.”
The team’s research was published July 17 in the journal Nature.