Highly eccentric exoplanet helps solve hot Jupiter mystery

An international team of astronomers has found a huge exoplanet with a “super-eccentric” orbit, more elliptical than any previously discovered.

The bizarre planet, which also orbits backwards, could help to solve the mystery of how “hot Jupiters” form.

The exoplanet, called TIC 241249530b, is about 1,100 light-years away from Earth and 5 times more massive than Jupiter.

The researchers have dubbed it a “Warm Jupiter.” They’ve published their findings in Nature.

“We caught this massive planet making a sharp, hairpin turn during its close passage to its star,” says co-author Professor Suvrath Mahadevan, an astronomer at The Pennsylvana State University (PSU), US.

“Such highly eccentric transiting planets are incredibly rare — and it’s really amazing that we were able to discover the most eccentric one.”

Hot Jupiters are gas giant exoplanets that orbit very close to their stars. Astronomers think it’s unlikely that these planets formed right next to their stars, but instead formed further away and migrated in.

This exoplanet could be on the way to hot Jupiter status because of its elongated orbit.

When it’s closest to its sun, the planet is ten times closer than Mercury is to our Sun. At its furthest distance, it’s slightly further than the Earth-Sun distance.

“We think that when this planet formed, it would have been a frigid world,” says Dr Sarah Millholland, an astronomer at the Massachusetts Institute of Technology, US.

“And because of the dramatic orbital dynamics, it will become a hot Jupiter in about a billion years, with temperatures of several thousand kelvin. So it’s a huge shift from where it started.”

Astronomers measure the shape of planetary orbits by “eccentricity”. An orbital eccentricity of 0 represents a perfect circle, while 1 is the most eccentric an orbit can be.

TIC 241249530b has an orbital eccentricity of 0.94, the highest ever recorded. The previous record was also set by a hot Jupiter, called HD 80606b, with an eccentricity of 0.93.

“This new planet experiences really dramatic changes in starlight throughout its orbit,” says Millholland.

“There must be really radical seasons and an absolutely scorched atmosphere every time it passes close to the star.”

The exoplanet also orbits in retrograde: that is, it orbits in a direction opposite to the way its star spins.

“This is the most eccentric transiting planet known and will prove to be as important as the previous record holder, HD80606b, which likewise has a wacky orbit highly misaligned with its host star’s spin,” says Professor Jason Wright, an astronomer at PSU.

“These two highly eccentric planets have been ‘caught in the act’ of evolving towards hot Jupiter status. Like HD80606b, this planet is many times Jupiter’s mass, suggesting this channel for forming hot Jupiters might be one only the most massive planets can take.”

The team first detected the exoplanet with NASA’s Transiting Exoplanet Survey Satellite (TESS) in January 2020, when the star TIC 241249530 dimmed slightly in a way that’s consistent with a gas giant moving in front of it.

They confirmed their observations with instruments at the WIYN 3.5-metre telescope at the Kitt Peak National Observatory, which is run by the US National Science Foundation’s NOIRLab.

“While we can’t exactly press rewind and watch the process of planetary migration in real time, this exoplanet serves as a sort of snapshot of the migration process,” says lead author Dr Arvind Gupta, a postdoctoral researcher at NOIRLab.

“Planets like this are hard to find and we hope it can help us unravel the hot Jupiter formation story.”