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There’s more to a distant gas giant than meets the eye.
Astronomers have discovered that the exoplanet, or “exoplanet,” not only has one of the strangest orbits ever discovered, but is also transforming into a “hot Jupiter” world. Understanding this transformation could help scientists better understand how worlds in this curious class form.
The exoplanet, designated TIC 241249530 b and located about 998 light-years from Earth, was first discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) in January 2020 when the planet passed in front 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. That proximity means it completes an orbit in just 15.2 Earth days. But that’s not what’s so extreme about this planet’s orbit.
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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 b has one of the most elongated and flattened orbits astronomers have ever seen. In addition, the Jupiter-sized planet orbits its star “backwards” compared to the star’s rotation.
Hot Jupiters, however, are exoplanets that orbit their host planets at distances that allow them to complete a 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 are puzzled by how hot Jupiters get so close to their parent stars, with scientists suggesting that these planets form farther out from their stars and then migrate inward.
Yet the early stages of this migration process remain frustratingly elusive, even after astronomers have observed and confirmed at least 5,600 exoplanets.
A team of astronomers used two instruments on the WIYN 3.5-meter telescope at Kitt Peak National Observatory (KPNO) to observe TIC 241249530 b, revealing that it is an early-stage hot Jupiter.
“Astronomers have been looking for exoplanets that are likely progenitors of hot Jupiters, or intermediate products of the migration process, for more than two decades, so I was very surprised — and excited — to find one,” team leader Arvind Gupta, a NOIRLab postdoctoral researcher, said in a statement. “It’s exactly what I was hoping to find.”
A hot Jupiter in the making
The scientists first used the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) to remove the “blinking” patterns caused by Earth’s atmosphere. They also reduced noise from other light sources that could pollute the signal from the star TIC 241249530 as its planet moves past the star.
They then measured the velocity of the exoplanet around the star using the NEID spectrograph to determine the shift in light from the star.
“NESSI gave us a sharper view of the star than would otherwise have been possible, 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 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 being a perfectly circular orbit and 1 being highly elliptical.
Dwarf planet Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25, for comparison, 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 orbit of any other exoplanet ever found using the transit method of detecting exoplanets.
Interestingly, there is another planet with a more flattened orbit, HD 20782 b, a gas giant 1,117 light-years away. Its orbit has an eccentricity of 0.956, but this world was not discovered by the transit method.
If TIC 241249530 b were placed in the Solar System, its orbit would bring it 10 times closer to the Sun than Mercury (which is about 3 million miles or 4.8 million kilometers) and right at 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 swing from that of a nice summer day on Earth to warm enough to melt lead.
The motion of TIC 241249530 b around its star also had another unusual feature. The planet orbits its star in the opposite direction to the star’s rotation, something called “retrograde motion.” This is something rarely seen in exoplanets and a feature only demonstrated by two planets in the solar system, Venus and Uranus.
Both aspects of TIC 241249530 b’s orbit gave the team a hint about its impending transformation into a hot Jupiter. The team thinks that as this highly eccentric orbit brings the planet closer to its star, its orbit will begin to fill in like the 2D shadow of an inflating beach ball. This is expected because tidal forces generated by the star’s gravity pull orbital energy away from the exoplanet.
As the planet’s orbit becomes more “circular,” it will also become smaller, bringing TIC 241249530 b closer to its star and giving the planet a year of less than 10 Earth days. This is a sign that the transformation into a hot Jupiter is complete.
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TIC 241249530 b is only the second exoplanet discovered that appears to be in the pre-migratory phase of a hot Jupiter. Both TIC 241249530 b and the previous example of such a hot Jupiter progenitor appear to support the transformation of higher-mass gas giants into hot Jupiters via their migration from highly eccentric orbits to closer and more circular orbits.
“While we can’t really press rewind and watch 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 story of how hot Jupiter formed.”
The team’s research was published July 17 in the journal Nature.