Planets and stars form within collapsing clouds of dust and gas. As gravity crunches the material into a smaller space, the cloud's spin increases, and it flattens out. At the center, where the greatest amount of material is concentrated, gravity forges hydrogen into helium to liberate energy, and a star is born. Planets spring from the remaining gas and dust in the disk and orbit in the equatorial plane of their parent star. Nearly all of the 4,868 extra-solar planets discovered to date circle their suns like runners on a circular race course.
Just the other day someone asked me if there were any planets that go around the poles of a star instead the equator. At the time I wasn't aware of any. But I'm happy to share that an international team, led by astronomers from the University of Geneva (Switzerland), recently announced the discovery of planets that orbit perpendicular to one another around the star HD 3167 in the constellation Pisces.
The innermost planet, named HD 3167b, is not much bigger than the Earth, with a diameter of 13,460 miles (21,662 km). It orbits an orange dwarf — a cooler and less massive star than the sun — within the star's equatorial plane in just 23 hours. That short period means it's incredibly close to its host sun. Intense radiation has probably stripped away its atmosphere, rendering the orb uninhabitable.
The system's other planets, HD 3167c and d, are larger and orbit in 29.8 days and 8.5 days, respectively. But instead of joining their sibling on the same plane they pass over the star’s poles, nearly 90 degrees above the equator. HD 3167c in particular stands out, with an inclination of 102° to HD 3167b. Seen from "b," the other two planets would travel up and down in the night sky instead of from right to left as the planets do in our sky.
HD 3167b and c were found by the transit method, where astronomers measure a dip in a star's light as the planet passes in front of it. HD 3167d betrayed itself by its gravitational tug on the host star, which makes it wobble slightly to and fro.
The smaller the planet the more difficult it is to detect no matter the method. Critical to the new discovery was a special instrument called ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations), which can measure the minute tugs a planet exerts on a star. Although the planetary trio had been detected several years earlier, when the team re-observed the system with ESPRESSO they were able nail down the orbit of the smallest, HD 3167b. They discovered that it circled around the star's equator perpendicular to the plane of its polar-orbiting siblings.
So what would cause planets to orbit at nearly right angles to each other? Remember that HD 3167b is on a very short leash, circling ultra-close to its host sun. So close in fact that the star's gravity has forced it to maintain its alignment since birth. The other planets, which orbit farther away, are less strongly bound. The team suspects that gravitational interaction with another yet-to-be-discovered planet gradually misaligned their orbits, nudging them from equatorial paths to polar ones.
This is the first cockeyed planetary system we know of. And as you might suspect, the search is now on for the suspected companion. In the meantime, when I'm asked about planets in perpendicular orbits I can happily nod and offer an example.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.