The exoplanet GJ436b is further proof that more often than we probably realize, nature likes letting its freak flag fly.
GJ436b has a hydrogen tail, much like a comet. [Image by Mark Garlick/University of Warwick]
GJ436b, or Gliese 436b, is a very strange planet indeed. It certainly doesn’t fit the usual model of what we think planets and their host stars are like, and how they interact. In our minds and visualizations, probably, planets orbit their host star in the star’s equatorial plane. The host star, meanwhile, has a spin axis aligned with the spin axes of of the orbits of its planets. While many things in our world may fit in neat, predictable little boxes, some things that break the mold pop up now and then.
This strange exoplanet is one such example. The fact that it’s strange isn’t news; researchers have already found that GJ436b sports a hydrogen tail, much like a comet. In addition to this, however, researchers have also found that the planet has a special “polar” orbit as well.
GJ436b is called a "warm Neptune" because of its similarity in size to Neptune.
GJ436b is about 22 times larger than Earth, a “warm Neptune” orbiting a red dwarf star named GJ436. In spite of the fact that it’s much larger than Earth, it orbits its host star at a distance much closer than the one between Earth and the sun.
The planet is just four million kilometers away from its host star, which is a small distance in cosmological terms. This small distance is also the reason why the planet leaves behind a trail of hydrogen, since the heat of the star causes the GJ436b’s atmosphere to burn off.
Researchers also discovered that the planet’s orbit is a tight oval that runs over the host star’s pole, unlike other planets that follow a circular orbit in their host star’s equatorial plane. A new study now offers more insight into why this orbit is the way it is.
"This planet is under enormous tidal forces because it is incredibly close to its star, barely 3 percent of the Earth-Sun distance," said Vincent Bourrier, one of the researchers. "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!"
The interesting thing about orbits is that they’re like a record of a planet’s history—the way the planet formed and evolved over its lifetime. If a massive object passed near the planet, or there is another massive object in the same star system, the planet’s orbit will keep a record of it. It’s possible that a massive, distant, still-unknown object in the system is the reason for GJ436b’s strange polar orbit.
There's a mysterious object lurking around GJ436b, skewing its orbit and pushing it closer to its host star. [Image by ESA/Hubble]
"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,” says Hervé Beust, the researcher who was in charge of orbital calculations. “That's exactly what we just measured!"
It’s also possible that this mysterious object lurking around GJ436b pushed it closer to its host star. The planet has not always been that close to its star, and it may have moved nearer to GJ436 in more recent years. The next step for researchers, therefore, is to find and identify this mysterious, massive object.
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