Space

This wackballs planet has an incredibly strange orbit around its host star


For the most part, our Solar System is wonderfully ordered. The Sun spins on its axis. The planets, including Earth, orbit it in the same direction, arrayed on a nearly flat plane around the Sun’s equator.

 

The orbit of our Milky Way galaxy around its black hole is similarly ordered, and it was sort of assumed that other solar systems would fall into the same pattern. But, as we have been discovering more and more exoplanets, we’ve also been discovering weirdly tilted orbits – and this latest discovery may be the weirdest of all.

The planet is called GJ436b, and its orbit is so tilted, it circles its host star’s poles, instead of its equator. Which gives it a tight, oval orbit. 

That’s weird enough, but it’s just the latest in a increasing number of weird things about GJ436b and its star, GJ436, a “warm Neptune” orbiting a low-mass red dwarf.

Around 22 times the mass of Earth, GJ436b orbits its sun at a much closer distance than Neptune’s and even Earth’s, just 4 million kilometres (2.5 million miles).

This is close enough that the star burns off the planet’s upper atmosphere, giving GJ436b a hydrogen tail, much like that of a comet, that eclipses the star.

Also, GJ486b seemed to follow a tight oval around its star’s poles, unlike most planets that follow a circular, equatorial orbit. Now astronomers finally have more insight into what’s going on.

 

“This planet is under enormous tidal forces because it is incredibly close to its star, barely 3 percent of the Earth-Sun distance,” said first author Vincent Bourrier of the UNIGE Faculty of Science.

“The star is a red dwarf whose lifespan is very long, the tidal forces it induces should have since circularised the orbit of the planet, but this is not the case!”

So what’s happening here? A previous study suggested that a large, nearby planet could be perturbing the planet’s orbit, even if there is no other evidence of such a planet.

Such a planet – or a nearby star – tugging at GJ486b is still the most likely explanation, the researchers believe, keeping GJ436b zooming around GJ436’s poles like some kind of stellar deviant.

“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!” said co-author Christophe Lovis.

The team’s calculations also suggest that the presence of another planet or star in the mix could explain the other weirdness in this system.

 

For example, the researcher suggest GJ436b wasn’t always so close to GJ436 – but that the second planet could have pulled it in closer. The planet wasn’t always a comet-planet, but became one after its rude companion got pushy.

If the extremely tilted orbit is being caused by another planet farther out from GJ436b, it would constitute an example of the Kozai Mechanism, whereby the orbit of one body is perturbed by the orbit of another.

However, the second planet may need to be in a weird orbit of its own to affect GJ436b in such a manner – which in turn begs the question, how did that happen?

But one step at a time.

“Our next goal is to identify the mysterious planet that has upset this planetary system,” Bourrier said.

Then they can try to figure out the three-body dynamics at play.

The research has been published in the journal Nature.

 



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