Astro Bob: astronomers discover solar system with planets at right angles


Planets and stars form in collapsing clouds of dust and gas. As gravity crushes the material in a smaller space, the cloud’s rotation increases and it flattens. In the center, where the greatest amount of matter is concentrated, gravity forges hydrogen into helium to release energy, and a star is born. The planets gush out of the gas and dust remaining in the disk and orbit in the equatorial plane of their parent star. Almost all of the 4,868 extra-solar planets discovered to date revolve around their sun like runners on a circular running course.

Our solar system (and many others) formed when an interstellar cloud of gas and dust collapsed under the force of gravity. As it compressed it would spin and flatten as well. The planets froze from clusters of matter inside the disc. Contribution / NASA, JPL-Caltech

The other day someone asked me if there were any planets circling around the poles of a star instead of the equator. At the time, I was not aware of it. But I am 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 each other around the star HD 3167 in the constellation Pisces.

The innermost planet, named HD 3167b, is not much larger than Earth, with a diameter of 13,460 miles (21,662 km). It orbits an orange dwarf – a star cooler and less massive than the sun – in the star’s equatorial plane in just 23 hours. This short period means that it is incredibly close to its host sun. Intense radiation likely destroyed its atmosphere, rendering the orb uninhabitable.


Using the European Southern Observatory's Very Large Telescope (VLT) in Chile, astronomers have elucidated the perpendicular orbits of the planets around the orange dwarf star HD 3167. Contributed / ESO

Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, astronomers have elucidated the perpendicular orbits of the planets around the orange dwarf star HD 3167. Contributed / ESO

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The other planets in the system, HD 3167c and d, are larger and orbit in 29.8 days and 8.5 days, respectively. But instead of joining their brother on the same plane, they pass above the poles of the star, almost 90 degrees above the equator. The HD 3167c particularly stands out, with an inclination of 102 ° compared to the HD 3167b. Seen from “b”, the other two planets would travel up and down in the night sky instead of right to left as the planets do in our sky.

HD 3167b and c were found by the mode of transport, where astronomers measure a drop in a star’s light as the planet passes in front of it. HD 3167d betrayed itself by its gravitational pull on the host star, causing it to oscillate slightly back and forth.


HD 3167, the star hosting the planets at right angles, shines at a magnitude of 9.0, bright enough to be seen in a small telescope or large binoculars.  Start at Algenib in the lower left corner of the Grande Place and work your way down to lower Delta in Pisces the Fish.  Then use the detailed map below to locate the star.  Contribution / Stellarium

HD 3167, the star hosting the planets at right angles, shines at a magnitude of 9.0, bright enough to be seen in a small telescope or large binoculars. Start at Algenib in the lower left corner of the Grande Place and work your way down to lower Delta in Pisces the Fish. Then use the detailed map below to locate the star. Contribution / Stellarium

The smaller the planet, the harder it is to detect, regardless of the method used. A special instrument called ESPRESSO (Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations Scale), which can measure the tiny tugs that a planet exerts on a star. Although the planetary trio was detected several years earlier, when the team re-observed the system with ESPRESSO, they were able to determine the orbit of the smaller one, HD 3167b. They discovered that it rotated around the star’s equator perpendicular to the plane of its polar orbiting siblings.


From Delta, "star jump" at HD 3167 and use your mind to imagine the three planets there.  Contribution / Stellarium

From Delta, “star jump” to HD 3167 and use your mind to imagine the three planets there. Contribution / Stellarium

So what would cause the planets to orbit almost at right angles to each other? Remember that HD 3167b is kept on a very short leash, spinning very close to its host sun. So close in fact that the star’s gravity has forced it to maintain alignment since birth. The other planets, which orbit further away, are less strongly linked. The team suspects that the gravitational interaction with another planet to discover gradually misaligned their orbits, causing them to pass from equatorial trajectories to polar trajectories.

It is the first flirtatious planetary system that we know of. And as you can imagine, the search for the suspicious companion is now on. In the meantime, when asked about planets in perpendicular orbit, I can happily nod my head and give an example.

“Astro” Bob King is a freelance writer for the Duluth News Tribune. Read more of his work on duluthnewstribune.com/astrobob.


Arline J. Mercier