Orbital mayhem around a red dwarf

December 18, 2017, University of Geneva
A stellar regatta around GJ436. Space boaters parked in the equatorial plane of the star wait for the hairy planet to emerge from this plane to 'ride' the planetary wind with the help of kite-surfing vessels. As they rise above the poles of the star, they obtain a breathtaking view of the entire planetary system and can glimpse at the mysterious disruptive planet, which appears as a bright spot in the background. Credit: © Denis Bajram

In the collective imagination, planets of a solar system all circle in the equatorial plane of their star. The star also spins, and its spin axis is aligned with the spin axes of the planetary orbits, giving the impression of a well-ordered system. But nature is capricious, as an international team led by researchers from the University of Geneva (UNIGE), Switzerland, has detected a planetary system turned upside down. This discovery is published this week in the prestigious journal Nature.

GJ436 is a star that hosts a planet nicknamed "the hairy planet," which evaporates like a comet. In this study, researchers at UNIGE showed that in addition to its huge cloud of gas, the planet GJ436b also has a very special . It is polar: Instead of orbiting in the equatorial plane of the star, the planet passes almost above the stellar poles.

The orbital inclination of this plane is the last piece of a puzzle that has baffled astronomers for 10 years. Unlike the planets of our solar system, whose orbits almost form perfect circles, tGJ436 forms a very flat or strongly eccentric ellipse—that is, its distance to the star varies along its orbit. "This planet is under enormous because it is incredibly close to its star, barely 3 percent of the Earth-sun distance," explains Vincent Bourrier, researcher at the Department of Astronomy of the UNIGE Faculty of Science. "The star is a red dwarf whose lifespan is very long, and the tidal forces it induces should have since circularized the orbit of the planet, but this is not the case," he says.

Orbital architectures of are fossil records that tell us how they have formed and evolved. A planet disturbed by the passage of a nearby star or by the presence of other massive in the system will keep track of it in its orbit. "Even if we have already seen misaligned planetary orbits, we do not necessarily understand their origin. This is the first time we've measured the architecture of a planetary system around a ," says Christophe Lovis, a UNIGE researcher and co-author of the study.

The existence of an unknown, more massive and more distant disturbing planet would explain why GJ436b is not on a circular orbit: "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," says Hervé Beust, who did the orbital calculations.

These same calculations also predict that the planet has not always been so close to its star, but might have come near it recently (on a cosmic scale). Thus, the "evaporating planet" would have been pushed towards the star by the gravity of a yet-undetected companion. For Vincent Bourrier, the hunt continues: "Our next goal is to identify the mysterious planet that has upset this planetary system."

Explore further: We've found an exo-planet with an extraordinarily eccentric orbit

More information: Vincent Bourrier et al, Orbital misalignment of the Neptune-mass exoplanet GJ 436b with the spin of its cool star, Nature (2017). DOI: 10.1038/nature24677

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Knecht
not rated yet Dec 18, 2017
Have astrophysicists considered capture of an unbound "rogue" planet into this bizarre orbit?
maholmes1
5 / 5 (1) Dec 18, 2017
Pluto and Eris have orbits that are quite eccentric (0.25 and 0.44 respectively) and inclined to the ecliptic (17 and 44 degrees, respectively) and Makemake and Haumea have orbits with eccentricity values of 0.16 and 0.19, respectively, and orbital inclinations of 29 and 28 degrees, respectively. Ceres' orbital eccentricity is 0.08 and its orbit is inclined 11 degrees to the ecliptic. I read some individual once stating that he always "knew Pluto wasn't a planet" by the eccentricity and marked inclination of its orbit. So is this object not a planet in mental-IAU-land, too, despite orbiting its sun independently and being non-self-luminous? I take it it's in hydrostatic equilibrium. But in the article it's referred to repeatedly as a planet.
rrwillsj
1 / 5 (1) Dec 18, 2017
I would assume (Oh, oh. That word!) that from what little data is available and yet to verified? The odds are this 'drunkard's walk' object will probably qualify as a planet.

As for the dubious guess that it may not be native to that star system? Try to estimate the amount of energy and what would be the source of that energy to capture an interstellar flyby. No. really. You do it, it's your guess and I'm too lazy.

How about a compromise? If this does turn out to be a multi-star system or with giant Jupiters or both or something entirely unexpected?

Perhaps the polar-orbiting planet was captured from a companion stellar?
TechnoCreed
not rated yet Dec 18, 2017
Pluto and Eris have orbits that are quite eccentric (0.25 and 0.44 respectively) and inclined to the ecliptic (17 and 44 degrees, respectively) and Makemake and Haumea have orbits with eccentricity values of 0.16 and 0.19, respectively, and orbital inclinations of 29 and 28 degrees, respectively. Ceres' orbital eccent... and bla, bla.

Except that this object is around 8000x as massive as pluto.
maholmes1
5 / 5 (1) Dec 18, 2017
Pluto and Eris have orbits that are quite eccentric (0.25 and 0.44 respectively) and inclined to the ecliptic (17 and 44 degrees, respectively) and Makemake and Haumea have orbits with eccentricity values of 0.16 and 0.19, respectively, and orbital inclinations of 29 and 28 degrees, respectively. Ceres' orbital eccent

Except that this object is around 8000x as massive as pluto.


Yet this object is presumably in hydrostatic equilibrium also, and orbits its parent star independently as Pluto does.
mackita
not rated yet Dec 18, 2017
The retrograde spins are selected out, since most objects have pro-grade spins, and most moons have prograde orbits of their respective planet. Retrograde (spin) planets decay inward: As it moves through a protoplanetary cloud, it's slowed down, but it's spin on the leading edge of the proto-planet would tend to kick dust outward, producing a net-inward acceleration, while slowing down it' tangential velocity. Thus such an object should begin to decay inward and fall into the Sun. The net angular momentum is still prograde, since much of it is in the form of the orbital velocity, so this does not slow the sun's rotation. Prograde proto-planets decay outward, because when they pass into an average cloud of material, their spin kicks some of the dust inward, which produces a net outward thrust. Their forward velocity is slowed, but that is OK: they remain stable because wider orbits require less tangential velocity to remain stable.
wduckss
1 / 5 (1) Dec 19, 2017
"Gliese 436b, 22 M⊕; Semimajor axis(AU) 0.0291 ± 0.0004; Orbital period(days); 2.64385 ± 0.00009; Eccentricity 0.150 ± 0.012; 4.33 ± 0.18 R" https://en.wikipe...y_system

Authors need to re-enter the basics of physics. They should observe the object 'Oumama and make sure that: "... If there is no rotation, there is also no orbit, no matter what the speed of the incoming object is." http://www.svemir...n-object

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