Astronomers propose a second, tilted disk to explain GJ 3090 b's backward orbit around a red dwarf

The case against a hidden perturber rests mostly on where the planets sit. GJ 3090 b is the closest to the star. A massive outer planet or an unseen binary companion strong enough to tip it over would have disturbed the more distant planets more, not less ^[12]. Those planets are not upside down ^[12]. "We looked for the kind of massive companion that could have forced the planet into such an extreme orbit," said Yann Carteret, a University of Geneva astronomer and co-author of the study ^[5]. "We may need to think differently about how this system acquired its unusual architecture," he said ^[6].

Planets normally turn the same way as the disk they form in, because reversing that rotation takes a lot of energy applied in the right way ^[2]. The team's proposal keeps that rule and swaps the disk. Gas and dust from outside the system may have fallen in and replaced the original disk with a second one at a different angle, and GJ 3090 b would then have formed from the newer disk ^[13]. The star kept its own spin because the inner planets hold less material than it would take to pull the star's rotation around, so the system stays misaligned ^[14]. "The idea that a planetary system could be rebuilt from a second, differently oriented disk is particularly exciting," said Vincent Bourrier, another Geneva astronomer on the paper ^[15].

The star made the measurement difficult. M dwarfs are small and cool, and so dim that even the nearest cannot be seen with the naked eye ^[8]. The 10 transits came from 23 hours of data, an average of about 2.3 hours per crossing ^[1]. From those observations the team placed the orbit 136 degrees out of line with the star's spin, among the largest misalignments recorded and 44 degrees short of a fully reversed orbit ^{[11] [2]}.

The thing this doesn't tell you is how often it happens. M dwarfs are about three-quarters of the Milky Way's stars ^[9], so a formation route common among them would matter for most of the galaxy's stars. One backward planet around one M dwarf shows the outcome can occur there. The report does not say how many M dwarf planets have had their spin-orbit angles measured, so there is no rate to set this one against.

In my view the aligned outer planets are firmer evidence than the companion search. The researchers state that result cautiously: the system does not seem to contain a massive object that could have changed the planet's orbit ^[4].