Giant planet slingshots around star
By Amanda Jermyn



There’s always something new under the Sun, and oftentimes well beyond it. Just recently, astronomers discovered a giant planet three times the mass of Jupiter that follows a long, elliptical (egg-shaped) path around its star. When planets coalesce from the disks of material left over after stars form they usually start out in circular orbits around their star. However, if this newfound planet were placed in our solar system, at times its orbit would be somewhere between Mars and Jupiter, and at others, beyond Neptune.

While other giant planets with highly elliptical orbits have been found around some stars none have been located at the outer reaches of their star systems like this one. According to Caltech’s Sarah Blunt, lead author on this study published in The Astronomical Journal, “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. Other planets detected far away from their stars tend to have very low eccentricities, meaning that their orbits are more circular. The fact that this planet has such a high eccentricity speaks to some difference in the way that it either formed or evolved relative to the other planets.”

The newfound planet, called HR 5183 b, orbits the star HR 5183 in the constellation Virgo. It was discovered using the radial velocity method, which detects the wobble of the host star in response to the gravitational pull of the planet. This method, however, usually requires observations taken of the planet’s complete orbit. For planets orbiting far from their stars, as this one does, this method is usually not practical as such planets can take hundreds of years to orbit their stars.

Astronomers from the California Planet Search, led by Caltech’s Andrew Howard, have been watching the star HR 5183 for two decades, but this should not have been nearly enough time to detect the new planet, given that it takes somewhere between 45 and 100 years to circle its star. Instead, they found it because of its peculiar orbit. According to Howard, “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. 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.”

We know that the circular orbits of planets can be disrupted by interactions with other planets, so scientists think the most plausible explanation for the newfound planet’s eccentric, elongated orbit is that it was given a gravitational push by a close encounter with another object, most likely a nearby planet of similar size.

The discovery of this planet with a peculiar orbit tells us that we have a lot to learn about the evolution of solar systems elsewhere in the universe, and that the ones we have yet to discover may be quite different from our own.

Join the Springfield Stars Club on Tuesday, October 22nd at 7pm at the Springfield Science Museum for a talk by Ed Faits on “Comets, Asteroids and other Wanderers.” Richard Sanderson will also give a brief talk on “The November 11th Transit of Mercury.” Ed Faits, an avid astronomer, is a founding director and past president of the Arunah Hill Natural Science Center in Cummington, MA and a past president of the Springfield Stars Club. Richard Sanderson is the recently retired curator of physical science at the Springfield Science Museum and a long time member of the Springfield Stars Club. He is an astronomy writer and co-author of the 2006 book, “Illustrated Timeline of the Universe.” Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

Also, on Friday, November 1st at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Crystal Mengele and Marguerite Seuffert will present “Stars Game Night,” an astronomy quiz show with audience participation. A fee of $3 for adults and $2 for children under 18 will be charged.