An international team of researchers led by scientists
from the University of Geneva has discovered an exoplanet which follows an
extraordinary elliptical orbit.


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.

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Gliese 436b was already a noteworthy planet; it was one
of the first hot Neptunes to be discovered, 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 round 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 thirty 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.


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