One nice thing about discovering exoplanets is that, once you work out how long their orbits take, they continue to show up like clockwork. The timing is so regular, in fact, that we can use variations to detect the gravitational influence of additional planets in the same exosolar system. And the precise timing is useful in that we can rely on it to point telescopes at the system when we know some of the light we detect has passed through the planet's atmosphere.
Except when we can't.
On Monday, researchers reported on an exoplanet that simply stopped showing up. TOI-2535b is a hot Jupiter-sized planet we've been observing as it transits between its host star, TOI-2535, and Earth for about seven years now. But it recently stopped showing up, and researchers suspect that's due to an unseen companion orbiting in a different plane.
Big changes
TOI-2535 is one of the many stars with exoplanet candidates identified by TESS, the Transiting Exoplanet Survey Satellite. It's a K dwarf, meaning it's somewhat smaller than the Sun. And TESS found signs of a Jupiter-sized planet orbiting it every eight days, an orbit close enough to make TOI-2535b a “hot Jupiter” exoplanet. At that point, there was nothing especially unusual about it.
But a closer look at the data showed two very strange things. First, the timing of when its transits occurred could shift by as much as six hours, a substantial variation for an orbit this brief. That's typically a sign of gravitational interactions with another planet; depending on the details of that second planet's orbit, it may slow the first one or accelerate it through a transit more quickly. Since the second planet doesn't show up in TESS data, it appears to orbit in a slightly different plane, so it doesn't pass between its host star and Earth.
The second oddity is that the transit duration grew progressively shorter in more recent observations. This suggests that, while early observations saw the planet transiting across the star's equator, later ones involved transits near one of the poles, ensuring that the planet spent less time between the star and Earth. This trend eventually reached its logical end point: TOI-2535b simply stopped transiting in the most recent observations.
Obviously, gravitational interactions with a planet orbiting in a different plane could potentially alter TOI-2535b's orbit so it no longer passed between TOI-2535 and Earth. There is also some precedent for the converse happening: a star around which no transits were originally detected but where transits began appearing in later observations. Still, as a Jupiter-sized planet, TOI-2535b should not be that easy to shift, 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 “would need to be either extremely massive or on a highly eccentric orbit,” the researchers noted. So they decided to figure out what sorts of configurations could lead to such rapid changes.
No clear answers
To do so, they modeled 300,000 different starting configurations and ran them forward in time. They found there wasn't a single favored configuration; as long as the planets were in an orbital resonance, things were relatively stable, at least over relatively short periods. So they took the 100 best solutions and ran them forward for a million years.
About two-thirds became unstable during that period, which ruled out a couple of the potential resonances. The remaining third shared a couple of features. One is that the second, undetected planet has to be pretty massive—roughly Saturn-sized at a minimum. The second is that the angle between the orbits of the two planets can't be that large. If it is, the difference would shrink over time, causing their orbits to become increasingly eccentric. Eventually, this results in a close approach between the two planets, followed by instability.
UCLA has posted a video of one of the better models if you're having a hard time visualizing all of this.
While a second planet hasn't been detected via the transit method, its gravitational pull on the star should create changes in the radial velocity we could potentially detect. Right now, however, we don't have enough clean data to detect it. The researchers also say there's enough noise in the transit timing variations that a third planet in the system can't be excluded.
The question we probably can't answer with further observations is how this system got into its current state. Large planets are thought to form farther out from the star and then migrate inward because of friction caused by the gas in the planet-forming disk. (This inward motion tends to stop when orbital resonances develop.) But this friction should also limit the angle between the planets' orbits. Without some additional event that mixed up the orbits, it's not obvious how this system could have come to be.
The Astrophysical Journal Letters, 2026. DOI: 10.3847/2041-8213/aeb4a4 (About DOIs).
