The planets we know best have almost circular orbits. Paths stretched to run from the inner Solar System to the icy outer reaches are restricted to small asteroids, and particularly comets. So astronomers were surprised to discover a planet with at least three Jupiter masses orbiting a star in a very elongated orbit.
On average HR 5183b orbits its sun-like star at a distance just less than Uranus’ from the Sun, but like the proverbial economist with feet in an icebox and head in an oven, averages can deceive. If superimposed on the Solar System, HR 5183b would voyage from the asteroid belt to beyond Neptune.
A perfectly round orbit has zero eccentricity, while anything above 1 means an unbound visitor like 'Oumuamua. Some comets have eccentricities of 0.99, but Mars is the highest of the Solar System's planets at 0.09, unless you count Pluto's 0.25.
The average eccentricity for planets that are part of multiple systems is just 0.04, but one estimate is that where a star has only one companion the average is 0.32. Even so, HR 5183b’s eccentricity of 0.84 is exceptional.
"This planet is unlike the planets in our solar system, but more than that, it is unlike any other exoplanets we have discovered so far," said Dr Sarah Blunt, who recently graduated from Caltech, in a statement.
Almost all the planets we have found were detected using methods that favor those close to their star. Finding planets using the “Doppler wobble” method, for example, involves tracking them for at least a complete orbit.
However, HR 5183b takes an estimated 74 years to return to its starting position, and we have less than 30 years of observations of its parent star. However, the extraordinary nature of HR 5183b’s orbit allowed astronomers to find it in that time.
"This planet spends most of its time loitering in the outer part of its star’s planetary system in this highly eccentric orbit, then it starts to accelerate in and does a slingshot around its star," explained co-author Professor Andrew Howard. "We detected this slingshot motion. We saw the planet come in and now it's on its way out. That creates such a distinctive signature that we can be sure that this is a real planet, even though we haven't seen a complete orbit." No planet has previously been found on such a long orbit using this method.
The finders propose HR 5183b once had a companion of similar mass and their interactions created the extraordinary orbit, destroying the other planet. In the process any smaller planets would have been annihilated, leading Howard to compare it to a wrecking ball.