An international team of researchers led by scientists from the University of Geneva has discovered that an already odd exoplanet is following an extraordinary elliptical orbit. 

The planet, dubbed GJ 436b, previously hit the headlines in 2015 when astronomers found its atmosphere was leaving a tail, rather like a comet.

You might think that planets orbit stars in a reasonably orderly way – that they stay in almost circular orbits around a star’s equator. This exoplanet is very different.

Gliese 436b was already a noteworthy planet. Discovered in 2004, it was one of the first “hot Neptunes” to be identified, a class of exoplanet similar in mass to Neptune but orbiting closer to its star than we do to our Sun.

Writers have called it the “strangest exoplanet ever” due to its hot ice and evaporating atmosphere. A paper published today in Nature bolsters its claim to being the most bizarre exoplanet out there.

While most exoplanets orbit in the equatorial plane, Gliese 436b whips around its star’s poles, and unlike most other exoplanets, Gliese 436b also has a highly eccentric (elliptical) orbit. It’s also just over 2.5 million miles away from its star – which might seem like a long distance, but our Earth is more than 30 times further away from the Sun.

What this means is that some parts of Gliese 436b experience a much larger gravitational force from the star than other parts – scientists call this a tidal force.

“This planet is under enormous tidal forces because it is incredibly close to its star, barely 3% of the Earth-Sun distance,” says Vincent Bourrier, a researcher from the University of Geneva. “The star is a red dwarf whose lifespan is very long; the tidal forces it induces should have since circularized the orbit of the planet, but this is not the case!”

Gliese 436b’s bizarre orbit might provide scientists with clues to the formation and evolution of the planetary system. For many years, astronomers have proposed that other planets are orbiting Gliese 436, but efforts to find them have proven inconclusive.

“Even if we have already seen misaligned planetary orbits, we do not necessarily understand their origin, especially since here it is the first time we measure the architecture of a planetary system around a red dwarf,” explains Christophe Lovis, one of the co-authors of the study.

In the absence of other planets, Gliese 436b’s orbit should have settled into a more circular, less eccentric shape. Could another, as yet unobserved planet be preventing this?

“If that is true, then our calculations indicate that not only would the planet not move along a circle around the star, as we’ve known for 10 years, but it should also be on a highly inclined orbit. That’s exactly what we just measured!” adds Hervé Beust, a researcher at IPAG (Grenoble Institute of Planetary Science and Astrophysics) who did the calculations.

Beust’s calculations also predict that Gliese 436b might have started life further out from its star, only being pushed closer inwards by another planet. “Our next goal is to identify the mysterious planet that has upset this planetary system,” says Vincent Bourrier.

While these calculations do not definitively prove that Gliese 436b has any companions, it gives astronomers some idea of what they might be searching for. The hunt for other planets circling Gliese 436b continues – and the results from this new paper might give it a better chance of succeeding.

Footnote: Gliese 436, the exoplanet’s host star, lies only  33 light-years away from us, in the constellation of Leo. But because it is a red dwarf, it is too dim to be seen with the unaided eye. You need a small telescope to spot it shining at magnitude 10.7.

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