There is 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 strangest orbits ever seen, but is also transforming into a “hot Jupiter” world. Understanding this transformation could help scientists build a better picture of how worlds in this special class form.
The exoplanet, designated TIC 241249530 ba and located about 998 light-years from Earth, was first detected by NASA’s Transiting Exoplanet Survey Satellite (TESS) in January 2020 as 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 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. Most planetary orbits are rather 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. Additionally, a Jupiter-sized planet orbits its star “backwards” relative to the star’s rotation.
However, hot Jupiters are exoplanets that orbit their 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. How hot Jupiters get so close to their parent stars is currently something of a puzzle for astronomers, with scientists suggesting that these planets form further away from their stars and then migrate inward.
However, the initial stages of this migration process have remained 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 ba and revealed it to be an early hot Jupiter.
“Astronomers have been looking for exoplanets that are likely progenitors of hot Jupiters, or that are intermediates in the migration process, for more than two decades, so I was very surprised – and excited – to find one,” team leader Arvind Gupta. NOIRLab postdoctoral researcher, said in a statement. “It’s exactly what I was hoping to find.
A hot Jupiter is brewing
The researchers first used the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) to remove “flickering” patterns caused by Earth’s atmosphere, as well as to reduce noise coming from other light sources that could contaminate the signal from the star TIC 241249530 as its planet passes its face.
Next, they measured the speed of the exoplanet around the star using the NEID spectrograph to determine the star’s shift in light.
“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 approximately five times that of Jupiter. The investigation 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.
For context, 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, an eccentricity greater than that of any other exoplanet ever found using the transit exoplanet detection method.
Remarkably, there is another planet with a more flattened 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 bring it 10 times closer to the Sun than Mercury (which is about 3 million miles, or 4.8 million kilometers) and then directly to Earth’s maximum distance from the Sun (about 95 million miles or 153 million kilometers). This would cause temperature changes at TIC 241249530 b to swing from nice summer days on Earth to hot enough to melt lead.
The motion of TIC 241249530 b around its star also had another unusual feature. A planet orbits its star in the opposite direction to the star’s rotation, which is called “retrograde motion”. This is something rarely seen in exoplanets, and only two planets in the solar system, Venus and Uranus, demonstrate the characteristic.
Both of these aspects of TIC 241249530 b’s orbit alerted the team to an impending transformation to a hot Jupiter. The team thinks that once this highly eccentric orbit brings the planet close to its star, the orbit will begin to fill in like the 2D shadow of an inflatable beach ball. This is expected to happen because tidal forces generated by the star’s gravity drain orbital energy from the exoplanet.
As the planet’s orbit “circles,” 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 discovered exoplanet that appears to be in the pre-migration phase of a hot Jupiter. Both TIC 241249530 b and a previous example of such a hot Jupiter precursor appear to support the transformation of higher-mass gas giants into hot Jupiters through their migration from highly eccentric orbits to tighter, more circular orbits.
“While we can’t exactly press rewind and watch the planetary migration process 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 this will help us unravel the story of hot Jupiter’s formation.”
The team’s research was published July 17 in the journal Nature.