This Planet Is Orbiting At Right Angles To Expectations

An artist's impression of the planet GJ436b and its parental star. It was already known the planet is evaporating away, but now we know its orbit goes over its star's poles. ESA/Hubble

A planet already considered one of the strangest we know has got weirder still. Not only is GJ436b evaporating like a comet, but its orbit takes it over the poles of its star, 90 degrees to the usual planetary path.

The planets of our own Solar System have well-behaved orbits. All are roughly in a plane, which is approximately aligned with the Sun's equator, something thought to be a product of the way planets form out of rotating disks.

Things get a bit messy out past the orbit of Neptune, which is contributing to the difficulty in finding planet nine, assuming it exists. However, it was anticipated inner planets around other stars would resemble our own in this regard, and most of them have. Systems with multiple planets like Trappist-1 and Kepler-90 exist in a plane that matches the rotation of their parental star.

So the University of Geneva's Dr Vincent Bourrier was surprised to discover that planet GJ436b has what is known as a polar orbit. This takes it over the poles of the star GJ436, only crossing the equator twice every orbit, rather than being permanently close to the star's equatorial region.

This is particularly odd, because GJ436b lies very close to its star, with an average distance of just 3 percent of that between the Earth and Sun. Polar orbits are unexpected anywhere, but as Bourrier notes in Nature: “Exoplanets orbiting close to cool stars are expected to be on circular, aligned orbits because of strong tidal interactions with the stellar convective envelope.”

GJ436b is certainly under phenomenal tidal forces, having previously puzzled astronomers with the observation that it has an unusually elongated orbit for such a close-in planet. Moreover, it is larger than its mass and location would predict, something astronomers explain as the product of hydrogen and helium boiling off it, puffing it up before they escape, and inspiring the nickname the hairy planet.

Bourrier and co-authors explain these strange features of GJ436b as the consequences of interactions with a much larger and more distant companion, which we have yet to detect otherwise. They propose that these interactions caused the Neptune-mass GJ436b to migrate inwards to its current location, which explains how it has not entirely evaporated away in GJ436b's 7-11 billion year lifetime.

GJ436 has a mass a little less than half that of the Sun. It rotates just once every 44 days, compared to the Sun's 24 days, increasing the difficulty of measuring the angle between its axis of rotation and the orbital plane of its planet. Having finally achieved this measurement, however, Bourrier is up for the next challenge of trying to find the more massive planet that has thrown GJ436b into such a strange orbit.

An imaginary fleet of spaceships orbiting GJ436, with an asteroid belt in a normal orbit, GJ436b in its polar orbit, and the planet that put it there off in the distance. Denis Bajram



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