The second curiosity is that the transit duration has become shorter and shorter in recent observations. This suggests that although early observations saw the planet crossing the star’s equator, later observations saw transits occur near one of the poles, ensuring that the planet spent less time between the star and Earth. This trend eventually reached its logical endpoint: TOI-2535b simply stopped transiting in the most recent observations.
Apparently, gravitational interactions with a planet orbiting in a different plane could potentially change TOI-2535b’s orbit so that it no longer passes between TOI-2535 and Earth. There is also a precedent for the opposite: a star around which no transits were originally detected, but transits did occur in later observations. Still, as a planet the size of Jupiter, TOI-2535b shouldn’t shift easily, and as the researchers note, “This is the fastest rate of inclination change observed for an exoplanet orbiting a single star.”
The second, unseen planet “must be either extremely massive or in a highly eccentric orbit,” the researchers noted. So they decided to find out which configurations could lead to such rapid changes.
No clear answers
To do this, they modeled 300,000 different starting configurations and ran them forward in time. They found that there was no single preferred configuration; As long as the planets were in orbital resonance, things were relatively stable, at least over relatively short periods of time. So they took the 100 best solutions and ran them for a million years.
About two-thirds became unstable during this period, ruling out some possible resonances. The remaining third had some similarities. For one thing, the second, undiscovered planet must be quite massive – at least about the size of Saturn. The second reason is that the angle between the two planets’ orbits cannot be that large. If this were the case, the difference would become smaller over time, causing their orbits to become more and more eccentric. This eventually leads to a convergence of the two planets, followed by instability.
