Star systems come in all shapes and sizes. Some have lots of planets, some have larger planets and others have no planets at all. But a particularly unusual system about 150 light-years from our own has scientists scratching their heads.
In 2016, astronomers discovered two planets orbiting the star HD 3167. They were thought to be super-Earths — between Earth and Neptune in size — and circled the star every one and 30 days. A third planet was found in the system in 2017, orbiting in about eight days.
What’s unusual is the inclinations of the outer two planets, HD 3167 c and d. Whereas in our solar system all the planets orbit in the same flat plane around the sun, these two are in polar orbits. That is, they go above and below their star’s poles, rather than around the equator as Earth and the other planets in our system do.
Now scientists have discovered the system is even weirder than they thought. Researchers measured the orbit of the innermost planet, HD 3167 b, for the first time — and it doesn’t match the other two. It instead orbits in the star’s flat plane, like planets in our solar system, and perpendicular to HD 3167 c and d. This star system is the first one known to act like this.
“It was clearly a surprise,” said Vincent Bourrier, from the University of Geneva in Switzerland, who led the discovery published last month in the journal Astronomy & Astrophysics. “This is something radically different from our own solar system.”
While none of the planets are thought to be habitable, were you to stand on one you would see a rather intriguing view of this peculiar system. “If you had a telescope and you were looking at the trajectory of the other planets in the system, they would be going vertically in the sky,” Dr. Bourrier said.
Finding exoplanets in polar orbits is not wholly unusual, said Andrew Vanderburg of the Massachusetts Institute of Technology, who led the initial discovery of HD 3167 c and d but was not involved in the new research. But the perpendicular nature of this system “is odd,” he said.
The latest discovery was made possible by an instrument on the Very Large Telescope in Chile, called ESPRESSO. Using extremely precise measurements of the star, the scientists were able to track the direction in which the innermost planet was passing in front of its star relative to us, known as a transit, and work out the angle of its orbit.
The misalignment in the system may result from an unseen object in its outer reaches. Shweta Dalal, from the University of Exeter in England, has studied the system and said there was evidence of a Jupiter-size planet orbiting the star every 80 days. The gravitational effect of this world could have pushed the outer two planets into their unusual orbits, while the innermost planet remained locked to the star owing to its tight orbit.
“A Jupiter-sized planet could be massive enough to tilt the planets,” Dr. Dalal said.
While our solar system has its own massive Jupiter, the wider orbits of our planets means the same fate has not befallen Earth or the other planets. In contrast, the planets that orbit HD 3167 “are all within the orbit of Mercury,” Dr. Dalal said, and thus close together, magnifying the effects of their interactions.
Upcoming observations may reveal more systems like this. The European Space Agency’s Gaia telescope, which is mapping billions of stars in the Milky Way, is expected to reveal data soon on thousands of giant planets in other star systems, including inclination data for those that transit. Dr. Bourrier and his team also hope to use ESPRESSO to make similar observations of other systems.
The unusual configuration of HD 3167 highlights just how weird and wonderful other stars and their planets can be. “It puts in perspective again what we think we know about the formation of planetary systems,” Dr. Bourrier said. “Planets can evolve in really, really different ways.”
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