In our solar system the Sun spins one way and every planet goes around in that same direction, close to the line of the Sun's equator. That is what you get when planets form out of a flat disk of gas and dust that turns with its star. GJ 3090 b breaks that rule. A team of 59 astronomers, with Yann Carteret as first author, measured the angle between the planet's orbit and its star's spin at about 136 degrees. Anything past 90 degrees means the planet is going around the wrong way. The paper came out in Astronomy & Astrophysics in September 2026.
The star, GJ 3090, is a red dwarf about half as wide as the Sun, some 22.5 parsecs away, which is about 73 light years. The planet is 2.18 times as wide as Earth and about 4.5 times as heavy, and it finishes a lap in roughly 2.9 days. It shares the system with at least one more planet, which takes 16 days.
Nobody can see the orbit itself, so the team watched the planet cross its star. A spinning star has one side coming toward us and one side moving away. When the planet covers a bit of it, the starlight shifts in a small but telling way, and the pattern shows which side the planet crosses first. This is called the Rossiter-McLaughlin effect. They caught six crossings with the NIRPS instrument on the 3.6 meter telescope of the European Southern Observatory at La Silla, in Chile.
Tipped orbits usually have a culprit. A giant planet or a second star nearby can slowly tug an orbit until it turns over. The authors point out that the five systems with several planets and a strongly tipped orbit known before this one all have a companion like that. Here they searched with years of measurements of the star's wobble, sharp imaging and data from the Gaia space telescope, and found no giant planet and no second star.
Their best explanation is that the planet never flipped. The disk it grew in may have been tilted from the start, maybe because the star gathered a second disk of gas late, one that did not line up with its spin. The planets would then have formed and drifted inward inside it, already going the wrong way.
