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There’s more to a distant gas giant planet than meets the eye.
Astronomers have discovered that an extrasolar planet, or “exoplanet,” not only has one of the most peculiar orbits ever seen, but it also evolved into a “hot Jupiter” world. Understanding this transformation could help scientists build a better picture of how this intriguing class of worlds came to be.
Designated TIC 241249530 b and located about 998 light-years from Earth, this exoplanet was first detected by NASA’s Transiting Exoplanet Survey Satellite (TESS) when it crossed, or “swept”, the face of its parent star in January 2020.
The planet orbits its star, TIC 241249530, at a distance of about 12% of the distance between Earth and the Sun. That closeness means it completes its 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 a degree of flattening that astronomers call “eccentricity.” TIC 241249530 b has one of the most taut and flattened orbits astronomers have ever seen. Additionally, the Jupiter-sized planet orbits “backwards” around its star compared to the star’s rotation.
However, hot Jupiters are exoplanets that orbit their planets at distances that would take them a year to complete in just 10 Earth days or less. This means that TIC 241249530 b is not a hot Jupiter — at least not yet. For now, how hot Jupiters get so close to their parent stars is a puzzle for astronomers, with scientists suggesting that these planets form farther away from their stars and then migrate inward.
But even after astronomers have observed and confirmed at least 5,600 exoplanets, the early stages of this migration process remain frustratingly unclear.
A team of astronomers observed TIC 241249530 b using two instruments on the WIYN 3.5-meter Telescope at Kitt Peak National Observatory (KPNO) and revealed that it is an early-stage hot Jupiter.
“Astronomers have been searching for exoplanets that are possible precursors of hot Jupiters or intermediate products of the migration process for more than two decades, so I was very surprised and excited when I found one,” team leader Arvind Gupta, a NOIRLab postdoctoral researcher, said in a statement. “It was exactly what I was hoping to find.”
A hot Jupiter is forming
Scientists first used the NN-EXPLORE Planet and Star Spot Imager (NESSI) to eliminate “glow” patterns caused by Earth’s atmosphere and reduce noise from other light sources that could contaminate the signal as it passes over the face of star TIC 241249530.
They then used the NEID spectrograph to measure the exoplanet’s speed around the star and determine the star’s change in light.
“NESSI gave us a sharper image of the star than would otherwise be possible, and NEID precisely measured the star’s spectrum to detect shifts caused by the influence of the orbiting exoplanet,” Gupta said.
The team’s analysis of this spectrum confirmed that TIC 241249530 b has a mass about five times that of Jupiter. The research 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 indicating a perfectly circular orbit and 1 indicating a highly elliptical orbit.
The dwarf planet Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25, for context, while Earth’s nearly perfectly circular orbit has an eccentricity of 0.02. TIC 241249530 b’s orbit has an eccentricity of 0.94, which is more eccentric than the orbits of all other exoplanets found using the transit method of exoplanet detection.
Of note, there is another planet with a flatter orbit, HD 20782 b, a gas giant located 1,117 light-years away. Its orbit has an eccentricity of 0.956, but it has not been discovered using the transit-world method.
If TIC 241249530 b were placed in the solar system, its orbit would bring it 10 times closer to Mercury (about 3 million miles or 4.8 million kilometers) and then it would reach Earth’s maximum distance from the sun (about 95 million miles or 153 million kilometers). This would cause temperature variations on TIC 241249530 b to range from a beautiful summer day on Earth to hot enough to melt lead.
TIC 241249530 b’s motion around its star had another unusual feature. The planet orbits its star in the opposite direction of the star’s rotation, a process called “retrograde motion.” This is something rarely seen in exoplanets and is a feature only exhibited by two other solar system planets, Venus and Uranus.
Both of these aspects of TIC 241249530 b’s orbit alerted the team that it was about to become a hot Jupiter. The team thinks 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 pushing orbital energy away from the exoplanet.
The planet’s orbit will shrink as it “circularizes,” bringing TIC 241249530 b closer to its star and giving it a year of less than 10 Earth days, signaling that its transformation into a hot Jupiter is complete.
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TIC 241249530 b is the second exoplanet discovered to appear to be in a hot Jupiter pre-migration phase. Both TIC 241249530 b and the previous example of such a hot Jupiter precursor appear to favor the transformation of high-mass gas giants into hot Jupiters via migration from highly eccentric orbits to tighter, more circular orbits.
“While we can’t press rewind and watch the planetary migration process in real time, this serves as a kind of snapshot of the exoplanet migration process,” Gupta concluded. “Planets like this are incredibly rare and elusive, and we hope they help us unravel the story of hot Jupiter formation.”
The team’s research was published in the journal Nature on July 17.