This Backwards Orbiting Planet Is The Most Eccentric We Have Ever Seen

A new planet has been discovered that has the most elongated orbit of any of the 5,600 we have found. On top of that, it’s going the wrong way. Strange as this is, it could provide the missing link between planetary formation, and the category that made up most of our initial discoveries of planets beyond the Solar System.

When the capacity to detect planets around other stars was first developed, astronomers experienced a shock. Instead of finding systems like our own, most of the early discoveries were what became known as “ hot Jupiters ”, gas giants that orbited closer to their stars than Mercury, producing tremendous temperatures.

This was an artifact of detection methods that could more easily pick up planets with larger masses and those with short orbits. As our planetary sample has grown, the proportion of hot Jupiters has shrunk, to now be less than 10 percent. Nevertheless, having nothing remotely like this in our Solar System, the frequency of such objects was unexpected. The search began to explain how such planets came to be, which makes finding one suspected of being a hot Jupiter in the making a major prize.

Such a world has now been found orbiting the star TIC 241249530, 1,100 light years from Earth. On average, the planet TIC 241249530b has an orbital distance of about two-thirds Earth’s and its year is 167 days, between that of Mercury and Venus. With a star 27 percent more massive than the Sun, that makes it a warm Jupiter at least.

However, like the old story of the billionaire who transforms the mean wealth of the people in a room by walking in, averages can be very deceptive. TIC 241249530b gets ten times closer to its star than Mercury does, before whipping out again to further out than the Earth.

Astronomers refer to how far a planet’s orbit differs from circular as its “eccentricity”. TIC 241249530b’s eccentricity is 0.94, where anything equal or greater than 1 leaves the system . We know of comets that are much more eccentric than TIC 241249530b, but the only candidate planet HD 20782b , has competing estimates .

This is an eccentric world in the colloquial sense as well. All the planets in our Solar System orbit in the same direction the Sun rotates, a big clue to common origins in the same spinning gas cloud. This is one respect in which the overwhelming bulk of planets we have found orbiting other stars resemble those we know. TIC 241249530b is a very rare exception indeed.

MIT’s Dr Sarah Millholland, part of the team that discovered TIC 241249530b, thinks the orbit will shorten until it’s a more regular hot Jupiter, although it will still be orbiting backwards.

“This new planet supports the theory that high eccentricity migration should account for some fraction of hot Jupiters,” she said in a statement . “We think that when this planet formed, it would have been a frigid world. 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.”

In the meantime, she noted; “There must be really radical seasons and an absolutely scorched atmosphere every time it passes close to the star.”

Our understanding of planet formation doesn’t allow very large planets to form close to their stars. Unless we have that very wrong, hot Jupiters must originate further out, at the sort of distances where our own gas giants now orbit, and migrate in. That creates two obvious questions: how does that happen? And why didn’t Jupiter and Saturn do the same thing?

The second puzzle is particularly crucial, because if a gas giant had migrated through the Solar System to take up residence close to the Sun, it would almost certainly have annihilated the Earth – or thrown it into deep space – in the process. It’s only in systems without hot Jupiters that there can be lifeforms capable of spotting them elsewhere and wondering what is going on.

TIC 241249530b is unlikely to provide a universal answer, but it does offer clues to the origins of some hot Jupiters. The star it orbits, TIC 241249530, is circled by a distant small star. Gravitational interference from this companion may have pushed the planet out of its conventional path at the sort of distance where gas giants normally form.

We know not all hot Jupiters are in binary systems, so there must be other explanations in some cases, but it makes such events less likely in a system like our own.

Despite TIC 241249530b’s rareness, it’s not unique. The previous record for planetary eccentricity is held by HD80606b, which is suspected of being at a later stage of a similar migration.

“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,” said Penn State Professor Jason Wright in a different statement .  The authors propose that smaller planets are more tidally disrupted, and move much more rapidly to their final hot orbit.

“Planets like this are hard to find and we hope it can help us unravel the hot Jupiter formation story,” said first author Arvind Gupta, a doctoral student at Penn State.

TIC 241249530b was first spotted by the Transiting Exoplanet Survey Satellite (TESS) detecting a dip in its star’s brightness in 2020. It was only when other instruments were used to confirm the observation that its exceptional characteristics were revealed, including the fact it is five times as massive as Jupiter.

Inevitably, the discoverers are hoping for time on the JWST to observe the effects of such a remarkable orbit. "We’re especially interested in what we can learn about the dynamics of this planet's atmosphere after it makes one of its scorchingly close passages to its star," Wright said.

The study is published in the journal Nature .