Gang of Gas Giants May Have Tilted Distant Solar System

By Ken Croswell

Published on New Scientist (August 31, 2016)

A massive planet in a far-flung system has swivelled the orbits of its smaller neighbours.

Planets usually orbit near the plane of their star’s equator. For example, Earth’s orbit is tilted to the Sun’s equatorial plane by just 7.2 degrees.

But since 2008, we have seen extrasolar planets orbiting at huge angles to this plane. Some are even revolving backwards – in effect, their orbits have flipped over.

Multiple planets in one system can be tilted. In 2013, Daniel Huber, now at the University of Sydney in Australia, and his colleagues used the Kepler spacecraft to find two planets whose orbits are aligned with each other but tilted at about 45 degrees to the equatorial plane of their star, Kepler 56. One of these planets is twice as massive as Uranus, the other twice as massive as Saturn.

Meanwhile, ground-based observations indicated that the star’s velocity was changing, suggesting a third object’s gravity was tugging at it. But these observations spanned only three months, insufficient to determine whether the object was a planet or a star, let alone its mass or orbital period. Still, astronomers suspected it could be responsible for the tilt.

Now, after analysing four years of Kepler 56 observations, Justin Otor at Princeton University and his colleagues conclude that the culprit is probably an even more massive unseen planet. With at least 5.6 times Jupiter’s mass, it is tugging on its companions from an orbit much farther out. Whereas the misaligned planets are closer to their sun than Mercury is to ours, the new planet would be just beyond Mars if part of our solar system. The object orbits every 2.74 years.

Simulations suggest that planet may be a relic of a former trio of outer worlds. “This one’s at the scene of the crime, but we still have the fingerprints of the other two,” says team member Benjamin Montet at the California Institute of Technology in Pasadena.

He cites simulations by Pierre Gratia and Daniel Fabrycky at the University of Chicago, which describe how three outer planets could interact gravitationally, tilting the inner duo. During this process, one outer planet gets ejected or collides with another, leaving just two outer planets.

If that’s the case, a fourth planet should be in orbit around Kepler 56. “It could be on a very far-out orbit,” says Gratia, with such a long period that observers have not yet detected it.

Ken Croswell earned his Ph.D. in astronomy from Harvard University and is the author of The Alchemy of the Heavens and The Lives of Stars.

"An engaging account of the continuing discovery of our Galaxy...wonderful." --Owen Gingerich, The New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"A stellar picture of what we know or guess about those distant lights."--Kirkus. See all reviews of The Lives of Stars here.